Clay Minerals (1984) 19, 509-520

F E R R O U S - I R O N - B E A R I N G V E R M I C U L I T E - S M E C T I T E S E R I E S F O R M E D D U R I N G

A L T E R A T I O N OF C H L O R I T E TO K A O L I N I T E , O T A G O S C H I S T , NEW Z E A L A N D

D. C R A W

Department of Geology, Unversity of Otago, Dunedin, New Zealand

(Received 25 July 1983; revised 24 February 1984)

A B S T R A C T: Interlayered (<20 #m scale) vermiculite-smectite minerals were found in drill-cores from altered Otago schist basement up to 2 m from the contact with the overlying sediment. Mineral compositions defined a series from Fe2+-rich trioctahedral vermiculite to Fe2+-bearing dioctahedral smectite, in these, tetrahedral substitution of AI for Si was unusually high (up to 3.3/8 in the formula unit). The minerals formed as intermediates during the alteration of chlorite to kaolinite under non-oxidizing, possibly acid conditions. The source of the altering solutions may have been overlying lignite seams.

The Haast Schist of Otago (Wood, 1978) consists of low- to medium-grade (maximum = garnet zone) meta-psammitic and metapelitic rocks with minor metavolcanic horizons. The schists were metamorphosed and multiply-deformed in middle Mesozoic time, then uplifted and eroded in the late Cretaceous and Tertiary with development of a widespread peneplain or erosion surface of low relief. Terrestrial sediments were deposited on this erosion surface in Central Otago. Late Cenozoic block-faulting disrupted the sediments and underlying erosion surface, and the sediments are now preserved mostly in isolated fault-angle depressions.

A notable feature of the erosion surface in nearly all localities where it is preserved is a highly leached kaolinite-rich zone up to 10 m thick. This zone has been attributed to deep weathering of the basement during the erosive process (Benson, 1935). Some exposures have fresh schist in contact with the overlying sediments where the leached schist was eroded from these surfaces before deposition of the Tertiary sediments.

The green phyllosilicate minerals described here were found in kaolinitized schist from the schist erosion surface at Roxburgh, Central Otago (Fig. 1). The schist is overlain by a thick (>120 m) sequence of terrestrial Tertiary sediments, including 55 m of lignite (Douglas, 1984). The mineral material was obtained from two driU-holes (d2001, d2071) which were drilled as part of a lignite exploration project funded by the New Zealand Government. Mineralogically similar material coexisting with chlorite has been observed in lower-grade schists near Taieri Mouth on the Otago coast (D. S. Coombs, pers. comm.). The following data and discussion show that the Roxburgh green phyllosilicate consists of interlayered smectites and vermiculites of variable composition.

R O C K D E S C R I P T I O N

The basement of the Roxburgh area is greenschist-facies quartzofel'dspathic schist of textural zone 4 (Bishop, 1972). The schist is composed mainly of alternating

�9 1984 The Mineralogical Society

510 D. Craw

' ."'::/ N """ " ' \ 4 x R O X B U R G H H Y D R O .... .."..: �9 '; ' " [

3",, -~i,,d2OO1 ..!~.'/ .,:" .. T ..-"..) \ \ ~'-d2071 "" ,,""/ / ' ~ /

,, ";,.,ROX,U~OH .... :~. I "' : .J

/.-!' ". . . . . . . - ' : - \ . .............. :----': .

�9 . . . . - . �9 , ~ . . -" . . . , . : ) . .

: �9 :'*-;'" �9 :: "~.4 " " 4.'" ....... " DUNEDIN

�9 " "i" ,":" : ,':"" �9 '

FIG. 1. Locality map of east and Central Otago predominantly underlain by Otago Schist. The locations of drill-holes d2001, d2071 are indicated. Boundary between textural zones 3 and 4 (Bishop, 1972) in the Otago Schist is shown as a dashed line. Cretaceous and Cenozoic cover is

stippled.

