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EUR 2 1 1 0 e A STUDY O F SOME NIOBIUM-BEARING MINERALS O F THE LUESHE CARBONATITE DEPOSIT (Kivu, Republic of Congo) by L. VAN WAMBEKE (Euratom) European Atomic Energy Community — E U R A T O M Joint Nuclear Research Center — Ispra Establishment (Italy) Materials Department — Metallurgy and Ceramics Service Contract EURATOM/N.V. Hollandse Metallurgische Industrie Billiton No. 017-61-9 ISPN Paper presented at the Symposium on the Mineralogy of Carbonatites and Alkaline Ultrabasic Rocks, New Delhi (India), December 1964 Brussels, January 1965, 31 pages, 6 figures New data are given on the mineralogy and on the geochemistry of some niobium-bearing minerals from the Lueshe carbonatite deposit : pyrochlore, hydrated K-Sr pyrochlore, fersmite, columbite, lueshite, aegirite, zircon and a probably new iron titanium mineral of formula (Fe 11 , Mn, M g ) 4 F e 2 m Tie O19. In

EUR 2110.e A STUDY O F SOME NIOBIUM-BEARING MINERALS O F THE LUESHE CARBONATITE DEPOSIT (Kivu, Republic of Congo) by L. VAN WAMBEKE (Euratom) European Atomic Energy Community — E U R A T O M Joint Nuclear Research Center — Ispra Establishment (Italy) Materials Department — Metallurgy and Ceramics Service Contract E U R A T O M / N . V . Hollandse Metallurgische Industrie Billiton No. 017-61-9 ISPN Paper presented at the Symposium on the Mineralogy of Carbonatites and Alkaline Ultrabasic Rocks, New Delhi (India), December 1964 Brussels, January 1965, 31 pages, 6 figures New data are given on the mineralogy and on the geochemistry of some niobium-bearing minerals from the Lueshe carbonatite deposit : pyrochlore, hydrated K-Sr pyrochlore, fersmite, columbite, lueshite, aegirite, zircon and a probably new iron titanium mineral of formula (Fe 11 , Mn, Mg)4 Fe2 U I Tie O19. In

EUR 2110.e A STUDY O F SOME NIOBIUM-BEARING MINERALS O F T H E LUESHE CARBONATITE DEPOSIT (Kivu, Republic of Congo) by L. VAN WAMBEKE (Euratom) European Atomic Energy Community — E U R A T O M Joint Nuclear Research Center — Ispra Establishment (Italy) Materials Department — Metallurgy and Ceramics Service Contract E U R A T O M / N . V . Hollandse Metallurgische Industrie Billiton No. 017-61-9 ISPN Paper presented at the Symposium on the Mineralogy of Carbonatites and Alkaline Ultrabasic Rocks, New Delhi (India), December 1964 Brussels, January 1965, 31 pages, 6 figures New data are given on the mineralogy and on the geochemistry of some niobium-bearing minerals from the Lueshe carbonatite deposit : pyrochlore, hydrated K-Sr pyrochlore, fersmite, columbite, lueshite, aegirite, zircon and a probably new iron titanium mineral of formula (Fe 11 , Mn, Mg) 4 F e 2 m Tie O19. In

the three last minerals, Nb and Ta occur as accessory elements. B a­goyazite was also found as mineral inclusions in some pyrochlores. The new hydrated K­Sr pyrochlore shows a large deficit of the elements in the A­positions and a parameter value of about 10.58 Å. Its origin is discussed. Two types of fersmites (I and II) were found : the first is associated to the columbitization of pyrochlore, the second replaces columbite and occurs in a later carbonatitic stage. Physical properties, X­ray data and a chemical analysis are given for the probably new iron titanium mineral. The geochemical distribution of Nb, Ta, rare earths, U, Th and other elements was studied in all minerals investigated. The Nb­Ta ratio widely varies and is much higher in the niobium minerals whereas the lowest values were found in the Nb­containing minerals. The Ce group dominates in the pyrochlores and in lueshite and the Y group in the fersmite II and in the zircon. The Ca­niobates are generally richer in U whereas Th is predominant in zircon and especially in lueshite. The processses of the columbitization of pyrochlore and the fersmitization of columbite are described. From these processes it is indicated that the carbona­ titic differentiation is characterized in a late stage by an enrichment with Ta, Fe, Mn, V and rare earths.

the three last minerals, Nb and Ta occur as accessory elements. B a­goyazite was also found as mineral inclusions in some pyrochlores. The new hydrated K­Sr pyrochlore shows a large deficit of the elements in the A­positions and a parameter value of about 10.58 Λ. Its origin is discussed. Two types of fersmites (I and II) were found : the first is associated to the columbitization of pyrochlore, the second replaces columbite and occurs in a later carbonatitic stage. Physical properties, X­ray data and a chemical analysis are given for the probably new iron titanium mineral. The geochemical distribution of Nb, Ta, rare earths, U, Th and other elements was studied in all minerals investigated. The Nb­Ta ratio widely varies and is much higher in the niobium minerals whereas the lowest values were found in the Nb­containing minerals. The Ce group dominates in the pyrochlores and in lueshite and the Y group in the fersmite II and in the zircon. The Ca­niobates are generally richer in U whereas Th is predominant in zircon and especially in lueshite. The processses of the columbitization of pyrochlore and the fersmitization of columbite are described. From these processes it is indicated that the carbona­ titic differentiation is characterized in a late stage by an enrichment with Ta, Fe, Mn, V and rare earths.

the three last minerals, Nb and Ta occur as accessory elements. B a­goyazite was also found as mineral inclusions in some pyrochlores. The new hydrated K­Sr pyrochlore shows a large deficit of the elements in the A­positions and a parameter value of about 10.58 Å. Its origin is discussed. Two types of fersmites (I and II) were found : the first is associated to the columbitization of pyrochlore, the second replaces columbite and occurs in a later carbonatitic stage. Physical properties, X­ray data and a chemical analysis are given for the probably new iron titanium mineral. The geochemical distribution of Nb, Ta, rare earths, U, Th and other elements was studied in all minerals investigated. The Nb­Ta ratio widely varies and is much higher in the niobium minerals whereas the lowest values were found in the Nb­containing minerals. The Ce group dominates in the pyrochlores and in lueshite and the Y group in the fersmite II and in the zircon. The Ca­niobates are generally richer in U whereas Th is predominant in zircon and especially in lueshite. The processses of the columbitization of pyrochlore and the fersmitization of columbite are described. From these processes it is indicated that the carbona­ titic differentiation is characterized in a late stage by an enrichment with Ta, Fe, Mn, V and rare earths.

EUR 2110.e EUROPEAN ATOMIC ENERGY COMMUNITY — EURATOM

A STUDY OF SOME NIOBIUM-BEARING MINERALS OF THE LUESHE CARBONATITE DEPOSIT (KIVU, REPUBLIC OF CONGO) by L. VAN WAMBEKE (EURATOM)

1965

Joint Nuclear Research Center Ispra Establishment — Italy Materials Department Metallurgy and Ceramics Service Contract EURATOM/N.V. Hollandse Metallurgische Industrie Billiton No. 017-61-9 ISPN Paper presented at the Symposium on the Mineralogy of Carbonatites and Alkaline Ultrabasic Rocks New Delhi, India — December 1964

CONTENTS I — The pyrochlores A — The unweathered pyrochlores a — Mineral impurities b — Physical properties c — X-ray data d — Chemical composition e —Geochemical data B — The weathered pyrochlores (hydrated K-Sr pyrochlores) a — Mineral impurities b — Physical properties c — DTA data d — X-ray data e — Chemical composition f — Geochemical data and origin of the weathered pyrochlores g — Nomenclature II — The fersmites a — Mineral impurities b — Physical properties c — X-ray data d — Chemical composition of fersmite II e — Geochemical data and origin of the fersmites

5 5 5 6 6 7 7 9 9 10 11 11 12 14 15 15 15 16 18 18 20

I I I — T h e columbites a — Mineral impurities b — Physical properties c — X-ray data d — Chemical composition of columbites e — Geochemical data

21 21 21 21 21 22

IV — The lueshite a — Chemical composition b — Geochemical data

23 23 24

V — Niobium-containing minerals A — Aegirite a — Chemical analysis b — Geochemical data B — Zircon a — Chemical analysis b — Physical properties c — Geochemical data C — The new iron titanate mineral a — Physical properties b — X-ray data c — Chemical composition d — Geochemical data e — Nomenclature

25 25 25 25 26 26 26 26 26 27 27 28 29 29

VI — Conclusions

29

VII — Bibliography

30

KEY TO THE TABLES Table Table

I — Density values and refraction indices of the unweathered pyrochlores. II — Parameter values of the unweathered pyrochlores.

