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 <3 m.
Genesis and composition of natural pyroaurite
299
THE GEOLOGICALSETTINGFORTHE OCCURRENCE
OF NATURALPYROAURITE
LEGEND
9
Hyperalkalinewells
Autochthonouslimestone massif
o
Hyperalkalinesprings
AilocMhonous limestone klippen
|
Pyroauritewell
Radiolarites and other oceansediments
Nz
Townof Nizura
eq
Village of Firq
,,'t
Mainfluvial channels
[ ~
Sheared harzburgite (mantle sequence)
Gabbro-dunite-pyroxenitecomplex
(crustal sequence)
Thin alluvium
I
FIG. 1. Geological setting for the occurrence of natural pyroaurite in the Sultanate of Oman.
R. M. Taylor
300
e t al.
0oOO o o o l
100% serpentinizedalluvium oOgoog~J
o o o o o ~ o o o2oo~^oO o o o o o . ~ o ooooO.CoO
o o o o o o ~ o o o - o o ~ o o o ~ o o ~ o o o ~ o o~o
o o ~o Q oOl
watertable
ZoneA:
HCO~type groundwater
Major dunite
with minor
harzburgite
and pyroxenite;
all partially
serpentinized
(40% to 90%)
exceptthe near
surface (about100%)
..................................
-iL
Zoneof mixing
ZoneB:
OH- typegroundwater
_[
NB: Depth of the well is about20m
Fl~. 2. A vertical section showing the hydrological environment associated with the natural
occurrences of pyroaurite in Oman,
Laboratory techniques
Pyroaurite synthesis.
The previously described pyroaurite
& Taylor, 1990) comprised
laboratory
synthesis (Hansen
the essential feature of the controlled oxidation (aeration)
FeCO3 (siderite) in a Mg-rich environment
of
at pH 8.5 and 35~
Table 1. Chemistry of the bicarbonate and hydroxide type water from the "pyroaurite well" area; (a) typical
"zone A"; (b) "zone B".
EC
(p,S)
T
~
Ca 2+ Mg2+ Na +
K+
7,2
7.7
7.7
8.3
8.2
8,5
1155
1510
568
1279
925
360
32
23
-33
30
20
57
44
56
41
38
31
71
92
38
90
69
14
77
138
43
107
86
90
3.5
6.9
2.0
5.8
5,7
5.7
--0.7
1.0
1-0
--
12-0
12.0
11.7
11.6
11,3
2510
2194
2290
1775
872
24
33
30
31
23
81
44
55
55
8.0
0-5
0.5
0.0
0.0
0.0
299
226
250
250
141
10.9
5.2
13.0
13,0
6,3
--0.2
0-2
--
Samples pH
CI-
5042
NO 3-
F
SiO2
247
410
293
358
359
198
194
218
45
121
137
117
128
199
57
138
99
53
-7-5
9-3
8-6
4-0
3.0
--0-2
0.2
0.2
--
---37
34
--
(OH)116
108
76
76
36
330
231
340
340
124
6
4
0
0
18
0.5
2.0
0.2
0,2
0-1
0-0
-0-2
-0.0 0.0
0.0 0.0
0.03 --
Sr2+
HCO3(mg/1- l )
(a)
1
2
3
4
5*
6
(b)
7
8
9
10
11
The zone B waters are dominated by low-temperature serpentinisation reactions which act as sinks for nearly all the
Mg 2+ and SiO2 by precipitation, leaving a high pH/Iow Eh residual solution. EC is the measured electrical
conductivity.
* Sample 5 is streamflow fed by near-surface groundwater. Sample 6 is "recharge" (ponded surface water). All
other samples were pumped from wells.
Genesis and composition of natural pyroaurite
301
X-ray diffraction (XRD). The silver and golden natural pyroaurites, as well as the
synthetic products with which they were compared (Hansen & Taylor, 1990), were
examined by XRD with Co-Ko~ radiation using Philips PW1710 and PW1800 computer
controlled diffractometers fitted with monochromators. Pressed powder samples were
scanned over the angular range 3-80 ~ 20 at 3 ~ per min.
To assist in determining differences in their compositions, the two natural samples were
given a treatment designed to exchange the interlayer anion with SO42- and then reexamined by XRD to determine line shifts. This exchange technique involved treatment
with heated glycerol containing dissolved Na2SO4 and is described separately (Hansen &
Taylor, 1991a).
