on the genesis and composition of natural pyroaurite - Mineralogical ...

Clay Minerals (1991) 26, 297-309

ON T H E G E N E S I S A N D C O M P O S I T I O N OF NATURAL PYROAURITE R. M. T A Y L O R ,

H . C. B. H A N S E N , * G. S T A N G E R t C. B E N D E R K O C H : ~

AND

CSIRO Division of Soils, Private Bag 2, Glen Osmond, South Australia, * Royal Veterinary and Agricultural University, Chemistry Department, Thorvaldsensvej 40, 1871 Frederiksberg, Copenhagen, Denmark, t School of Earth Sciences, Flinders University, Bedford Park, South Australia and $ Laboratory of Applied Physics, Technical University of Denmark, Building 307, DK-2800 Lyngby, Denmark (Received 12 July 1990; revised 3 October 1990)

ABSTRACT: Samples of the mineral pyroaurite, formed from the weathering of partially serpentinised harzburgite (olivine + pyroxene) were found in an arid region of the Sultanate of Oman. These were either golden or silver in colour depending on the horizon from which they were derived. Chemical analysis showed that the colour variation was primarily due to the differing conditions in the hydrologicalenvironment. The golden colour was attributed to small Fe(lll) oxide particles detected by M6ssbauer spectroscopy. In addition, the samples were examined by X-ray diffraction, scanning electron microscopy,and glycerolintercalation. These results were compared with a syntheticpyroaurite sample prepared under conditions (previouslyreported) similar to those in nature. These conditions are shown to approximate to those found in the hydrological environment in the zones of the natural pyroaurite formation.

Pyroaurite [Mg6FeIH2(OH )16]2+[CO 3 4H20] 2 has been previously synthesized by coprecipitation of iron(III) and magnesium(II) hydroxides followed by ageing of the precipitate (see Feitknecht, 1942; Hashi et al., 1983). A more recent technique, called induced hydrolysis (Taylor, 1984), involved the formation at constant p H (pH = 8.85) of relatively crystalline pyroaurite from a 0.01 i suspension of ferrihydrite in a 0.05 M magnesium nitrate solution. Although pyroaurite formed within a short period of time (several hours) by this method the rapid transformation of ferrihydrite to other more crystalline iron oxides means that there were some limitations to the applicability of the process to the natural environment (Hansen & Taylor, 1990). To explain the natural occurrences of pyroaurite, Hansen & Taylor (1990) suggested a reaction involving a low rate of oxidation in a magnesium rich solution at p H 8.5 of Fe(II) derived from the dissolution of precipitated iron(II) carbonate (siderite). These authors justified their approach with the observation that pyroaurite was often associated in nature with brucite, Mg(OH)2, and magnesite, MgCO3, and that siderite was sometimes also found with magnesite, its isostructural analogue. Desautelsite (Dunn et al., 1979), the Mn analogue of pyroaurite, was synthesized by a similar technique (Hansen & Taylor, 1991b), whereby the Mn(III) in solution was supplied by the controlled oxidation of suspended synthetic rhodochrosite (MnCO3) in a magnesium solution at constant pH. The nature of the final product depends greatly on the rate of dissolution and supply of the divalent oxidizable cation in relation to its subsequent rate of oxidation, and other factors such as the Mg 2+ concentration, temperature and pH. 9 1991 The Mineralogical Society

298

R . M . Taylor et al.

The occurrence and environment of formation of a natural pyroaurite showing colour variations have been reported by one of us (GS) (Neal & Stanger, 1983, 1984, 1985; Stanger, 1986a,b; Stanger et al., 1988). This environment can be related to that simulated by Hansen & Taylor (1990). This work examines the composition of the natural samples and particularly the cause for their variation in colour, and compares their formation environment with that postulated during synthesis experiments.

ENVIRONMENT,

MATERIALS

AND METHODS

Conditions in the natural environment Two natural pyroaurite samples were taken from the mantle sequence of the Semail Nappe in the Sultanate of Oman. This is an exceptionally large and well preserved ophiolite sequence. The arid conditions have resulted in the complete absence of soil, and rapid infiltration of groundwater, whilst infrequent recharge results in a large rock to groundwater ratio, long residence times, and hence relatively stable hydrochemical conditions for most of the time. Groundwater recharge occurs as calcium bicarbonate dominated flash floods, typically every two to three years, from the high limestone massif of Jebel Akhdaar, shown to the north of the "pyroaurite well" in Fig. 1. The host rock to the pyroaurite is in the relatively low relief mantle sequence which, in general, consists of heavily sheared, partially serpentinised harzburgite (olivine + orthopyroxene). However, in the spoil heap from the specific well in which the pyroaurite was observed, the host rock was mainly dunite, i.e. olivine only, in which a variable amount, an estimated 40-90%, of serpentinisation (magnesium silicate hydration) had occurred (see Fig. 2). The pyroaurite formed in a zone beneath the water table and its formation appeared to be restricted to depths near and below the presumed zone of mixing of two different water layers of different composition, zones A and B in Fig. 2. Zone A water is in contact with the atmosphere and is not, therefore, a reducing environment. Mg 2+ is the dominant cation and HCO3 with relatively high amounts of SO42 and C1- are the main anions in solution whilst the pH of the water is - 8 , cf. Table 1 (after Neal & Stanger, and Stanger, op. cir.). In contrast, the water in the lower zone B is highly alkaline, around pH 11-12, and is dominated by Ca with minor Mg. The conditions are extremely reducing with most of the sulphate, nitrate and iodate reduced to sulphide, nitrite and iodide, respectively. Fracture faces in the dunite host rock from the zone of mixing of the two water types were coated with silver coloured pyroaurite up to a thickness of --1 mm. This mineral was itself often coated, to a thickness of several mm, with yellow (chromian) brucite. A golden coloured pyroaurite-type mineral without any associated chromian brucite was also found in zones slightly above the silver-coloured pyroaurite but still within the presumed zone of mixing of the surface and deeper waters. These coloured variations will henceforth be referred to as the silver and golden pyroaurite. It is not known exactly how much of this dunite zone (B, Fig. 2) is pyroaurite bearing, but from its abundance in the spoil heaps and its presence between specific horizons, the vertical extent of the pyroaurite-rich zone appears to be 40 /~m in cross section (Figs. 4a and 4b). This is in contrast to the synthetic HT47 which occurred as thin plates showing a high degree of intergrowth (Fig. 4c). E D X A results from clear areas on the platy surface of both the silver and golden natural samples show that both varieties contain Fe, Mg, C1 and slight traces of Si. In addition, S was identified in the golden sample, consistent with IR and chemical analyses. Fig. 4d showed the acicular surface deposits on a sample of the golden pyroaurite, although these deposits were also present to a lesser extent on the silver variety. E D X A revealed that the composition of these needles was essentially that of a Mg silicate with possible smaller amounts of Fe. Together with the X R D analysis of the residues after acid digestion which showed spacings of --7.24 and 3.6 •, these compositions suggest the presence of a serpentine material; this is consistent with the report by Thomassin & Touray (1982) who noted the formation of aluminous serpentines from hydrotalcite-like precursors, and the observations of Stanger (personal communication) that serpentine is a c o m m o n low-temperature precipitate on brucite formations in this region.

Genesis and composition of natural pyroaurite

305 o

e-

9

"2

.x:

Q

e~

e-

R.M. Taylor et al.

306 I

g

I

I

I

',..,...,,,

k.. O o'J

I

I

.-.. a ' . + , ,

9

I

,'4.-'.z.

I

I

9

"" :.

x