muscovite-chlorite-epidote lamellae and quartz-albite segregation lamellae up to 1 cm thick (Fig. 2). The rocks are cross-cut by quartz veins, commonly with chloritic selvedges. The minerals are medium-grained (~0.5 mm), and finer-grained accessory minerals (graphite, tourmaline, pyrite, sphene and apatite) are also present. Natural outcrops in the Roxburgh vicinity show various degrees of modern weathering (Brown, 1967; Craw, 1981) and lithologic studies are consequently difficult. However, freshly blasted rock at the Roxburgh hydro-electric dam, about 5 km from the drill-holes, is relatively fresh. This rock is more phyllosilicate-rich (pelitic) than typical Central Otago quartzofeldspathic schist (cf. Brown, 1963), and contains thin (about 5 cm) layers of quartz-albite-chlorite-epidote greenschist. The schist immediately beneath the terrestrial sediments in the drill-holes is kaolinitized throughout the recovered core lengths (2 m in d2001, lm in d2071), but is otherwise apparently similar though softer than the rock seen in fresh outcrop.

Schist from both drill-holes contains a very distinctive deep green phyllosilicate, hereafter referred to as vermiculite-smectite. This material is very much more common in d2071, where it is an important rock-forming mineral, than in d2001. Its presence is in strong contrast to the situation in most kaolinitized schists in which all the green (iron-bearing) minerals have been totally oxidized and altered to goethite. The vermiculite-smectite is especially common in, and marginal to, quartz veins and segregations, and it clearly cross-cuts the schist foliation in some almost monomineralic veins up to 5 mm wide. The grain size of the vermiculite-smectite is approximately the same as that of phyllosilicates in the host schist, i.e. 0.05-0.5 mm. Flakes along the margins of quartz-albite segregation lamellae are oriented parallel to the foliation defined by contiguous muscovite-rich lamellae. More rarely, flakes of the vermiculite-smectite are found within the muscovite lamellae.

The kaolinite is present as cross-cutting veins, and in places is interlayered with muscovite and vermiculite on a scale of 1-10 pm. Chlorite is not recognized in these kaolinitized rocks. Apart from the presence of the green vermiculite-smectite and kaolinite, and the absence of chlorite, the mineralogy of the altered rocks is the same as that

Interlayered vermiculite-smectite series 511

FIG. 2. Photomicrographs of (a) unaltered schist from Roxburgh Hydro, and (In) altered schist from d2071. Chlorite (C) in (a) forms foliation lamellae and clusters of flakes near the margins of quartz-albite segregation lamellae and quartz veins (white). In (b) vermiculite-smectite (V) mimics the chlorite textures. Expansion and in-filling with epoxy resin during thin-section preparation of (b) is arrowed. Apart from vermiculite-smectite apparently replacing chlorite, the mineralogical compositions of (a) and (b) are identical (see text). Horizontal field of view in both

photographs is 4 mm.

of the fresh rocks described above. Textures of the vermiculite-smectite in altered rock mimic chlorite textures in fresh rock. Thus, although the drill-holes did not penetrate the fresh rock to allow more detailed study, it seems highly likely that the vermiculite-smectite has replaced chlorite.

O P T I C A L P R O P E R T I E S

Optical properties of the vermiculite-smectite are summarized in Table 1. Variability o f measured y refractive index is due to variable chemistry (see below) and possibly to the

512 D. Craw

TAaLE 1. Optical properties of green phyllosilicate.

Optic sign Sign of elongation Birefringence 2 Va Dispersion Refractive indices

Pleochroism

Extinction

negative, (pseudo-)uniaxial positive ~0.025; second-order interference colours 0-5 ~ very weak y ~ f l 1.600+0.003

1.606 + 0.003 1.590 + 0.003

a greater than Canada Balsam, ~ 1.57 (estimated from y and birefringence).

ct pale green-pale yellow brown fl ~ y deep olive green parallel, mottled.

TABLE 2. Microprobe analyses of phyllosilicates.