Chemical techniques. All analyses were carried out in duplicate or triplicate. The
interlayer carbon dioxide evolved during acid dissolution of a known weight of air-dried
sample was absorbed in a 0-05 M barium hydroxide-0-2 M barium chloride solution (Larsen,
1949). The excess hydroxide was back titrated with hydrochloric acid using phenolphthalein
as indicator. Metal concentrations in the acid digest were determined by atomic absorption
spectroscopy (AAS). Iron(II) was determined by the o-phenanthroline colorimetric
method (Schilt, 1969). The methods of Marczenko (1986) were used to determine nitrate in
the natural samples and chloride from both acid digestion and from extraction with
CO2-free boiled water.'
The techniques used for the analyses of natural waters have been described by Neal &
Stanger (1985). Bicarbonate, carbonate and hydroxyl ion concentrations were measured by
acidimetric titration and pH determinations at the time of collection. The samples were
stored and analysed by standard colorimetric and spectrographic methods.
Infrared (IR) spectroscopy. The possible presence of sulphate or nitrate anions in the
samples was first checked by IR spectroscopy using a Perkin Elmer 580 A instrument with
transmitted light. Test samples consisted of 1.5 mg of sample in 200 mg KBr disks.
Scanning electron microscopy (SEM). Powdered samples of the golden and silver
pyroaurites were carbon coated and examined by SEM using a Cambridge Stereoscan 250
equipped with an energy dispersive X-ray analyser (EDXA) for elemental determinations
and distributions.
MOssbauer spectroscopy. M6ssbauer spectra of the two samples were made to determine
the valence state of the Fe in the samples and to determine whether or not some Fe oxide
impurity phases were present. The spectra were obtained using a constant acceleration
spectrometer with a source of 57Co in Rh. The spectrometer was calibrated at room
temperature using a 12.5 ~m o~-Fefoil and the isomer shifts are given relative to the centroid
of this absorber. Spectra were obtained at absorber temperatures of 295, 80 and 12~ A
synthetic pyroaurite was also run to establish that both natural and synthetic samples
displayed essentially the same features.
RESULTS
XRD analysis
The diffractograms obtained for the silver and golden varieties of natural pyroaurite
together with a synthetic sample HT47 (Hansen & Taylor, 1990) are shown in Fig. 3. The
high intensity and relatively sharp basal spacings of the natural samples ca. 7-7-7-9 A. (003)
and 3.9 A (006) in relation to the much reduced intensities of other reflections indicate well
R.M. Taylor et al.
302
HT47 Synthetic
0
O
0
0
X
A
J
v
. n
r
9
0
Silver
I
I
6.00
12.00
f
I
I
18.00
24.00
30.00
36.00
Co Ko~ radiation Angle (deg) 20
FIG. 3. X R D traces of a synthetic pyroaurite (HT47) and the natural silver and golden pyroaurites,
developed crystals with a morphology which favours preferred orientation, further
evidenced by the lubricious feel and coarse flaky appearance of the hand specimens.
There is a single peak for the 003 basal spacing for the silver pyroaurite (Fig. 3) at
--7.79 A., compared with 7-77 A. listed for pyroaurite (JCPDS 25-521). Modifying the
intensity scale and angular range allows the resolution of two further small peaks, not seen
at the scale of the diffractogram shown. One peak occurs at --8-09 A in the low-angle tail of
the peak and the other appears as a small shoulder at -7-85 A. However, the second order
is clearly resolved into two peaks, a stronger one at - 3 - 9 3 A and a higher spacing appearing
as a shoulder at - 3 . 9 7 A.
In contrast, the main 003 peak (for a non-oriented sample) for the golden sample occurs
at a slightly hi~her spacing of - 7 - 8 8 A, with an unresolved shoulder indicating a peak near
7-78 A, near to the d003 of the silver pyroaurite. Other shoulders suggest further peaks at
- 9 . 1 3 and 8.09 A, this latter peak also being observed from the silver pyroaurite. There
were also two broad, low intensity peaks at --6-43 and 5.64 A. in the d003 tail. Two well
resolved second order peaks occurred at - 3 - 9 6 and 3.905 A with shoulders indicating
further peaks at - 3 . 9 6 and 3-85 ~ . Weak reflections at ~4.03 and 3.64 A were also detected
in the high- and low-angle tails of this complex second order reflection.
