experiments on phase transformations and ...

365 Elscvier

Scientific

Publishing

EXPERIMENTS REACTIONS GRINDING:

(Received

Company.

Amst~rdai~-Printed

in The Netherlands

ON PHASE TRANSFORMATIONS

OF MECHANICALLY PETROGENETIC

September

ACTIVATED

AND CHEMICAL MINERALS

BY

IMPLICATIONS

17, 1980; revised version

accepted

September

11, 19XI )

ABSTRACT

Dandurand,

J-L.. Gout.

reactions

R. and Schott,

of mechanically

activated

J., 1982. Experiments minerals

by grinding:

on phase

transformations

and chemical

petrogenetic

implications.

Tec~ronc$r~~~/c~.

x3: 365-386 Prolonged

grinding

mu* surfaces can

increases

and internal

result

in polymorphic

solid-state

transformation

transformation dehydration

the energy

defects.

Moreover,

transformations of metastable

of low-pressure

of solids by the production grinding and

mineral

temperatures

phases

of stored Here

(aragonite+

calcite.

(calcite -aragonite);

in the form of pressures

which

we demonstrate anatase-

and

(siderite -magnetite

of minerals

energy

quasi-hydrostatic

decomposition.

to stable polymorphs

to high-pressure

and decarbonation

also generates

the

rutile):

the lowering

or hematite,

the

of the

diaspore-

corundum). In the presence reaction

of a fluid phase. stored energy

rates and, more importantly,

transformations:

ground

calcite-aragonite

(at

dolomite-

aragonite

Assuming consequences

Mg2+

calcite

can be released,

resulting

In this paper we demonstrate

(at low MgZC concentration

concentration),

ground

magnesite-

in accelerated the following

in solution),

hydromagnesite,

and

ground ground

+ Mg*+

an analogy

between

on the release

transformations. aragnnite

calcite -magnesian high

from grinding

phase transformations.

laboratory

of hydrothermal

As examples

and d&spore-bauxites

the possible

and natural fluids, role

grinding,

tectonic

the solubilization

of deformation

on

activity

may have important

of minerals

and on solid-state

the formation

of metamorphic

is discussed.

INTRODUCTION

The experimental study of the mechanical activation of solids by intensive grinding is of great interest in geoscience because minerals are often subjected to naturally occurring mechanical strains. Therefore, knowledge of the effects of mechanical activation should permit a better underst~ding of certain mineral transformations which may occur together with tectonic events. 0040- 195 1/g2/OOOC-C@OO/%O2.75

kp 1982 Elsevier Scientific

publishing

Company

366

During

grinding,

energy

entering

stored.

Storage

boundaries) cracks,

During rapidly

non-hydrostatic

of energy

is linked

and accumulation

twinning

of minerals

essentially

the solid some is rapidly

strains

released

both

of internal

to formation strains

is the sum of these two different

reaches

grinding

a limit.

the increase

Indeed

the rate of agglomeration

of fine

and some is

particles

(grain

in the lattice (dislocations,

processes

increase

micro-

of free energy

of storage.

of free energy due to grain-size

after few hours

and surface

to solids. Of the

phenomena)

etc.. . ). As a result the measured

boundaries

prolonged

are applied

(elastic

the grain-size

area of the powder

reduction remains

reduction rate equals

constant.

Ag-

glomeration produced by cold welding (Bradshaw, 1951; Benjamin, 1976) contributes to the accumulation of defects at the boundaries of grains coalesced by this process. This increase of free energy due to surface energy can be calculated if the particle-size distribution and interfacial free energy of the mineral are known. The relative importance of the two forms of stored energy depends upon the mode of the mechanical actions, their duration and the nature of the material. Thus, it is known (Dandurand about

and Jauberthie,

by grinding

are more important

than with those of prevailing In addition

1976; Dandurand,

to stress

with minerals

ionic-type

effects,

1978) that structural

bonds

grinding

of covalent

to measure

the relative

bonds

can locally

generate

importance

almost

of particle

1957; Perami, 1971; Dandurand et al., 1972). However, as pointed (e.g., Gla’ngeaud, 1947; Termier and Termier, 1956) very little due to stored energy. This investigation

tal results of some types of mechanically which are compared

EXPERIMENTAL

Techniques

induced

physico-chemical

the more

size, stress,

and

from the analysis

Relations between mechanically induced micro-fissures in rocks, liquids, and mineral formation have been studied to a certain extent

transformations

hydrostatic

polymo~hs,

hydrostatic pressure effects which occur during grinding. However, of selected examples their relative consequences may be defined.

natural

(e.g., quartz)

(e.g., calcite).

pressures. This effect favors the formation of high-pressure easily as pressures are applied on strained minerals. It is difficult

changes brought

migration of (e.g., Lafitte,

out occasionally is known about

presents

experimen-

transformations,

to field observations.

TECHNIQUES

of grinding

Mechanical activation has been carried out with a Dangoumau-type percussion grinder (Fig. 1). The grinding vessel is a steel or plastic beaker, which contains a ball, 20 mm in diameter. The beaker is shaken vertically at 730 c/min. The grinding energy can be modified by using balls of different densities. energies, relative to a steel ball, produced by ~uminum and tungsten carbide

Kinetic ball are

367

0.4 and 2.2, respectively.