Table

III — Chemical analyses of unweathered pyrochlores from Lueshe.

Table

IV — Powder pattern of the barian-goyazite from Lueshe.

Table Table

V — Density values and refraction indices for weathered pyrochloies. VI — Parameter values of weathered pyrochlores.

Table VII — Powder patterns of two weathered pyrochlores. Table VIII — Chemical analyses of weathered pyrochlores from Lueshe. Table Table Table

IX — Powder pattern of fersmite II. X — Chemical composition of fersmite II. XI — Chemical analyses of columbites.

Table XII — Chemical analyses of lueshite and of vermiculite from Lueshe. Table XIII — Chemical analysis of the aegirite from Lueshe. Table XIV — Chemical analysis of the brown zircon from Lueshe. Table XV — Powder pattern of the Fe-Ti mineral from Lueshe. Table XVI — Chemical analysis of the Fe-Ti mineral from Lueshe.

K E Y TO T H E F I G U R E S Fig. 1 — DTA curve of a hydrated potassium-strontium pyrochlore. Fig. 2 — Greenish hydrated K-Sr pyrochlore : Ilmenite and rutile inclusions. Fig. 3 — Columbite I : Laths and needles of fersmite T. Fig. 4 — Columbite II : Veinlets of fersmite II replacing the columbite. Fig. 5— Fersmite II : Relicts of columbite. Fig. 6 —Columbite II : Substitution of columbite II by fersmite II.

A S T U D Y O F SOME N I O B I U M - B E A R I N G M I N E R A L S O F T H E L U E S H E CARBONATITE DEPOSIT (Kivu, Republic of Congo)

The Lueshe carbonatite is one of the richest pyrochlore-apatite deposits in the world. A geological and petrological description of the alkaline rocks and the carbonatites was given by de Bethune and Meyer (1). A preliminary mineralogical and geochemical report on some minerals of the søvite was published by the author (2). A new niobium mineral, lueshite (NaNbOs) was also found in this deposit (3). This study is a contribution to the mineralogy and the geochemistry of some Nb-bearing minerals of Lueshe. Besides pyrochlore, which is the most important niobium mineral of the carbonatites, columbite and fersmite occur locally in the NW part of the deposit, whereas lueshite is found in a vermiculite-rich contact zone between cancrinite-syenite and carbonatite. Zirconium and titanium minerals, mainly zircon, ilmenite, rutile and also some silicates such as aegirite, which is an important component of the søvite, also contain small amounts of Nb. The weathered pyrochlores, the columbites and the fersmites were collected from prospection trenches in the residual soils and eluvials; the unweathered pyrochlores, the zircon and a probably new Fe-Ti mineral in the alluvials; the aegirite and the lueshite in the original rocks.

I — THE PYROCHLORES The pyrochlores, mostly found as single octahedrons up to 1 cm, occur as an important accessory mineral of the søvite. Based on their chemical composition and also by their parameters, the pyrochlores from Lueshe can be divided in two groups : I o The unweathered pyrochlores found mainly in the søvite and in the alluvials, 2° The weathered pyrochlores mainly coming from the eluvials and from the residual soils. Generally, both types of pyrochlores contain numerous mineral inclusions, sometimes showing a zonal distribution. All the pyrochlores are radioactive. A — The unweathered pyrochlores The unweathered pyrochlores from Lueshe are characterized by a great diversity of colour : green, pink, brown, white. The green pyrochlores (Pi) are generally rich in mineral inclusions such as ilmenite, rutile and also contain small amounts of anatase and brookite. The brown zoned variety is partially metamict and the radioactivity is mainly caused by the presence of U up to more than 1.5%. a — Mineral impurities Magnetic and gravimetric separations were widely used for the study of the numerous mineral inclusions found in these pyrochlores. The different concentrates obtained were carefully examined mostly by X-ray diffraction and the determination of the ilmenite and other titanium mineral contents was approximately estimated in polished sections on several crystals. Many diffi-

culties were encountered in this case because of the irregular distribution of the titanium minerals inside the crystal itself and also from one crystal to another. The mineral inclusions in the pyro­ chlores are essentially composed by the normal components of the søvite : calcite, aegirite, fluor­ apatite, pyrrhotite. The titanium minerals occur as exsolutions in the unweathered pyrochlores when the TiC>2 content exceeds about 4%. Traces of goyazite were also found in some green pyro­ chlores. b — Physical propert ies Density measurements using the pycnometer method corrected for mineral impuritie s have given close values for the different varieties of unweathered pyrochlores. These values also well agree with the calculated density derived from the chemical formula and the lattice constant. The refraction indices measured are comprised between 2.04 and 2.15. Table I gives the measured and calculated densities and the refraction indices for the different varieties of pyrochlore. TABLE I Density values and refract ion indices of t he unweat hered pyrochlores

Type of pyrochlores

Green pyrochlore Pi Brown­black pyrochlore Pu Pink pyrochlore P m White pyrochlore Piv

Density measured corrected for impurities

Density calculated

4.15 4.16 4.05 4.15

4.19 4.20 4.11 4.12

Refraction indices

2.11 2.04 2.06 2.15

± 0.02 ­ 2.05 ­ 2.07 ±0.02

c — X-ray dat a The lattice constant for the different types of unweathered pyrochlores from Lueshe is comprised between 10.41 and 10.43 Å. The values of the parameter are given in Table II : TABLE II Parameter values of t he unweat hered pyrochlores

Type of pyrochlores Green pyrochlore Pi Brown­black pyrochlore P u Pink pyrochlore P m White pyrochlore Piv

ao ao no a0

= = = =

Parameter 10.415 ± 0.004 10.415 ± 0.004 10.422 ± 0.002 10.414 ± 0.003

 ŠA Å

The dispersion of the parameter values for the different types of pyrochlores is very small compared, for example, to that of the Kaiserstuhl carbonatites, where it varies from 10.38 to 10.42 Â (4a) and where the dispersion is related with the variations of the chemical composition and especially with the Ti and Fe111 contents. The values of the cell dimension for the weathered Na­Ca pyrochlores from Lueshe well agree with those found for the same mineral in several other carbonatite deposits in the world. These data are summarized in the excellent study of pyro­ chlore made by van der Veen (5). A mean value of 10.41 ±0.03 Å can be accepted for the lattice constant of Na­Ca pyrochlores of carbonatitic origin corresponding to the formula A2­:cB 2 07­ 2/ where χ and y do not exceed respectively 0.6 and 1 and the Ti content 7%. We must also note that this mean value approximates to the cell dimension of many Na­Ca microlites of granitic and of pegmatitic origin.