The residue obtained after dissolution of the golden pyroaurite in 3 M HC104 for chemical
analysis gave a peak at - 7 - 2 4 ~ . The previously observed peaks at 8.09, 4.03 and 3-64
were also still present. This 7.24 ~ peak, originally masked by the more intense pyroaurite
d0o3, and its second order could probably arise from surface formations of serpentines of the
lizardite-chrysotile group, especially in view of the surrounding ultramafic environment.
Exchanging the interlayer anion with sulphate caused a significant change in the positions
of the X R D peaks in both the silver and golden samples (Table 2). The main 003 peak for
both the silver and golden samples increased to - 9 . 1 - 9 . 2 ,~, a spacing that was originally
present to a slight extent in the golden sample before treatment. Spacings corresponding to
the most intense 003 spacing in the untreated samples (where CO32 was the dominant
Genesis and composition of natural pyroaurite
303
interlayer anion) were still present, indicating only a minor component. Repeating the
Na2SO4-glycerol treatment enhanced the 9.1-9.2 A reflections and further reduced those
due to CO32- pyroaurite,
Chemical analysis
The mean values of replicates of chemical analysis of the natural samples are given in
Table 3. The formula of each sample was calculated from the average analysis data by
assuming that (1) no vacancies existed in the cation sites, and (2) that there were twice as
many hydroxyls as metal cations in the octahedral sheet. This assumption does not allow
other anions to substitute for O H in the octahedral sheet. The formulae were calculated to
a constant (OH)16. The sulphate content was not measured, but was calculated from the
balance of charge using the measured carbonate and interlayered chloride contents and the
16 hydroxyls per formula unit. This is reasonable because the presence of sulphate was
confirmed both by I R (see below) and by the effects of the anion interchange experiments.
The nitrate determination in the acid extracts gave the same value as aqueous extracts. This
suggests that the N O 3 content can be ascribed to salt impurities in the samples and that
probably none is present in the interlayer region. Apart from the small amount present in
the interlayer, soluble chloride salt was also associated with both samples. There were 34-4
and 30-9/~mole C1-1 per 100 mg for the silver and golden pyroaurites, respectively. Due to
the presence of this and other unidentified soluble salts and the presence of a serpentine
impurity, a reliable estimate of the water content cannot be given. The silver sample may
then be described by the formula:
Mg6.09Fem 1.82A10.09(OH) 16(CO3)0.87(C1)0.03(SO4)0.07
and the golden by:
Mg6.10Felnl.81A10.09(OH) 16(CO3)o.66(C1)0.07(SO4)0.2.
No Fe(II) or Cr(III) was detected (chemically) in any of the replicates of either sample. The
Table 2. Basal spacingsfor the syntheticand natural silver and golden pyroaurites before and after
treatment with sodium sulphate + glycerol.
Pyroaurite
sample
Silver
Golden
HT47 Synthetic
XRD spacings in ~*
Untreated
003
7-79
7.85
8-51
8.09
7.88
7.78
9.13
8.09
7.64
5042 + glycerol treated
006
3.93
3-97
3-90
3-98
3-85
3-96
3.84
* XRD spacingsgiven in descending order of intensities.
003
9-09
8.67
7.75
7.87
9.17
7-85
006
4-558
4-34
3-93
4-59
R. M. Taylor et al.
304
Table 3. Chemical compositionsof silver and golden pyroaurites.
Pyroaurite
Wt
(rag)
Mg2+
Fe 3+
AI 3+
sample
Silver
Golden
29-30
31-65
235.4
228.9
70-6
67.9
3.4
3.3
CO32- Clmicromoles
33-7
24.5
1.11
2.6
80420.07
0.02
The Mg 2+, Fe 3+, AI 3+ and CO3e results are the mean of triplicates for the
silver sample and of duplicates for the golden sample. The CI- content is the result
of a single determination on both samples, and the SO4e is calculated by
difference for charge neutrality.
serpentine impurity was not considered to have been sufficiently attacked to contribute to
the analysis.
IR analysis
The I R spectrum of the silver pyroaurite contained peaks at 1370 and 665 cm-1 indicative
of CO32 . A small peak at 1385 cm -1 is consistent with the trace NO3 impurity, and no
evidence for SO42- was found. For the golden sample, CO32- was indicated by peaks at
1365 and 665 cm -1 and SO42- by the frequencies 1110, 1160, 630 and 440 cm 1
OH-stretching absorptions were found at 3480 and 3430 cm -1 and H - O - H bending
absorptions at 1640 and 1645 cm -1 for the silver and golden samples, respectively. The
glycerol-sodium sulphate treatment to exchange the interlayer anion removed the 1370 and
650 cm -1 CO32- peaks and caused new absorptions at 1110, 990 and 440 cm 1, attributable
to SO42 , to occur. The golden pyroaurite, after the sulphate exchange, also showed
sulphate peaks at 1110,620 and 440 cm-a; no carbonate peaks were detected. Peaks arising
from glycerol at 2940, 2870-2880 and 960-965 cm 1 were also seen in the spectra of both
treated samples.