1" 2 3 4 5 6 7 8 9 10 11 12t

SiO z

A120~ TiO z FeO$ MnO MgO CaO Na20 K20 Total

26.7 26.6 30.4 31.0 32.2 35.3 37-8 39.4 40.2 42-0 45.4 46.6 20.4 16.2 15.2 15.2 19.9 20.4 22.2 23.2 21.4 29.6 28.5 39-5

- - 0.00 0.01 0.02 0.02 0.00 0.00 0.00 0.02 0.01 0-02 27.5 29-9 25.4 26-5 25.1 22.6 21.2 18.6 19.9 11.6 10.5

0.44 0.29 0.64 0.69 0-54 0-42 0.10 0.15 0.52 0-10 0.23 14-0 8.4 8.4 8.4 6.9 6.0 5.52 4.6 5.6 2.5 2.1

- - 0-35 0-64 0-65 0.48 0-52 0-28 0.27 0-68 0.39 0.49 - - 0.12 0.16 0-12 0.18 0.13 0.16 0.15 0.11 0.20 0-13 - - 1.2 1.4 1.9 0-45 0.21 0.94 0.39 0-60 0.50 0.13

89-04 83.06 82.25 84.48 85-77 85.58 88.20 86.76 89.03 86.90 87.50 86-1

Cations recalculated on the basis of 22 oxygens (anhydrous)

Si A1 w

A1 vI

Fe3+ Fe 2+

Mn Mg Ca Na K

4.68 5.26 5,25 5.22 5.60 7-74 5.95 5.99 6.07 6.42 3.32 2.74 2,75 2.78 2.40 2.26 2.05 2.01 1-93 1.58 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8-00 8.00

0.04 0.36 0-28 1.02 1-40 1.71 2.08 1-75 3-11 3.17 1.57 1.23 1-25 1.13 1.00 0.90 0.79 0.83 0-47 0.42 3-13 2.45 2.50 2-26 2.00 1.79 1-57 1-66 0.93 0.83 0-04 0.09 0- 10 0.07 0.06 0.01 0.02 0.07 0.01 0.03 2-20 2.15 2-12 1.67 1-42 1-25 1-04 1.24 0.54 0.44 0.07 0.12 0.12 0.08 0.09 0.05 0.04 0.11 0.06 0.07 0-04 0.05 0.04 0.06 0.04 0.05 0-04 0.03 0.06 0.04 0-27 0.32 0.41 0.09 0.04 0.18 0.08 0.11 0-09 0.02 7.36 6.77 6.82 6.38 6.05 5.94 5.66 5.80 5-27 5.02

* Chlorite analysis from Brown (1967); t ideal kaolinite; $ all Fe as FeO.

p r e sence o f in te r leaved kaolinite. M i n o r biaxial i ty o f s o m e gra ins m a y be cau s ed by

cu rva tu re o f flakes.

C H E M I C A L C O M P O S I T I O N

T h i r t y - o n e m i c r o p r o b e ana lyses (see C r a w et al., 1982, for ope ra t ing cond i t ions ) were

m a d e f r o m th ree d i f ferent levels in the a l tered schis t in d2071 , at 10, 40 and 90 c m be low

Interlayered vermiculite-smeetite series 513

the schist erosion surface. There is no significant difference between analyses obtained at the three levels: results are variable at all levels (Table 3). Ten representative analyses are presented in Table 2. A typical Otago chlorite analysis (Brown, 1967) and ideal kaolinite analysis are added for comparison. The analyses indicate that the green phyllosilicate has a very wide range of compositions. Semi-quantitative scanning traverses across grains show that these variations are the result of interlayering of flakes of different compositions on a scale of 10-20/tm (Fig. 3).

Sub-micron interlayering, beyond the level of microprobe resolution, may exist also. In fact, the optical uniformity (especially birefringence) of the vermiculite-smectite flakes on the 10-20 #m scale strongly implies that some finer interlayering does exist. Thus the analyses obtained are not necessarily representative of pure mineral phases. However, the total AI vs. Si and K20 vs. SiO 2 plots (Fig. 4) for the data show a scatter which precludes the existence of simple mixing of two discrete mineral species of constant composition.