d — Chemical composition The different types of pyrochlore were analyzed by the N. V. Hollandse Metallurgische Industrie Billiton on the purest material available, selected under binoculars and examined under the microscope and by X-ray diffraction. A preliminary analysis was made by us with an X-ray Philips spectrometer using an original system of autodiscrimination (6). The results of each chemical analysis were verified afterwards in our laboratory and completed especially for the rare earth distribution. Table III gives the analysis of the unweathered pyrochlores taking account of the mineral impurities found. The samples were not treated by acid to avoid partial dissolution of possible ions of the pyrochlores. The deduction of the mineral impurities is based on chemical analyses made on aegirite (see below), on a concentrate of fluorapatite and on calcite which contains about 1 % of SrO (2). The ratios P2O5/F and F/Cl for the fluorapatite are respectively 19 and 6, and the refraction indices : Ng = 1.638 - Np = 1.634. From the chemical composition of the unweatered pyrochlores we can derive the following formulas : I o Green pyrochlore : [Cao.98 Nao.so Sr0.o4 REo.oi (K, Ba, U, Fe, Mn, Th)0.o2] [Nbi.83 Tio.i6 (Zr, Ta, V)o.oi] 06.38 [F0.36 OH0.22I 2° Brown-black pyrochlore : [Cao.88 Nao.72 Sro.os U0.02 (Fe, Th, RE, Ba, K, Pb, Mn)0.o3] [ Nbi.80 Tio.is (Zr, Ta, V, Fe m ) 0 .o2] 06.28 [OH0.53 F0.30] 3° Pink pyrochlore : [Cao.9o Na0.74 Sr0.o4 (RE, Fe, K, U, Ba, Th, Mn, Pb)0.o3] [Nbi.si Tio.17 (Zr, Ta, V, Fe m ) 0 .o2] 06.26 [OH0.43 F0.21] 4° White pyrochlore : [Cao.89 Nao.80 Sro.os (K, Fe, RE, Ba, Th, Mn, Pb, U)0.02] [Nbi.82 Tio.175 (Zr, Ta, V)0.oos] 06.24 [OH0.50, F0.29] The calculated formulas show that the unweathered pyrochlores analyzed have about the same chemical composition and are essentially titaniferous niobates of Ca and Na with minor amounts of Sr, the latter element being characteristic of carbonatite deposits (2). Taking the general formula of pyrochlore (5) : A2-zB2 07±j, the value of χ for the analyzed pyrochlores is comprised between —0.15 and —0.30 and of y between - 0 . 1 0 and +0.10. The good uniformity of chemical distribution seems to indicate that all the pyrochlores analyzed, despite their differences of colour, were formed from carbonatitic differentiates having nearly the same composition. It is however possible that other generations of pyrochlores with a different composition are also present at Lueshe. e — Geochemical data The unweathered pyrochlores which were analyzed are characterized by a relatively weak geochemical dispersion for all the elements with the exception of U, and differ therefore consider­ ably from the pyrochlores of the Kaiserstuhl carbonatites where the geochemical dispersion is

TABLE III Chemical analyses of unweathered pyrochlores from Lueshe

Green pyrochlore

Brown black zoned pyrochlore P u

Pi

Elements analysed

Na20 K20 CaO BaO SrO MgO FeO MnO Th02 U3O8

PbO Ce203 La 2 03 Nd203 Pr203 Sm203 Y2O3

Gd203 Nb205 Ta 2 O s

v2o5

Ti02 Zr02 Fe203 A1 2 0 3 F H2O+

H20-

co2

S1O2 P2O5

S

Analysis of the mineral

6.33 0.09 15.8 0.16 1.05 Pr ^ Sm with a mean value of the ratio Ce/La/Nd of about 4/2/1. For the Y group, Y is generally the predominant element. The ratio CaO/SrO varies between 10 and 18. The unweathered pyrochlores generally contain more U than Th but the ratio Th/U widely varies from 0.25 to 11. Β — The weathered pyrochlores These pyrochlores are found principally in the weathering zone of the carbonatite and also sometimes in the stream sediments. They have a greenish colour and frequently their octa­ hedrons are strongly corroded. a — Mineral impurit ies The same procedure as described in the preceding paragraph was used for the study of the mineral impurities. B esides inclusions of ilmenite, rutile (Fig. 1), the following minerals were detected : brookite, anatase, calcite, fluorapatite, barian­goyazite, goethite, kaolinite, and some­ times traces of aegirite and pyrrhotite. The goethite content for the material analyzed was deter­

Fig. 1 — Greenish hydrated K­Sr pyrochlore: ilmenite and rutile inclusions (niçois: / / ; magn. 200).

mined by chemical treatment. A new mineral for this deposit, barian­goyazite, was identified by X­ray diffraction and occurs quite frequently in the weathered pyrochlores. Table IV gives the powder pattern of this mineral. B arian­goyazite sometimes is associated with fluorapatite and

has a specific gravity of about 3.25. X-ray fluorescence and chemical tests show that the goyazite of Lueshe contains variable amounts of Ba : for example, in the analysed pyrochlores Pvi and Pvii, the values of the ratio SrO/BaO are respectively 6.5 and 4.5. However not all weathered pyrochlores show inclusions of goyazite; some have only fluorapatite as phosphate minerals. TABLE IV Powder Pattern of the barian-goyazite from Lueshe (CuKoc) Mineral of Lueshe

Goyazite of Binnen ASTM

Uh

d{k)

Ilio

5.71 4.89 3.50 2.94

60 10 50 100

2.74 2.44 2.20 2.16 2.00 1.89 1.75

30 30 40 30 10 40 30

1.66

10

1.62

10

5.73 4.91 3.49 2.96 2.83 2.75 2.45 2.20 2.16 2.00 1.89 1.75 1.71 1.66 1.64 1.62

100 20 80 100 20 20 20 80 20 20 60 40 5 20 5 20

d{k)

b — Physical properties Density measurements corrected for mineral impurities have given values that are in good agreement with the calculated density derived from the chemical formula and the lattice constant, especially if we take account of the frequent presence of holes inside the crystals. For heavy weathered pyrochlores, the density decreases to 3.30. A particular case is shown by a partially weathered pyrochlore (Pv) which has a mean density of 3.75, value situated between the unweathered and the weathered pyrochlores. The low density values are characteristic for highly hydrated pyrochlores. The refraction indices are comprised between 1.93 and 1.99 and density values are appreciably lower than the corresponding measurements for the unweathered pyrochlores. The density values and refraction indices for some weathered pyrochlores are summarized in Table V. TABLE V Density values and refraction indices for weathered pyrochlores Type of pyrochlores

Density measured

Partially weathered pyrochlore Pv

3.75

Greenish pyrochlore Pvi Greenish white pyrochlore Pvii Greenish grey pyrochlore P v m

3.48 3.42 3.40

10

Density calculated

3.52 3.48

Refraction indices

(1.98 external zone) (2.09 internal zone) 1.985 ± 0.01 1.95 ± 0.02 1.97 ± 0.02

The Table V shows that the partially weathered pyrochlore Pv has two refraction index values, that of the internal zone corresponding to the value found for unweathered pyrochlores. The reflectivity for the weathered pyrochlores is 13.3% (measured by A. H. van der Veen) but locally decreases to 11%, especially in the most weathered zone. The hardness (A. H. van der Veen) varies between 315 and 366 with a mean of 351 V.H.N. This corresponds to a hardness between 4 and 4­J­ on Moh's scale. c — D TA dat a (Fig. 2) The weathered pyrochlores show an endothermic peak at 325°C and exothermic reactions at 560 and 860°C. The endothermic peak is the dehydration peak. The DTA curve is very similar to that obtained with the B a­Sr pyrochlore of Panda­Hill (7). DTA CURVE OF HYDRATED K-Sr PYROCHLORE

^V

—J—

ι

100

1 300

1

1

1

500

1

1

700

1— 900

Toc

Fig. 2 — DTA curve of a hydrated K­Sr pyrochlore.

d — X-ray dat a By their structure, the minerals belong undoubtedly to the pyrochlore group and do not give any forbidden reflections. The cell dimensions measured on several weathered pyrochlores vary between 10.56 and 10.59 Å. These values are the highest observed for the pyrochlore group. Parameter values are given in Table VI. They were found by linear extrapolation to the zero value of the function cos20/sin(3 ­f cos0/0. TABLE VI Parameter values of weat hered pyrochlores

Parameter values

Type of pyrochlores ίαο = \a0 = ao = a0 = ao = a0 = a0 = a0 =

Partially weathered pyrochlore Pv Greenish pyrochlore Pvi Greenish white pyrochlore Pvii Greenish grey pyrochlore Pvm Other weathered pyrochlores I II III

11

10.415 ± 0.005 A for the internal zone 10.585 ± 0.005 A for the external zone 10.584 ± 0.004 A 10.580 ± 0.004 A 10.570 ±0.003 A 10.568 ± 0.003 A 10.577 ± 0.003 A 10.588 ± 0.003 A

Tables V and VI show clearly that the green pyrochlore Pv coming from alluvials constttutes an interesting case : the internal zone has a refraction index and also a parameter value corresponding closely to the normal unweathered green pyrochlore Pi already described in the preceding paragraph, whereas for the external zone these values are similar to the weathered green pyrochlores. Table VIII gives the interplanar spacings, the intensities of the lines and the plane hkl for two weathered pyrochlores (Pvi and Pvm). TABLE VII Powder patterns of two weathered pyrochlores (CuKa)