Scanning electron microscopy
Both the silver and golden pyroaurite samples were seen to be composed of tabular platy
crystals generally >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
<c
I
9 -- ' . - . . . " :
."-
:g
9 9
#
I
-8
I
-4
I
0
I
4
I
8
I
-8
I
-4
I
0
I
4
I
8
Velocity (mm/s)
Fro. 5. MOssbauerspectra of the two samples obtained at 12 K. Left: golden; right: silver. The upper
traces show the full spectra with maximum absorption at 12.5 and 15-7% for the golden and silver
samples, the lower traces show the spectra with enhanced background.
MOssbauer spectroscopy
The M6ssbauer spectra obtained at 12 K (Fig. 5) are d o m i n a t e d by a Fe 3+ doublet with
MOssbauer p a r a m e t e r s IS = 0.46 mm/s and QS = 0.48 mm/s similar to the p a r a m e t e r s of
Fe 3+ in synthetic pyroaurite. The absorption at 2.8 mm/s is presumably line 2 of a F e z+
doublet, where line I is hidden in the intense Fe 3+ doublet. The origin of the Fe 2+ is not
known with certainty; it may be a substitution in the serpentine, but from the literature
substitution of Fe 3+ appears to be the rule. Substitution within the pyroaurite structure is
also possible but the extent to which this may occur is not known.
In addition, four very weak absorption lines due to the outer four lines in a magnetically
split sextet are seen. This c o m p o n e n t has b r o a d (F1,6 ~ 0-99 mm/s) and non-Lorentzian
lines indicating substitution and/or p o o r crystallinity. The hyperfine field is approximately
51 T at 12 K suggesting the presence of an Fe3+-oxide compound. F r o m the fitting of spectra
it is seen that the Fe 2+ doublet and iron(III) oxide contents of the golden sample (2-5 and
5 % , respectively) were higher than those found in the silver sample (1 and 3 % ,
respectively). In contrast, the Fe 3+ doublet content was higher in the silver sample (96%)
than in the golden (92.5%). The spectra was 295 and 80 K are similar to that at 12 K except
that no magnetically split c o m p o n e n t is observed. Preferred orientation of the crystals in the
absorber is partly responsible for the noted higher intensity of line 2 of the ferric doublet
relative to line 1. Spectra obtained with the absorber at the magic angle show line 1 to be the
most intense and it is likely that relaxation causes this asymmetry.
DISCUSSION
Interchange of tetrahedra18042 for CO3 2 goes to completion with successive treatments,
and the resultant change in X R D spacings allow several observations to be made. Both
Genesis" and composition o f natural pyroaurite
307
forms show the same swelling behaviour and there is no evidence for non-removable
"pillars" (Fe, Cr, A1 or silicate interlayers). Moreover, with this interchange the multiple
basal XRD peaks shown by the original golden material tend towards a single feature,
suggesting variable interlayer anion rather than mixed mineral phases. The XRD spacings
for the glycerol-sulphate treated natural pyroaurite samples (Table 2) are slightly higher
(9-1-9.2 A) than the range (8.66-8.95) observed for SO42--exchanged takovite at 30%
relative humidity or 5042 hydrotalcites (Bish, 1978; Miyata & Okada, 1977; Drits et al.,
1987). Bish (1978) showed that the basal spacing increases from 8-9 to 11 A. in
SO42--exchanged takovites with increasing humidity. It seems reasonable, therefore, to
suggest that the broad, weak 9-1-9-2 A peak in the untreated SO42 -containing golden
sample (Fig. 3, Table 2) may be due to layers with an excess water content. The persistence
and enhancement of this spacing after the glycerol-SOn2- reaction is probably unrelated
and arises from residual glycerol whose presence in confirmed by IR (Table 3).