Wet-chemical analyses for FeO by the method of Wilson (1955) were carried out on 10- and 50-mg hand-picked mineral separates from between the top and middle of d2071. The results are presented in Table 4. These data can only reflect average FeO content in what are highly variable mineral compositions. However, they serve to indicate a relatively high FeO content in the minerals. If the average FeO (total iron) content for the green material between the top and the middle of the d2071 schist core is taken as 19 wt% (average from Table 3), the wet-chemistry average of 13% FeO implies that about ] of the FeO is ferrous iron. This very approximate figure has been used in calculations of structural formulae in Table 2. These values of FeO can be considered as minima only, as some oxidation of iron may have occurred after formation of the mineral (cf. Sudo, 1954).

TABLE 3. Means and standard deviations of microprobe analyses of SiO 2 and FeO (total iron) at three levels (~50 cm apart) in the

altered schist of d2071.

Wt% SiO/ Wt% FeO

Top (11 analyses) 38.5 (2.6) 18.2 (3.5) Middle (12 analyses) 37.3 (5.0) 20-3 (5-4) Bottom (8 analyses) 36.4 (8.5) 26.8 (4-5)

30"

FeO"

10-

"25

-45

SiO 2

2 o

o 16o pm 260 36o FIG. 3. Semi-quantitative microprobe scanning traverse across a grain of green phyUosilicate from d2071. 'Noise' on each profile is indicated by error bars. Approximate calibration from

spot analyses is included on vertical axes. Beam diameter was 2 ~trn on periclase.

514 D. Craw

a

.5

AI

3

m m mmm

~

O0

$i �9 sS 6

b m

K2O

2

I

3O ! !

[]

SiO 2 4O !

FIG. 4. Element-element plots for green phyllosilicate analyses. (a) Aluminium vs. silicon (axes graduated in formula units. Total A1 vs. Si (squares) shows considerable scatter which precludes a simple mixture of two components. When analyses are recast into smectite structural formulae, octahedral AI vs. Si (dots) show a more ordered trend, (b) K20 vs. SiO2(wt%) plots shows

considerable scatter. Approximate analytical error is indicated by the hatched rectangles.

TABLE 4. FeO determinations from d2071 (near middle of altered zone).

Sample size (mg) FeO (wt%)

Sample 1 10 12.1 Sample 2 I0 14.1 Sample 3 10 11.9 Sample 4 50 12.5 Sample 5 50 13-1 Sample 6 50 12-0 Sample 7 50 13.2 Sample 8 50 15.1

average (s.d.) 13 (1.1)

X-RAY D I F F R A C T I O N A N A L Y S I S

Pure samples of the d2071 and d2001 green phyllosilicate material gave moderately broad diffraction peaks at 14.4, 7.2 and 3.59 A. No other peaks were observed. The width of the peaks probably reflected the variability in composition of the layers which make up the flakes. Progressive heating of the samples caused lowering of intensity and broadening of these peaks, and a shift in peak position towards 11 ~_. The mineral swelled to give a broad 17-18 A peak with both glycerol and ethylene glycol, and to 15 A with water. Diffraction traces following most of these treatments are presented in Fig. 5. The 7 and 3-6 A peaks move in sympathy with the 14 /k, peak, precluding the existence of random interstratification of minerals with other than 14 /~ basal spacings (cf. McEwan et al.,

Interlayered vermiculite-smeetite series

t

. . , i . . . . . I ' I

I

i glycol

glycerol i i

300~ . . . . . . , m

i

i

400~ l

i i . _

~ I

! I I

I

18A

" ' I

- - - - ~ l ~ m l l I I l I m

FIG. 5. X-ray diffraction traces showing responses of the green phyllosilicate material to heating and ethylene glycol and glycerol saturation. Cu-Karadiation, Ni-filter, scan speed 1/4~

515

1961). No change in diffraction behaviour was observed after treatment with potassium acetate or 1 ~a K +, NH4 + or Mg 2+ solutions for one week at 40~ or after one week with 1 M Mg 2+, followed by 1 M K + (one week).

The green mineral was attacked by 10% HC1 overnight, 1% HC1 over two weeks, and 10% acetic acid over one month, leaving an amorphous white residue. There was no change in 0.1% HC1, or 1% or 0.1% acetic acid over one month. A 10% NH~OH solution destroyed the structure over one week.