Pvm

Pvi

a 0 = 10.570 ± 0.003 A

«o = 10.584 ± 0.004 A

d(A)mes

Ilio

rf(A)mes

Ilk

6.16 3.21 3.07 2.66 2.44 2.17 2.045 1.875 1.794

87 77 100 15 4 5 18 46 19

6.10 3.19 3.04 2.64 2.43 2.16 2.034 1.865 1.787 1.751 1.613 1.594 1.525 1.480 1.376 1.322 1.247 1.223 1.214 1.171 1.161 1.108 1.079 1.063 1.018 0.986

97 62 100 15 4 3 18 49 19 1 15 37 8 11 8 3 0.5 1 9 8 2 3 4 2 5 0.5

1.617 1.599 1.532 1.485 1.380 1.325

9 30 9 11 8 2.5

1.217 1.174

15 13

1.111 1.082 1.065 1.025 0.988 0.955 0.9255 0.8982 0.8950 0.8828 0.8754 0.8506 0.8372 0.8099 0.8068 0.7976

6 6 4 2 1 1 3 4 7 5 3 2 4 1 2.5 2

0.9237 0.8966 0.8935 0.8808 0.8717 0.8491 0.8357 0.8084 0.8059 0.7967 0.7830

3 2 5 4 2 1 2 1 2 1 0.5

hkl

(111) (311) (222) (400) (331) (422) (511) (333) (440) (531) (600) (442) (533) (622) (444) (711) (551) (731) (553) (800) (822) (660) (751) (555) (662) (840) (911) (753) (931) (844) (933) (771) (755) (666) (953) (11.1.1) (971) (11.3.1) (973) (10.6.2) (12.0.0) (884) (777) (975) (12.4.0) (11.5.5) (13.1.1) (10.6.6) (12.4.4) (11.7.3)

e — Chemical composition The samples were carefully selected and only uncorroded crystals were taken for analysis. The same procedure was used as described before for the analysis of the unweathered pyrochlores. 12

TABLE VIII Chemical analyses of weathered pyrochlores from Lueshe

Elements analyzed

Partially weathered green pyrochlore Pv

Partial analysis of the mineral

Na20

κ2ο

CaO BaO SrO MgO FeO MnO Th02

3.81 1.0 8.6 0.2 2.10

u3o8

PbO Ce203 La203 Nd203 Pr203 Sm203 Y2O3 Gd203 Nb205 Ta205

v2o5

Ti02 Zr02 Sn02 Fe203 A1 2 0 3 F H20+ H20-

1.1 4.1

co2

S1O2 P205 S

Mineral impurities Rutile, Ilmenite, Fluorapatite, Calcite

Greenish pyrochlore Pvi

Analysis of the mineral

Analysis corrected for impurities and reduced to 100%

0.29 2.3 0.84 0.43 3.8 0.1 0.7 0.07 0.11 0.09 0.01 0.22 0.08 0.025 0.025 n.d. 0.006 n.d. 66.40 0.05 0.021 5.6 0.56 n.a. 0.73 3.0 0.32 9.9 1.65 0.4 0.3 2.3 n.a.

0.33 2.68 0.39 0.21 2.52 0.11 0.27 0.08 0.12 0.10 0.01 0.25 0.10 0.03 0.03

—

0.007

—

77.39 0.06 0.023 3.79 0.64

—

0.05 0.17 0.37 10.28

— — — — —

Greenish white pyrochlore PVII

Analysis of the mineral

0.50 2.35 1.0 0.53 3.1 0.1 0.7 0.05 0.15 0.07 0.02 0.21 0.11 0.04 0.04 n.a. 0.02 0.009 68.2 0.1 0.029 5.5 0.31 0.05 0.53 2.7 0.1 10.1 0.27 0.7 0.22 2.3 n.a.

10.0327 O = F 0.135

100.108 O = F 0.042

100.192

100.066

Mineral impurities Goyazite 7.8% Ilmenite 2.0% Rutile, Brookite, Anatase 1.3% Calcite 0.9% Kaolinite 0.55% Goethite 0.3% Traces of aegirite

13

Analysis corrected for impurities and reducrd to 100%

0.57 2.70 0.15 0.25 1.92 0.11

—

0.055 0.17 0.08 0.02 0.24 0.12 0.045 0.045

—

0.022 0.01 78.25 0.11 0.033 3.90 0.36 0.06 0.15

—

0.11 10.52

— — — — —

Mineral impurities Goyazite 7.5% Ilmenite 1.9% Rutile, Anatase 1.1% Calcite 1.6% Kaolinite 0.4% Goethite 0.2%

The chemical compositions of two weathered pyrochlores are reported in Table VIII and the material analyzed was not treated by acid to avoid partial dissolution, especially in this case. However, acid treatment was made on a part of the submitted material in order to determine the goethite content. The main difficulties for the calculation of the chemical formulas of the samples are caused mainly by the irregular distribution of the Ti mineral inclusions, and also by the variations of the Sr and Ba contents of goyazite. Therefore, it was absolutely necessary to have a complete analysis of the samples in order to make the minimum of error in the formulas. The impurities were deduced from the chemical analysis itself, from partial chemical assays, from microscopic observations especially in the case of the Ti minerals and from the analysis of some characteristic minerals already mentioned. A first analysis of a weathered pyrochlore is reported in the work of van der Veen but is unfortunately not complete and therefore cannot be used for the calculation of the chemical formula. From the chemical composition given in Table VIII and by deduction of the impurities, we can derive the following formulas for the weathered pyrochlores : Greenish pyrochlore Pvi : [Ko.is Sro.os Nao.03 Cao.02 Alo.oi Feo.oi (Mg, RE, Ba, Mn, Th, U)0.o3] [Nbi.83 Tio.15 Zro.oi7 (Ta, V)0.oo3J O5.19 [H 2 0i. 8 o F0.03] Greenish white pyrochlore PVII : [Ko.is Sro.06 Nao.06 RE0.01 (Ca, Mg, Ba, Mn, Th, U)0.o2] [Nbi.83 Tio.15 Zro.oi ( F e m , Ta, V, Sn)0.oi] O5.12 [H2O1.82 F.001] The formulas compared with normal pyrochlore formula : CaNaNb206F = A2B206F indicates a large lack, especially in Na-Ca atoms; only one-fifth to one-sixth of the A positions are occupied. In the A group, K dominates largely over all other elements and the entry of this atom with larger ionic size in the structure explains the high values found for the parameter of all weathered pyrochlores. The Sr content is also slightly increased compared to the unweathered pyrochlores analyzed. On the contrary, the Na and Ca contents are very low and F is largely replaced by [H2O] molecules. Compared to panda'fte (7), it is probable that in this structure the water is present as [H2O] molecules and not as (OH) groups. The greenish weathered pyrochlores of Lueshe are thus hydrated potassium-strontium pyrochlores with a large deficit in the A positions. These minerals are also characterized by a quite good exchange capacity as is pandarte. f — Geochemical data and origin of the weathered pyrochlores The partial chemical analysis of Table VIII made on the weathered pyrochlore Pv together with the results of the X-ray data and the physical properties for this mineral (Tables V and VI) show clearly that the main geochemical processes leading to the formation of hydrated K-Sr pyrochlores are the leaching of Na, Ca and F and an enrichment of K, Sr and H2O in the external zone as indicated not only by X-ray diffraction but also by X-ray fluorescence and chemical assays. These geochemical processes seem to have acted only by the weathering of the normal greenish pyrochlores of type Pi in the eluvials and in the residual soils of the carbonatitic zone. The circulation of meteoritic water appears to play, therefore, an important role in these geochemical processes, causing not only leaching and hydration but also ion exchange with modification of the primary structure, this more particularly by the entry of K atoms. The values of the ratios Nb, 05/Ta20ö, Nb205/Ti02, Nb20s/V20s and the distribution of the rare earths, the last charac14

terized by a predominance of the Ce group, are in good agreement with those found for the unweathered pyrochlores, taking account of possible small analytical errors in these complicated analyses and of the difficulties in the estimation of the Ti minerals content. The weathering processes seem not to have affected the distribution and the ratio values of the rare earths except perhaps La and Nd which have some lower values. Also U seems to have been partially leached. The large deficit in the elements of the A group in the structure has also produced higher values for Nb content in the analyses (Table VIII). g — Nomenclature The weathered pyrochlores of Lueshe can be considered as a new variety of pyrochlore, but it belongs to the Commission for New Minerals of the International Mineralogical Association to decide in a definitive way if these pyrochlores must be or not considered as a new species. Until present, there is no unanimity and agreement on the nomenclature of the pyrochloremicrolite-betafite group. The same problem already occurred for the lead-microlite already described by us (11), and on which nomenclature no clear answer has been obtained yet. A complete revision of the nomenclature of the pyrochlore-microlite-betafite by the IMA is absolutely necessary. However, the same problem will certainly be settled in the future on other Nb-Ta-Ti-Zr mineral groups and therefore also needs an urgent solution.