XRD, IR and chemical analyses show that the major difference between the golden and
silver samples is the degree of replacement of CO32- by SO42 in the interlayer region. The
OH-stretching frequency ca. 3450 cm 1 is, however, lower than expected from the IR
results of related synthetic compounds. Hernandez-Moreno et al. (1985) showed that for
M(II) : M(III) = 3 : 1 the OH-stretch varies from 3500-3550 cm -1, whereas a divalent : trivalent cation ratio of 2 reduces the range to 3420-3450 cm -1. In contrast, these natural
pyroaurites have a cation ratio >3 but the OH-stretching frequency is <3500 cm -1.
Although the CI- content of the pyroaurites is low (Table 3), the higher concentration of
interlayer C1- occurs in the golden sample as for the SO42 . The silver sample, however,
has a higher total C1- content when soluble salts are included. This is in accord with the
higher C1- concentration in the waters of zone B (Table 1). The silver pyroaurite can be
regarded as containing virtually no SO42- because its concentration, calculated as 0.07
moles per 16 OH, is within experimental error negligible in contrast to the relatiyely
significant amounts in the golden sample. Because interlayer anion exchange enhances the
SO42 XRD features and almost completely eliminates those arising from the CO32- anion
without producing a colour change, the presence or absence of SO42 cannot be regarded as
causing the colour difference.
However, the colour differences could be explained by our interpretation of their
genesis. At the high pH (11-12) of water in zone B, virtually no Fe(II) exists in solution. In
zone A, dissolution of primary or secondary phases proceeds under oxidizing conditions,
again precluding the availability of Fe 2+ species in a Mg2+-rich solution. This zone of mixing
is known to fluctuate in response to recharge and recession. The vertical range of this
interface embraced the few metres in which the pyroaurite occurred. It is therefore
inferred that the mixing of the reducing and oxidizing water layers of different ionic
compositions gave rise to pH and compositional conditions conducive to pyroaurite
precipitation. A striking feature of the two pyroaurite types was the common occurrence
of brucite as coatings on the lower (silver) pyroaurite and its total absence from the
overlying (golden) material. Although the brucite was clearly of later paragenesis, its
absence at higher horizons is probably due to a pH-dependent solubility, consistent with a
pH zonation (decreasing upwards) within the overall mixing zone. Aqueous precipitation of
the Mg end-member, brucite, without pyroaurite, has also been noted from the same
geological environment, at the interface between bicarbonate and hydroxide groundwaters
(Neal & Stanger, 1984).
The similarity of the Mg/Fe(III) and the Mg/[Fe(III) + AI(III)] ratios in both the silver
308
R.M.
Taylor et al.
and golden samples suggests formation under one set of conditions in this zone of mixing
which must, of course, be a zone of limited oxidation. Both SO42 and CO32- may be
initially incorporated into the mineral interlayers, but zonal fluctuations due to a rising
water table, will, to a large extent, cause SO42 to be replaced by CO32- in the lower more
reducing and alkaline horizons of higher CO32- concentrations. This will happen more
readily than the reverse exchange. This would therefore explain not only the formation and
similarities between the two samples, but also the difference in interlayer anions.
MOssbauer spectroscopy revealed no major differences between the two samples except
that a greater amount of iron(Ill) oxide was present in the gold sample. The golden colour
possibly arises from this (excess) iron(III) oxide in a similar way to the golden lustre
imparted to micas by small iron(III) oxide clusters. Environmental conditions responsible
for the variation in interlayer anion could also explain these excess of iron(III) oxide
responsible for the observed colour difference. In earlier syntheses (Hansen & Taylor,
1990), ferrihydrite was often identified in brown-coloured pyroaurite as an impurity which
could be removed by treatment with sodium dithionite or refluxing in a sodium carbonate
solution. With a rising water table, the (slightly) more alkaline and reducing waters that
caused the replacement of interlayer SO4 2 by CO32- could cause the reductive dissolution
of the Fe(III) on these precipitates. The golden material thus loses its surface colour and
becomes silver (white), while in the higher horizons, where the original formation
conditions still persist, the golden material retains its coloured Fe(III) surface contamination and partial satisfaction of the interlayer charge with sulphate anions. Only small
amounts of Fe are considered to be involved and have little apparent effect on the analytical
results, which show a slightly higher Mg(II)/Fe(III) ratio and lower Fe(III) content for the
gold sample.
Neal & Stanger (1985) maintained that the release of Mg 2+ during olivine dissolution is
central to the process of low-temperature serpentinisation occurring in this area. However,
the olivine is not pure forsterite (Mg end-member), and some Fe(II) is also liberated during
silicate dissolution. Since this is not incorporated in the precipitated serpentine structure, it
remains as the obvious source of soluble Fe(II), when exposed to mixing zone Eh-pH
conditions. Hence the limited oxidation of this Fe(II) in the Mg2+-rich environment (the
zone of mixing) closely resembles the pathway suggested in the synthesis work.