C A T I O N E X C H A N G E

The cation exchange capacity of the green phyllosilicate material was determined by the method of Blakemore et aL (1972). Duplicate samples of ~0-5 g were used in this determination. Samples contained a high proportion of green phyllosilicate but the degree of purity of samples was unknown. Kaolinite flakes, both macroscopic and microscopic, formed the most common contaminant. Both samples yielded CEC values of 18.9 mEq/100 g.

516 D. Craw

Qualitative analysis of the leachate by atomic absorption spectrometry yielded approximate Mg/Fe ratios of 60 and 70 for the two samples. These data suggest that magnesium is the dominant interlayer ion in the vermiculite-smectite material.

D I S C U S S I O N

Identity of the mineral species

The optical properties and variable chemistry of the green phyllosilicate imply interlayering of phyllosilicates of differing compositions. Response to glycol and glycerol treatments implies that smectite and/or vermiculite are present. The response to heating (previous section) and the significant potassium content in the low-silica analyses (e.g. analysis 2, Table 2) precludes the presence of chlorite as a major phase. Lack of response to potassium acetate (Wada, 1961) and the substantial Fe and Mg and low A1 in the high-silica analyses (e.g. analysis 11, Table 2) show that kaolinite does not make up a significant proportion of the flakes, apart from the obvious interlayered flakes seen in thin-section. Approximately regular increase of AI with Si (Fig. 4) implies that fine-grained (< 1/tm) amorphous silica is not present between flakes.

The microprobe scan traverses (Fig. 3) reveal interlayering on a scale of about 10-20 pm and possibly even finer. Movement of whole XRD peaks in response to treatments (especially swelling of basal spacings) suggests that the layers are all of one mineral group, the vermiculite-smectite group.

Mineral formulae

Recalculation of the green phyllosilicate analyses of Table 2 as smectite (22 oxygens, anhydrous) results in the mineral formulae presented in the lower part of Table 2. Plots involving octahedral and tetrahedral site occupancy based on these formulae (e.g. Fig. 6) show up more ordered series than the Al(total)-Si and SiO2-K20 plots (Fig. 4). The Fe 2+ vs. octahedral A1 data approach a straight line of gradient --3/2 (Fig. 6). This close approach, coupled with the close approach of the Mg vs. Fe z+ data to a straight line of gradient 1, implies a mineral series in which 3(Fe,Mg) vI substitute for 2AI w. Substitution of A1 w for Si increases as Fe 2+ and Mg substitution increases, but not at the same rate (least squares regression line gradient 0.73 (Fig. 6)). These data imply that the vermiculite-smectite series involves tetrahedral and octahedral substitutions which are effectively independent of each other. Independence of tetrahedral and octahedral substitutions confirms that the studied material is not merely an interlayered mixture of two discrete minerals. Deviations from these ideal substitutions (i.e. deviations from the lines in Fig. 6) may be due to variable Fe2+/Fe 3+ ratios and submicron interlayering of vermiculite-smectite lamellae of differing compositions. In addition, the presence of submicron-scale interlayered kaolinite or relict chlorite in some analyses cannot be discounted.

It is not possible to confidently recast cations into octahedral and interlayer positions because Fe2+/Fe 3+ ratios are not known with certainty for each mineral analysis. Hence the non-tetrahedral cations are all listed together in Table 2, with an assumed Fe2+/Fe 3+ ratio of 2:1 (see above). The maximum octahedral cation sum is 6.00 (trioctahedral

lnterlayered vermiculite-smectite series 517

'3

.2

\ I . - "

\ \ A l V i ...'/ ', \ .=-";*= / /

, S~

�9 - .~= // \ \,, ./r /

ii /.-'~11 e, o / " s

../ A~, ,,/ Al iV . . / � 9 \ / � 9 ~s S [ ]

s s ~ �9

-" 9 / e \ / �9

Mg/"e \ / \

/ \

o / \ \

Fe2+ ' , / k

," 1 2 / I , &t

FIG. 6. Element-element plots for green phyllosilicate analyses recast into smectite structural formula (see text). Lines through the octahedral AI (triangles) and Mg (dots) data are drawn at gradients of -3/2 and 1.0 respectively, and the data closely approach these lines. The line through the tetrahedral AI data (squares) is a least-squares regression line. Hatched rectangle

shows approximate analytical error.