II — THE FERSMITES Several types of fersmites were detected by us in columbite concentrates, coming from the NW part of the carbonatite deposit. Fersmite is a new mineral for this deposit and was never reported in the Republic of Congo. This rare niobium mineral was observed in some carbonatite deposits, in alkaline igneous rocks and pegmatites, and more rarely in contact deposits (8), (9), (10) and (5). In the columbite concentrates studied, fersmite occurs in two different types : 1. Fersmite I, often idiomorphic, closely associated with columbite I, mineral itself pseudomorphic after pyrochlore; 2. Fersmite II, as pseudomorphs after the "octahedral" columbite. The first type of fersmite generally exhibits well developed crystal forms as shown in Figure 3 and is associated to the columbitization process of pyrochlore. This fersmite is mainly concentrated along the columbite masses and appears more frequently with the black columbite I. Its colour is brown-red. The second type offersmite, generally characterized by very fine crystal skeletons, is much more hydrated, as indicated by lower refraction index and reflectivity, than the first one, and also by a chemical analysis. Fersmite II replaces columbite with formation of goethite and is particularly abundant with the brown columbite II. The fersmitzation process of the columbite is clearly proved by the development of fersmite veinlets inside the columbite masses (Figures 4 and 6) with subsequent replacement of the last mineral and by relicts of columbite in process of replacement found in the "octahedral" fersmite II (Fig. 5). Its colour is generally brown to yellowbrown. a — Mineral impurities The mineral impurities found in the "octahedral" fersmite II are represented essentially by columbite, talc, apatite, rutile, goethite, and calcite. Ilmenite is more frequent in the fersmite I. 15

Fig. 3 — Columbite I : Laths and needles of fersmite I (dark grey) in a geode. Columbite (light grey) and fersmite I contain inclusions of ilmenite and rutile (white) (niçois : / /; magn. 400; oil immersion).

Fig. 4 — Columbite II : Veinlets of fersmite II (dark grey) replacing the columbite (grey). Both minerals contain inclusions of ilmenite and rutile (white) (niçois : / /; magn. 400; oil immersion).

b — Physical properties In polished sections, the fersmite I is often well crystallized in prismatic sections and in needles (Fig. 3) and exhibits a week greyish pleochro'r'sm. Its reflectivity is lower than the columbite but generally higher than the unweathered pyrochlores. The internal reflections are generally strong when observed under crossed niçois. By its microscopic properties this mineral can easily be distinguished from pyrochlores and from both types of columbite. Fersmite II shows a lower reflectivity than the first type and the internal reflections are generally much more moderate. It exhibits little needles but when it is not well crystallized it is difficult to distinguish from pyrochlore which also possesses internal reflections. 16

¿m Fig. 5 — Fersmite II : Relicts of columbite (white grey) in the fersmite II (grey) (niçois : / /; magn. 400; oil immersion).

Fig. 6 — Columbite II : Substitution of columbite (white grey) by fersmite II (grey) with formation of goethite (light grey) (niçois : / /; magn. 400; oil immersion).

Density measurements using the pycnometer were made on several small samples of both types of fersmite, selected under the binocular. The specific gravity values varies largely from 4.45 to 4.0 for the fersmites I and from 3.80 to 3.69 for the fersmites II. The indices of refraction measured in Na light for a sample of fersmite I having a specific gravity of 4.45 were the following : Ng = 2.02 biaxial negative N p = —1.91 The values of the indices of refraction of the fersmites II show also variations between about 1.97 to 1.92 for Ng and 1.89 to 1.82 for Np. 17

The values found for the specific gravity and for the indices of refraction are notably lower than those measured on the fersmites of Montana (9) but this can be easily explained by a higher degree of hydration and also for the fersmite II by a deficit in the elements of the cations group. c — X-ray data The presence of fersmite in the columbite concentrates was confirmed by X-ray diffraction. The lines obtained are generally quite diffuse. Table IX gives the X-ray powder pattern for a sample of fersmite II. Its diffraction pattern is very similar to that of fersmite I. TABLE IX Powder pattern of fersmite II (CuKoc) o-(A)

Ilio

7.43 5.32 3.74 3.42 3.04 2.86 2.68 2.615 2.513 2.479 2.24 diff 2.144 2.003 1.965 1.929 1.912 1.873 1.799 1.782 1.767 1.627 1.579 1.521 1.475

4 4 21 4 100 5 1 8 6 7 4 4 1 1 4 2 5 7 4 7 3 5 9 3

hkl

(020) (110) (040) (111) (121) (131) (200) (220) (002) (201) (060) (211) (231) (061) (132) (142) (251) (202) (310) (080) (232) (311) (087) (341) (251) (043) (082) (261) (223)

These lines well agree with those found by H . D . Hess and H.J. Trumpour for the fersmite of Montana (9).

d — Chemical composition of fersmite II Some good "octahedral" crystals of fersmite II were submitted to a chemical analysis after a careful selection under binoculars and after verifications of the mineral inclusions by X-ray diffraction and by microscopic observations. Chemical assays have also shown the presence of about 0.5% goethite. The analysis was made by the N. V. Hollandse Metallurgische Industrie Billiton and was completed in our laboratory, especially for the distribution of the rare earths. The deduction of the numerous mineral impurities (about 10%) is based on quantitative measurements made in polished sections for columbite and rutile. The apatite, the talc and the calcite contents, minerals previously found by X-ray diffraction, were respectively determined from the data of the chemical analysis for P2O5, S1O2 and CO2. We have considered that all the S1O2 content found in the analysis belongs to the talc but it is possible that a part of it enters into the structure of fersmite. IÍ

Table X gives the results of the chemical analysis of fersmite II. TABLE X Chemical composition of fersmite II

Elements analyzed

Na20 CaO SrO MnO PbO Th02 UsOs FeO Ce203 La203 Nd203 Pr203 Sm203

y2o3

Gd203 Er203 Dy203 Yb203 Ho203 Tb203 Fe203 Nb205 Ta205

v2o5

Ti02 Zr02 Sn02 H 2 0+ H20Si02

p2o5

MgO

co2

Analysis of the mineral

Analysis corrected for mineral impurities and reduced to 100%

0.2 6.4 0.16 0.08 0.14 0.23 0.50 2.3 1.2 ] 0.20/ 0.80 ) 2.6 0.13 I 0.27/ 1.9 \ 0.85 ]

0.23 5.74 0.18 0.09 0.16 0.26 0.57 2.05 1.38', 0.22/ 0.92 \ 2.98 0.15 I 0.31 ) 2.18 \

Mineral impurities found

Columbite Talc Apatite Rutile Goethite

4% 2.75% 1.15% 2.00% 0.50%

0.98 J

0.5 I

0.57 f 0.81 \ 4.91 0.23 ( 0.11 ) 0.03/ 0.06 75.06 0.39 0.30 4.21 0.46 0.05 2.31

0.7 \4.28 0.2 1 0.1 ] 0.03/ 0.50 68.5 0.4 0.26 5.9 0.40 0.04 2.2 0.94 1.8 0.5 0.8 0.6 99.73

From the chemical composition and by deduction of the mineral impurities, we can derive approximately the following formula for the fersmite II : [Cao.33 REo.17 Feno.o9 Na0.o2 (U, Sr, Mn, Pb)o.oie] [Nbi.80 Tio.ι? Zro.oi V0.oi (Ta, Fe111, Sn)0.oi] O5.63(OH)0.4i This formula compared with the normal fersmite formula CaNb 2 06 = ΑΒ 2 0« indicates a lack of Ca atoms; only two-thirds of the A-positions are occupied and a part of the O is replaced by (OH) groups or by [H2O] molecules. The fersmite II, macroscopically pseudomorphic after the "octahedral" columbite, is thus a partially hydrated calcium — rare earth fersmite, with only about two-thirds of the A-positions occupied by atoms. 19