REFERENCES
BISHD.L. (1978) SO42 -takovite at low humidity. Bull. B.R.G.M. ll, 293-301.
DRITSV.A., SOKOLOVAT.N., SOKOLOVAG.V. & C~ERKASaINV.I. (1987) New members of the hydrotalcitemanasseite group. Clays. Clay Miner. 35, 401-417.
DUNNP.J., PtACORD.R. & PALMERT.D. (1979) Desautelsite, a new mineral of the pyroaurite group. Am. Miner.
64, 127-130.
FEITKNECHI"W. (1942) fiber die Bildung yon Doppelhydroxydenzwischenzwei- und dreiwertigen Metallen. Helv.
Chim. Acta 25, 555-569.
HANSE~qH.C.B. & TAYLORR.M. (1990) Formation of synthetic analogues of double metal-hydroxycarbonate
minerals under controlledpH conditions:I. The synthesisof pyroaurite and reevesite. Clay Miner. 25, 161-179.
HANSENH.C.B. & TAYLORR.M. (1991a) The use of glycerol intercalates in the exchange of CO32- with SO42-,
NO3- or CI- in pyroaurite type compounds. Clay Miner. 26, 311-327.
HANSENH.C.B & TAYLORR.M. (1991b) Formation of synthetic analogues of double metal-hydroxycarbonate
minerals under controlled pH conditions: II. The synthesis of desautelsite. Clay Miner. (in press).
HASmK., KIKgAWAS. & KOtZUMIM. (1983). Preparation and properties of pyroaurite-like hydroxyminerals. Clays
Clay Miner. 31, 152-154.
Genesis and composition o f natural pyroaurite
309
HERNANDEZ-MORENOM.J., ULmARRLM.A., RENDONJ.L. & SERNAC.J. (1985) IR characteristics of hydrotalcite
like compounds. Phys. Chem. Miner. 12, 34-38.
LARSEN S. (1949) An apparatus for the determination of small quantities of carbonate. Acta Chem. Scand. 3,
967-970.
MARCZENKOZ. (1986) Separation and spectrophotometric determination of elements, 2nd ed., pp. 225-227 and 420422. Ellis Horwood, Chichester.
MIYATAS. 8z OKADAA. (1977) Synthesis of hydrotalcite-like compounds and their physico-chemical properties- the
systems Mg2+-AI3+-SO42- and Mg2+-A13+-CrO42 . Clays Clay Miner. 25, 14-18.
NEAL C. & SXAN6ERG. (1983) Hydrogen generation from mantle source rocks in Oman. Earth Planetary Sci. Lett.
66, 315-320.
NEAL C. & SXAN6ERG. (1984) Calcium and magnesium hydroxide precipitation, from alkaline groundwater in
Oman, and their significance to the process of serpentinisation. Mineral. Mag. 48, 237-241.
NEAL C. & STAN6ERG. (1985) Past and present serpentinisation of ultramafic rocks; an example from the Semail
ophiolite nappe of Northern Oman. Pp. 249-275 in: The Chemistry of Weathering (J.J. Drever, editor).
D. Reidel Pub. Co. (NATO Advanced Workshop on Weathering).
SCHILXA.S. (1969) Analytical Applications of l,lO-Phenanthroline and Related Compounds, pp. 56-59. Pergamon
Press, Oxford.
STAN6ERG. (1986a) The hydrogeology of the Oman Mountains. PhD thesis, Open Univ., UK.
SXAN6ER G. (1986b) Hydrology and Water Resources of the Greater Manah Area. In: W.S. Atkins Int., Rep.
CCEWR 86-9, Oct. 1986. Council for Conservation of the Environment and Water Resources, Sultanate of
Oman.
SXANCERG., NEAL C. & LAVERJ. (1988) Black carbonaceous calcite associated with serpentinite from Oman.
Mineral. Mag. 52, 403-408.
TAYLORR.M. (1984) The rapid formation of crystalline double hydroxy salts and other compounds by controlled
hydrolysis, Clay Miner. 19, 591-603.
THOMASSINJ.H. & TOVRAYJ.C. (1982) L'hydrotalcite, un hydroxycarbonate transitoire precocement forme lors de
l'interaction verre basaltique-eau de mer. Bull. Mineral. 105, 312-319.