mineral) while the minimum is 4.00 (dioctahedral mineral). While the exact content of the interlayer region is not known, the non-tetrahedral cation sums for low-Si analyses show that there is a substantial interlayer charge on those minerals. The lack of more 'normal' interlayer cations Ca, Na, and K in the analyses (Table 2) implies that Mg and minor Fe are present in the interlayer, as indicated by the CEC results. High-valency, small cations (e.g. Mg 2+) in the interlayer region inhibit cation exchange (Grim, 1968). This may explain the low CEC determined, and the lack of change in basal spacing after treatment with various cations (cf. Walker, 1961).

Comparison with 'normal' vermiculites and smectites

The most notable feature of these mineral formulae is the unusually high tetrahedral AI (and low Si) compared to most smectites. Ferrous iron-rich vermiculite does not appear to have been reported in the literature. However, replacement of vermiculite-Mg by Fe 2+ is theoretically reasonable. Ferrous iron-rich smectites are rare. Up to 22% FeO in saponite was suggested by Konno & Akizuki (1976) who assumed considerable post-formation oxidation of ferrous to ferric iron (of. Sudo, 1954).

The Roxburgh analyses define a series from trioctahedral vermiculite to dioctahedral smectite through a region of composition normally considered to be outside the smectite range (Weaver & Pollard, 1973; Curtis & Smillie, 1981). While some of the analyses may be of combinations of more than one layer, the series is apparently continuous, involving independent substitutions in tetrahedral and octahedral sites. Hence

518 D. Craw

the evidence implies the existence of a mineral series mixed on a scale of 10--20 #m or finer, with compositions varying from high A1 ~v ferrous iron-rich vermiculite to high A1 Iv ferrous iron-bearing dioctahedral smectites. This mineral series is analogous to the celadonite-vermiculite series described by French et al. (1977). Nahon et al. (1982) reported similar interlayered smectites of varying compositions which formed during weathering of olivine. Likewise, similar mixed intermediates between saponite and vermiculite were described by Curtis (1976).

O R I G I N

There seems little doubt that the vermiculite-smectite in d2071 and d2001 formed by alteration of chlorite. The mineral series produced shows a trend towards higher Si and A1 with increasing alteration, and interleaved kaolinite is commonly found. Kaolinite is the most common clay mineral in the alteration zone. Thus it is likely that the green mineral forms an intermediate stage in the alteration of chlorite to kaolinite. This is very similar to the origin of the Fe2+,Fe3+-rich smectite griffithite (Arikas, 1974) which formed as an intermediate in the alteration of chlorite to kaolinite. Arikas gave a single analysis of pure griffithite based on wet chemistry, but this could possibly represent an average of finely interleaved layers of differing composition, similar to the Otago vermiculite-smectite.

The conditions of formation of the Otago vermiculite-smectite are unusual. As outlined above, alteration at the schist erosion surface normally involved oxidation of chlorite in the very early stages, followed by alteration to kaolinite and brown iron hydroxides. Churchman (1978, 1980) and Brown (1967) described the formation of vermiculite from chlorite under oxidizing conditions in the Otago Schist. However, the Roxburgh minerals were obviously formed under non-oxidizing conditions. This non-oxidizing vermiculite formation is in strong contrast to the experimental work of Ross (1975) and Ross & Kodama (1976) which suggests that oxidation of iron is an important mechanism in alteration of chlorite to vermiculite. Perhaps the process may occur without oxidation over long (geological) time periods, but not in short (laboratory) time periods. Vincente et al. (1977) described the formation of vermiculite and smectite from mica under acid conditions. A similar process, operating on chlorite, may have occurred at Roxburgh with an Fe2+-rich product resulting from an Fe2+-rich (chlorite) parent. Malcolm et al. (1969) reported the formation of montmorillonite from a chlorite-like mineral under low-pH, organic-rich conditions, possibly analogous to those at the Roxburgh erosion surface. Kaolinitization of smectite has been demonstrated by Poncelet & Brindley (1967)under acid conditions, and Urabe et al. (1970) experimentally transformed vermiculite to kaolinite under acid conditions. Natural transformation of smectite to kaolinite was described by Herbillon et al. (1981).