Compared to the chemical analyses published for two fersmites, one from the Ravalli County (9), the other from the Vishnevye Mountains, USSR (8), the fersmite II of Lueshe is characterized by a large deficit in Ca atoms, by a higher rare earths content (7.90% RE oxides) and by a partial replacement of O atoms by (OH) group or [H 2 0] molecules. The mineral analyzed has also a higher content of radioactive elements with a value of the ratio U/Th of about 2 and an appreciable content of V. e — Geochemical data and origin of the fersmites The major characteristic difference between the fersmites of both types and the pyrochlores from Lueshe is certainly the geochemical distribution of the rare earths. Not only the total rare earths content for both fersmites is much higher than in the pyrochlores from Lueshe, but also the ratio ZCe/EY is much lower. In the fersmites II, the Y group dominates over the Ce group as indicated by X-ray fluorescence and chemical analysis. In the fersmites I, we find the inverse relationship with a ratio ZCe/ZY varying between 1.1 to 2.0 (see also the chemical analysis of columbite I). The geochemical distribution of the rare earths inside each group remains about the same for both fersmites. For the fersmite II, the distribution is as follows : Y > Gd > Dy > Er > Yb > Ho > Tb Ce > Nd > Sm > La > Pr In relation to pyrochlores, the major changes in the distribution of the rare earths affect the Ce group, characterized especially by a very low La content compared to the Ce and the Nd. This geochemical distribution in the fersmites II is also very different from that found for the fersmites of Ravalli Cy (9) and of Vishnevye (8), where the Ce group dominates over the Y group. However, these last fersmites have a different origin, the first one coming fiom late magmatic solutions emplaced in a buff-colored marble, the second crystallized from alkaline pegmatite. For fersmites II, the major changes in their geochemistry, compared to the replaced mineral columbite, are certainly the variations observed in the values of the ratios Nb20s/Ta205 and Nb205/V2C>5. These values for the fersmites Hare respectively about 190 and 250 against about 35 and 71 for the columbites (see chemical analyses of columbites) and both are also much lower than those found for pyrochlores. The process of replacement of columbite by fersmite II is thus accompanied by a decrease in the Ta and V contents, assuming that this last element is structurally fixed mainly in the columbite. No other mineral was found to contain V, except traces in goethite. These changes in the Nb and V contents at the time of the replacement, affecting the elements in the B-positions, seem to occur only by magmatic solutions or rather by hydrothermal alterations in this case because weathering processes do not affect the elements in the B-positions and especially Nb and Ta. On the other hand, the deficit of the elements in the A-positions (mainly Ca) and the degree of hydration of the fersmite II are partially due to weathering processes. As a matter of fact, we must note that limonite pseudomorphs after pyrite are frequently found in the columbite-fersmite concentrates. The fersmitization of columbite is accompanied by the formation of goethite (limonite), which is especially well developed in the polished sections of the brown columbite II. This process of fersmitization can be explained as follows : 1st stage F e N b 2 0 6 > C a R E N b 2 0 6 + Fe 11 2nd stage F e " OXÎdalion > Fe"*. In the first stage, Ca and the rare earths with minor amounts of other elements take the place of Fe11 in the columbite structure with some change in the parameter values; the Fe 11 liberated is then rapidly oxidized into goethite (limonite). 20

I I I — T H E COLUMBITES Columbite was first identified at Lueshe by Safiannikoff. In all the samples investigated the columbites are pseudomorphic after pyrochlore which is generally completely replaced. As already mentioned above, there are two types of columbites : I o The black columbite I, 2° The brown columbite II. The black columbites generally show a lower degree of limonitization and minor amounts of fersmite II, whereas in the brown columbites the limonitization and the fersmitization processes are well developed (Fig. 4 and 6). a — Mineral impurities The mineral impurities inside the "octahedral" columbites are very numerous. They were studied by microscopy and by X-ray diffraction after a combined magnetic-gravimetric separation. In polished sections, all columbites contain many inclusions of ilmenite and more rarely rutile. Both types of fersmites are also present, but fersmite II is more frequently associated with columbite II. The amount of fersmite varies greatly from one sample to another. Goethite was also found in both columbites and more particularly in the brown variety. The following minerals, in order of approximative decreasing frequency, were observed also : talc, fluorapatite, micas (hydrobiotite and muscovite), calcite, Na-Ca pyrochlore, anatase. Traces of villiaumite (NaF) seem to be indicated by the X-ray diffraction analysis, but the presence of this mineral nevertheless remains doubtful. b — Physical properties In polished sections, columbite, often in prismatic crystals, exhibits a strong amsotropy (clear to deep grey-brown) and a moderate pleochroism, and therefore this mineral can easily be distinguished from both types of fersmite and from pyrochlore. It has a higher reflectivity also. c — X-ray data Several samples of both types of columbite were analyzed by X-ray diffraction and all of them have given the characteristic diffraction lines of columbite. With the exception of fersmite, and sometimes of pyrochlore, no other niobium mineral was found in the X-ray diffraction patterns. Mossite was not observed, either by X-ray or by microscopic investigations. d — Chemical composition of columbite For a chemical analysis, we have carefully selected the purest material available. However, the mineral impurities remain so abundant, even in the purest material, that we can only have an idea of the chemical composition of columbite. Nevertheless, these analyses have brought some interesting results. The chemical analyses are reported in Table XI. The main difficulties arise to the large variations of the chemical composition of both types of fersmite as indicated by the physical properties of this mineral, to the irregular distribution of this mineral and to the presence of pyrochlore in some columbites. Only an electron probe analysis is able to determine the average chemical composition of both types of' columbite, but unfortunately it was not possible for us to make such an analysis. From the chemical analyses of columbites we can outline the following facts : 1. Both types of columbite contain about 2.5% of ilmenite and 0.5% of rutile as mineral inclusions; 21

TABLE XI Chemical analyses of columbites (purest material available) Elements analyzed

NaoO CaO SrO

u3o8

Th02 PbO Ce203 La203 Nd203 Pr203 Sm203 Y2O3 Gd203 Dy203 Er203 Yb203 Ho203 Tb203 MgO MnO FeO Fe203 Nb205 Ta205

v2o5

Ti02 ZrOo SnOu Si0 2

p2o5

Al 2 O a Cr03 H 2 0+ F H20"

Black-columbite I

Brown-columbite II

0.09 0.78 0.018 0.19 0.13 0.010 0.27 0.06 0.11 0.02 0.04 0.19 0.10 0.07 0.04 0.02 0.01 0.004 n.a. 0.50 10.7 1.19 70.3 1.9 1.0 5.5 0.69 0.10 2.8 0.4 0.15 0.004 0.7 n.a. 0.39

0.11 1.4 0.01 0.36 0.16 0.008 0.51 0.08 0.25 0.05 0.07 0.70 0.27 0.18 0.11 0.04 0.05 n.d. 1.9 0.52 9.0 1.5 68.2 1.5 0.80 5.4 0.70 0.08 3.6 1.0 0.18 0.004 1.4 La > Nd > Sm

Pi­

The value of the ratio ΣΟε/ΣΥ, which is about 600, is much higher than in the pyrochlores of this deposit and in the niobian perovskites of the Kaiserstuhl. Th is the major radioactive element and the ratio Th/U has a value of about 100, whereas in most perovskites U dominates over Th. 24

V _ NIOBIUM-CONTAINING MINERALS Nb appears as trace element in all Ti and Zr minerals and in some silicates. The most common niobium-containing minerals are aegirite and zircon. During our investigations, a Fe-Ti mineral, quite similar in appearance to ilmenite, was examined by microscopy, and by X-ray diffraction and fluorescence. The results of this study indicate that this mineral may be a new species. A — Aegirite Aegirite is an important accessory mineral of the søvite. Prismatic crystals were carefully selected under binoculars after magnetic separation and acid treatment. The concentrate obtained was powdered, and again treated by HCl to avoid calcite and apatite contamination. a — Chemical analysis Table XIII gives the chemical analysis of the aegirite of L ueshe. The trace elements were determined by X-ray fluorescence. Rare earths were not detected in this mineral. b — Geochemical data The aegirite of L ueshe shows an abnormal content of Nb, Sr, Ta and Zr. The first two are characteristic elements of carbonatite deposits. Compared to the numerous published analyses (about 650) for major and trace elements in the ortho- and clinopyroxenes of different origin which were studied statistically in this laboratory (13), it seems that these four elements may be characteristic of aegirite associated to carbonatite. At L ueshe, all the samples of aegirite show about the same content for these elements. From the data given in the litterature concerning the clinopyroxenes, the mean content calculated for Nb, Ta, Sr and Zr is, respectively, 2.10^3%