When it hits the material

ball is ES = 0.18 J. Assuming obviously

Fig.

that all energy is transferred

inaccurate-one

I. Dangoumau-type

powder-for

to be ground,

can estimate

percussion

that

the energy

the energy of a steel

to the powder-w.hich communicated

is

to 1 g of

grinder.

log of material

in the beaker-after

1 h of grinding

is of the order of

300 J (or 60 J/m2 of powder). These numbers must be considered as limiting values. If not otherwise specified, all data refer to grinding with a steel ball. Grinding at elevated temperatures was realized either by attaching a water jacket to the grinding vessel (T < SO’C) or by using a vertical electrical furnace (T > SO‘%). Ana&ical

techniques

The mechanically (Philips

activated

PW 1010, filtered

material

has been examined

Ka Cu-radiation

or C.G.R.

by X-ray diffractometry

sigma 2070 monochromatic

Kcu, Co-radiation), differential thermal analysis (heating velocity: 700”C/h), and by using a thermo-balance (SETARAM G.70, heating velocity: 500’C/h). The specific area of the grains were measured by B.E.T. analysis (N2 adsorption). tion were obtained by sedimentation (sedigraph 5000 D). RESULTS

AND

Sizes distribu-

DISCUSSION

Experimental results analyzed here deal with the behavior aqueous solutions and with transformations which occur poIymo~~c transformations and mineral decomposition.

of ground minerals in during dry grinding-

36X

Behavior

of ground materials

The behavior increase

in aqueous solution

of minerals

in water

may

be modified

of free energy which makes the material

This may result in an enhancement enrichment

of certain

With carbonates

dissolved

by grinding

due

to an

less stable and thus more reactive.

(either continual

on transitory)

of solubility

and

species.

taken as an example,

the purpose

of this study is to measure

relative importance of the two forms of stored energy of the ground materials in various aqueous solutions.

and to examine

Estimation of surface and strain energies due to grinding Calcite of 2-4 mm grain-size was carefully washed

the

the behavior

in deionized

water.

Its

in pure water was then compared with calcite ground for 24 hrs. The changes caused by grinding of calcite, are illustrated by the X-ray

behavior structural

diagrams of Fig. 2. Previous to dissolution runs, three methods grain size of ground particles: sedimentation,

were used in order to determinate specific surface area and X-ray

the line

broadening. Size distribution deduced from sedimentation of materials of less than 40 pm and of strongly ground materials after energetic disaggregation by soneration are reported ground

and at about Calcite

(calcite

ground

or modal

powdered

for 24 hrs presents

and a density

shape are assumed,

0.4 pm for this ground runs). This is similar

sizes are at about

for 24 hours and dolomite

15 ym for less energetically

ground

measurement) cubic

in Fig. 3. Maximum

materials

a specific

* materials surface

of 2.620. If homogeneous

one can calculate material

9pm

ground

for strongly for 100 hours)

(< 40 pm).

area of 5.6 m2/g grain-size

from surface

and

area a mean grain size of

(see Table I for other materials

to results from X-ray line broadening,

(B.E.T.

distribution

used in dissolution

where a mean dimension

of.0.2 pm for coherent diffraction domains has been calculated for calcite ground for 24 hrs using the method of Stokes and Wilson (1944). These lower values are probably

more reliable

than size distribution

deduced

from sedimentation

because of

agglomeration of grains during grinding. Dissolution experiments on calcite were made at 25°C and a total pressure of I atm in a constant temperature bath as described by Menschel and Usdowski (1975). CO, -gas saturated with water vapor was continuously bubbled through the liquid phase ( Pco, = 0.97 atm.). Figure 4 shows the changes two different

calcite

varieties.

In both

cases pH

tends

of pH obtained towards

a value

with the of 6.03.

However, with ground calcite a maximum at pH = 6.17 is observed at the beginning of dissolution **, and after a period of 6 months this calcite recrystalhzes (Fig. 2).

* In the following

this material

** Note that the attainment placed

in solution

will be referred

of the maximum

be dissolved.

to as gently ground. pH value requires

that at least 25% of the powder

initially

369

1

i-‘

,

I

I

I~.

Fig. 2. X-ray diffraction ground

~

Wrticles

diagrams

for 24 hrs, 3=calcite

Fig. 3. Size distribution Curve

I

15

20

e"25

f =materials

3=dolomite

ground

The degree

of calcite (Co, Ka, ). I =normal

being in water

deduced gently

for 6 months

from sedimentation

reduced

to powder

calcite, 6-10 pm grain size. .?=calcite

after grinding

of some materials of less than

size (micrometers)

for 24 hrs. used in the solubility

40 pm;

I=calcite

ground

experiments. for 24 hrs,

for 100 hrs.

of supersaturation

of the solution

with respect

to normal

calcite

is

given by the maximum pH value. It can be used to calculate the total increase of the free energy of the ground calcite. The calculation yields an order of magnitude of 29X.15 ) =

‘cAG;

TABLE

I

Results

of specific

dissolution

2380 J/mole.

surface

area,

On

the other

density

and

hand,

calculation

it is possible

of mean

grain-size

Surface area