The alteration process therefore involved non-oxidizing conditions which were possibly acidic. One possible source of such conditions could have been the Tertiary coal-forming swamps overlying schist. Down-leaching of acidic non-oxidizing solutions from the peaty material either during peat formation or after burial could have initiated the kaolinization process (Staub & Cohen, 1978; Rimmer & Eberl 1982). Such peaty material could subsequently have been removed from the bedrock surface by erosion, or could perhaps still be represented by a 55 m lignite seam 40 m above the schist erosion surface at drill-hole d2071, (Douglas, 1984). This hypothetical process could occur only if unweathered chlorite-bearing schist (i.e. eroded to fresh rock) was present beneath the Tertiary sediments.

lnterlayered vermiculite-smectite series 519

C O N C L U S I O N S

A green phyl losi l icate found in kaol in i t ized schist f rom two drill-holes at R o x b u r g h , Otago ,

consis ts o f chemica l ly var iab le layers in ter leaved on a scale o f 1 0 - 2 0 Ftm or finer. The

phyl losi l icate has the X - r a y diffract ion character is t ics o f smect i te a n d / o r low- layer -charge

vermicul i te , i t is r icher in F e 2+ and te t rahedra l A1 than n o r m a l smect i tes or vermicul i tes .

The minera l compos i t ions fo rm a series f rom t r ioc tahedra l vermicul i te t h rough to

d ioc tahedra l smecti te. This series appears to be in te rmedia te in the a l tera t ion o f chlori te to

kaolini te, under non-oxid iz ing condi t ions. The al terat ion m a y have been caused by

down- leach ing of solut ions f rom over ly ing Ter t i a ry peat / l igni te deposits .

A C K N O W L E D G E M E N T S

I am grateful to Dr R. Morgan who conducted the CEC determinations, and Dr A. Reay for help with atomic absorption analysis. Financial support for this work was provided by a University of Otago research grant. Discussions with B. J. Douglas and C. A. Landis proved very rewarding. Comments by C. A. Landis, D. S. Coombs and an anonymous referee greatly improved the manuscript.

R E F E R E N C E S

ARIKAS K. (1974) Die 'Viridite' im Tholeyit von Tholey (Saar). Neues. Jahrb. Mineral. ,4bhdl. 120, 83-99. B ENSON W.N. (1935 ) Some land forms in southern New Zealand. ,4 ustraL Geog. 2, 3-23. BIsHop D.G. (1972) Progressive metamorphism from prehnite-pumpellyite to greenschist facies in the Dansey

Pass area, Otago, New Zealand. Geol. Soc..4 m. Bull. 83, 3177-3198. BLAKEMORE L.C., SEARLE P.L. & DALY B.K. (1972) Methods for chemical analysis of soils. New Zealand

Soil Bureau Scientific Report 10.4. BROWN E.H. (1963) The geology of the Mt Stoker area, eastern Otago, part I: metamorphic geology. N.Z.J.

Geol. Geophys. 6, 847-871. BROWN E.H. (1967) The greenschist facies in part of eastern Otago, New Zealand. Contrib. Mineral. Petrol.

14, 259-292. CRAW D. (1981) Oxidation and microprobe-induced potassium mobility in iron-bearing phyllosilicates from

the Otago schists, New Zealand. Lithos 14, 49-57. CRAW D., COOMBS D.S. & KAWACHI Y. (1982) Interlayered biotite-kaolin and other altered biotites, and their

relevance to the biotite isograd in eastern Otago, New Zealand. Mineral. Mag. 45, 79-85. CURTXS C.D. (1976) 'Unmixed' Ca2+/Mg 2+ saponite at Calton Hill, Derbyshire. Clay Miner. I l, 85-89. CURTIS C.D. • SMILLIE G.W. (1981) Composition and origin of smectite in soils derived from basalt in

Northern Ireland. Clays Clay Miner. 29, 277-284. DEER W.A., HOWIE R.A. & ZUSSMAN J. (1963) Rock-forming Minerals, vol. 3 Sheet Silicates. Longman,

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