TABLE XIII Chemical analysis of the aegirite from Lueshe

Analysis of the material

Elements analyzed

12.87 0.06 3.2 0.05 0.03 1.35 0.36 2.15 27.6 0.30 0.09 0.04 51.5 0.21 0.05 0.032 0.001 0.1 0.05

Na20

κ2ο

CaO SrO MnO FeO MgO A1 2 0 3 Fe203 Ti02 Zr02 Sn02 Si0 2 Nb205 Ta305 V2Os Cr03 H 2 0+ H20-

100.042

25

4.10~4%, 5.10~3% and 6.310^3%. The geochemical dispersion of these elements in this group is large but remains always under the corresponding contents found for the aegirite of L ueshe. The value of the ratio Nb/Ta which is about 4 for the aegirite is in good agreement with the mean value found for the clinopyroxenes. It indicates that during the differentiation of the søvite the major part of the Ta is selectively concentrated in aegirite, whereas the Nb is mainly distributed in the pyrochlores. Β — Zircon Zircon occurs frequently as bi-pyramidal prisms in the stream sediments at L ueshe. Crystals have a size up to 1.5 cm and a colour from red-brown to white-brown. All zircons are radioactive. a — Chemical analysis Table XIV gives the results of a chemical analysis of a brown zircon. The analysis was made by the N. V. Hollandse Metallurgische Industrie Billiton and was completed by us for some trace elements including the rare earths. From this analysis, we can derive the following formula : [Zro.93 Alo.oi Fe ni o.oi (Fe11, Hf, Ca, Nb, RE, Th, enz.)0.o5] [Si04]o.89 [OH]0.ii The mineral analyzed is a weakly hydrated zircon. b — Physical properties The refraction indices for this zircon are : Ng = 1.92, Np = 1.90. The specific gravity measured is 4.13. All these values are lower than normal for zircon and may be due to the hydration of the mineral. c — Geochemical data The brown zircon of L ueshe is geochemically characterized by the presence of small amounts of Nb and Ta. The ratio Nb/Ta is about 13. Compared to pyrochlore, Ta is strongly concentrated in this mineral. The Y group dominates largely over the Ce group with a ratio ECe/ΣΥ of 0.1. If the geochemical distribution for the Ce group remains similar to that of pyrochlore, this is not the case of the Y group which is characterized by a large predominance of Y over the other Y rare earths. X-ray fluorescence has shown the following geochemical distribution for the Y group : Y > Dy > Er > Yb > Gd > L u > Ho > Tb. Th is the main radioactive element and the ratio Th/U is about 2. The value of the ratio Hf/Zr which is 0.0178 is in good agreement with the value found for zircon from alkaline rocks (14).

C — The new Fe-Ti mineral This mineral was found in the stream sediments at L ueshe. It is macroscopically similar to ilmenite and occurs in tabular masses. Its colour is metallic black and its streak brown. 26

a — Physical properties The hardness of this mineral is about 5.5. Its specific gravity measured by a hydrostatic method under vacuum is 4.33 ±0.03 (M.F. Chantret, CEA). TABLE XIV Chemical analysis of the bro wn zircon from Lueshe Analysis of the mineral

Elements analyzed

0.14 0.04 0.35 0.015 0.022 0.15 0.31 0.02 \ 0.015 / 0.048 0.01 0.003 ) 0.31 0.045 1 0.04 ƒ 0.04 [ 0.485 0.02 1 0.015 \ 0.01 1 0.005 0.86 0.70 0.005 0.30 0.02 0.09 64.3 1.0 30.12 1.15 0.32

CaO SrO FeO MnO PbO U308 Th02 Ce203 La203 Nd203 Sm203 Y2O3 Dy203 Er203 Yb203 Gd203 LU0O3

Ho203 Tb203 Fe203 A1 2 0 3 Cr03 Nb205 Ta205 Ti02 Zr02 Hf02 SiOo H 2 0+ H20-

100.505

The optical properties are the following : the mineral has the reflectivity of columbite and shows à yellow brown pleochroism. Its anisotropy is masked by brown-red internal reflexions. The mineral is homogeneous but contains some rutile inclusions (about 1 %). By its optical properties, this mineral is very different from ilmenite. b — X-ray data Two diffraction patterns of the Fe-Ti mineral were taken with a Guinier camera using the CoKa radiation and an Al-filter and with a Debye-Scheirer camera using the MoK« and CrKa radiations. Both patterns show diffuse lines due to the pooi crystallinity of the mineral and only three strong lines. The ¿/-values for the Fe-Ti mineral of Lushe are given in Table XV. The diffraction patterns obtained do not indicate the presence of ilmenite, hematite, goethite, magnetite, brookite, anatase nor that of the normal mineral components of the carbonatite and of the alkaline rocks of Lueshe. 27

The diffraction patterns show some analogy with rutile and niobian rutile principally for the three most important lines at 1.70, 2.49 and 2.19 Å but differs also by the presence of some weak lines (especially at 3.85, 2.77, 2.71, 2.24, 1.90 and 1.44 Å) which cannot be indexed on a rutile or bi-rutile unit cell as well as by a much lower intensity of the main line of rutile at 3.233.26 Å. This weak line could also correspond only to the rutile inclusions present in the Fe-Ti mineral. The other weak lines belong to the mineral which is homogeneous as indicated by the optical investigations. TABLE XV Powder pattern of the Fe-Ti Mineral from Lueshe

d(k)

Remarks

Int.

4.6 diff. 3.85 diff. 3.26 2.77 2.71 2.489 2.312 2.241 2.191 1.898 1.704 1.44 diff. 1.32 1.25 1.15 1.10 1.05

v.w. v.w. w. w. v.w. strong w. w. strong v.w. very strong w. v.w. v.w. w. w. to medium w.

\ masked mainly by | the scattering

c — Chemical composition A chemical analysis of the mineral was made by the N. V. Hollandse Metallurgische Industrie Billiton and was completed by us for some elements. The results are given in Table XVI. TABLE XVI Chemical analysis of the Fe-Ti mineral from Lueshe Elements analyzed

Analysis of the mineral

MgO MnO FeO Fe203 Ti02 Nb 2 O ä Ta205 Cr203 HoO

0.41 1.2 29.0 16.71 50.7 0.38 0.1 0.003 0.01 98.50

Traces of Sn, Pb and Al are also present. Impurities about 1 % rutile.

28

From the chemical analysis we can derive the following formula for the Fe­Ti mineral : (Fe11, Mn, Mg)4 F e m 2 , Ti 6 O19 The chemical analysis and the formula indicate that the mineral is different from rutile and espe­ cially from its ferrian variety nigrin which contains much more Ti and only 11 % Fe 2 03. On the other hand, the chemical analysis is quite similar with the ferrian ilmenite but the Fe 111 content is much higher than any analyses already published. It is generally admitted that no more than about 6% Fe2Û3 enters into the ilmenite structure (15) and that a higher Fe 2 0.j content is due to inclusions of iron oxides, which is not the case for the Fe­Ti mineral from Lueshe. d — Geochemical data The mineral analyzed contains small amounts of Nb and Ta with a ratio Nb/Ta of about 3.5. As with the other niobium­containing minerals, this ratio is very low compared to the pyro­ chlores and other Nb minerals. e — Nomenclat ure The optical properties, the X­ray and chemical data given above indicate that the iron titanate found in the stream sediments of Lueshe may be a new mineral. It differs notably from ilmenite and from rutile and its ferrian varieties as already mentioned. Other iron titanates to take into consideration are : högbomite (Fe, Mg)ö (Al, Fe)i6 TÌO32, pseudobrookite (Fe 2 Ti05) and kalkowskite (Fe2TÌ30g?). The first two minerals are well described and in comparison with the Fe­Ti mineral from Lueshe, they differ in their diffraction pattern, their chemical composition and also in their optical properties. Kalkowskite was first mentioned by Rimann (16) in 1925. Its description is incomplete. Its optical properties in reflected light are not given and, as far as we know, no diffraction pattern of this mineral has been published. Its formula is uncertain and probably near Fe2TÌ309. The chemical analysis indicates clearly the presence of unknown mineral impurities and the Fe content seems to have been determined only as Fe203. Kalkowskite is therefore a doubtful mineral. We will submit this mineral to the Commission for New Minerals of the I.M.A. for discussion before proposing a new name.

VI — CONCLUSIONS This study has brought new data on the mineralogy and the geochemistry of some Nb­ bearing minerals of the Lueshe carbona r ite deposit. The Nb­bearing minerals studied can be divided in two groups, the first where Nb constitutes a major component, the second where Nb appears as an accessory element with a content Nb20s + Ta20s up to 0.5%. From the mineralogica! point of view, a new type of hydrated K­Sr pyrochlore is described as well as an iron titanate, which probably is a new mineral with formula (Fe11, Mn, Mg)4 Fe1,11 ΤίβΟΐ9. Physical properties, X-ray and chemical data are given for both minerals. The formation of the hydrated K-Sr pyrochlore is related to the weathering of the normal greenish pyrochlores in the eluvials and in the residual soils of the carbonatites. The presence of two types of fersmites (I and II) and of a barian-goyazite, has not been reported in the Republic of Congo. Columbite and fersmite II occur macroscopically in octahedral crystals, pseudomorphic after pyrochlore. In fact, however, fersmite II replaces the "octahedral" columbite, wheieas fersmite I is found often macroscopically as idiomorphic prismatic crystals associated to the columbite. The geochemical distribution of Nb, Ta, rare earths, V, Th and some other elements was studied for the different Nb-bearing minerals investigated. A high Nb-Ta ratio (385-30) charac29

terizes the niobium minerals whereas the value of this ratio is much lower in niobium-containing minerals such as aegirite, zircon, and the probably new iron titanate (13-3). In the søvite, the main part of the Nb is thus concentrated in the pyrochlores, whereas the Ta is fixed mainly in the aegirile. The highest Nb/Ta ratios are found in the pyrochlores with a mean value of 385 and decrease to 250 for lueshite, to 162 for the fersmite II and to 30 for the columbites. The geochemical distribution of the rare earths also widely varies. A predominance of the Ce group and especially Ce and La is found in both types of pyrochlores, lueshite and probably also occur in fersmite 1, whereas the Y group and principally Y dominates in the fersmite II and in zircon. The highest ZCe/ZY ratios are observed in the lueshite and in the pyrochlores and the lowest in the zircon with a value of about 0.1. Fersmite II is particularly rich in rare earths (7.8% RE oxides). The radioactive elements, U and Th, are widely distributed amongst the Nbbearing minerals such as both types of pyrochlores and fersmites, lueshite and zircon. Generally, U dominates in the pyrochlores and in the fersmites, whereas Th is the principal radioactive element in zircon and especially in lueshite (Th/U = 100). Until now, the radioactivity of this last mineral has not been reported. Due to the irregular distribution of the radioactive elements in many Nb-bearing minerals, the radioactivity cannot be used for the approximative determination of the Nb content of the carbonatites in mining work. In addition, we must also note that an iron-rich-thorogummite with a very low U content is sometimes present in the carbonatite samples. Columbitization of the pyrochlores ana fersmitization of the columbites are well developed principally in the NW part of the deposit. The columbitization of pyrochlore is not only accompanied by major structural modifications and by the replacement of Ca-Na by Fe-Mn but also by an appreciable enrichment with Ta and V. Part of the Ca and Na liberated during this process is fixed in the fersmite I, which is also enriched in rare earths. The fersmitization of the columbite is clearly demonstrated by the replacement of columbite by fersmite II. This fersmitization process produces some variations of the parameter values, due mainly to the substitution of Fe-Mn by Ca-RE in the columbite structure and also by the decrease of the Ta and V contents. The processes of columbization and of fersmitization are associated to a late carbonatitic stage, characterized b\ an enrichment with Ta, Fe, Mn, V and rare earths. Such a geochemical evolution was also observed during the carbonatitic differentiation at the Kaiserstuhl (4b) and also in some Russian carbonatite deposits (10).

ACKNOWLEDGEMENTS The writer is grateful do Dr. A. Safiannikoff, who provided the mineral samples already before 1960, and to the N. V. Hollandse metallurgische Industrie Billiton for the chemical analyses made under the contract EUR No. 017-61-9 ISPN. He is also indebted to Dr. A.H. van der Veen of the Billiton and to Dr. F. Chantret of the mineralogicai laboratory of the CEA at Fontenayaux-Roses, France, for physical measurements on some mineral samples.

BIBLIOGRAPHY 1. A. MEYER and P. DE BÉTHUNE, 1960 : The Lueshe Carbonatite (Kivu, Belgian Congo). Int. Geol. Congress. Proc. of section 13, pt XI11, pp. 304-309. 2. L. VAN WAMBEKE, 1960 : Geochemical Prospecting and Appraisal of Niobium-Bearing, Carbonatites by X-ray Methods, Econ. Geol.. Vol. 55, No 4, June-July, pp. 732-758. 3. A SAFIANNIKOFF 1959 : Un nouveau minéral de niobium, Académie Roy , Sc Outre-Mer, Série V-6, Bruxelles, pp. 1251-1255.

30

4. L. VAN WAMBEKE, J.W.

BRINCK, W. DEUTZMANN, R . GONFIANTINI, A. HUBAUX, D. MÉTAIS, P. OMENETTO,

E. TONGIORGI, G. VERFAILLIE, K. WEBER, W. WIMMENAUER, 1964 : Les roches alcalines et les carbonatites du

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Kaiserstuhl, Bade, Allemagne. a) L. VAN WAMBEKE : Géochimie minérale des carbonatites du Kaiserstuhl (Chap. V). b) L. VAN WAMBEKE : La géochimie des roches du Kaiserstuhl (Chap. VI). To be published as a Euratom report shortly. A . H . VAN DER VEEN, 1963 : A study of pyrochlore, Verband, van het Kon. Neder l. Geol. M ¡¡nb. Genootschap, Geologische serie, deel 22, 188 pp. K. WEBER and J. MARCHAL, 1964 : A device for coupling a pulse height discriminator to a scanning X-ray spectrometer, Journ. of Sc. Instr., Vol. 41, pp. 15-22, Rep. EUR. 572e. E. JÄGER, E. NIGGLI, A . H . VAN DER VEEN, 1959 : A hydrated barium-strontium pyrochlore in a biotite rock from Panda Hill, Tanganyika, Min. Mag., Vol. 32, No. 244, pp. 10-25. E. M. BOHNSTEDT-KUPLETSKAYA, T. A. BUROVA, 1946 : Fersmite, a new calcium niobate from the pegmatites of the Vishnevye Mountains, Central Urals. Compte rendus Acad. Sc. URSS, Vol. 52, pp. 69-71. H . D . HESS and H . I . TRUMPOUR, 1959 : Second occurrence of fersmite, Am. Min., Vol. 44, No. I, pp. 1-8. A.G. ZHABIN and V. S. GAIDUKOVA, 1962 : The interrelation of niobates — pyrochlores, fersmite and columbite in alkaline syenite and carbonatite rocks (in Geology of ore deposits), Acad. Se. URSS, Vol. 4, No. 1, pp. 87-98. A. SAFIANNIKOFF and L. VAN WAMBEKE, 1961 : Sur un terme piombifere du groupe pyrochlore-microlite, Bull. Soc. Franc, de Min. et de Crist., Vol. LXXXI V, pp. 382-384, Rapp. EUR 45.f. M. DANØ and H. SØRENSEN, 1959 : An examination of some rare minerals from the nepheline syenite of S.W. Greenland, Med. om Grönland, Bd. 162, Nr. 5-35. S. LUGINBUHL, 1963 : Studio statistico sulla geochimica dei pirosseni. Internal Report EUR not published. M. FLEISCHER, 1955 : Hafnium content and hafnium-zirconium ratio in minerals and rocks, US Geol. Sure, Bull. 1021 A, 11 p. C. PALACHE, H. BERMAN, C. FRONDEL : Dana's system of mineralogy, 7th edition, Vol. I.

16. RIMANN, 1925 : Kalkowskyn, CM. Min., 18.

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