EUΓ. J. Mineral. 1996, 8, 847-852

Occurrence of nacrite in the Lodève Permian basin (France)

MARTINE D. BUATIER*'1), JEAN-LUC POTDEVIN1^ MICHEL LOPEZ1* and SABINE PETIT2*

! ) Université de Lille I, Laboratoire de Sédimentologie et Géodynamique, URA 719, 59655 Villeneuve d'Ascq, France

2) Université de Poitiers, Laboratoire de Pétrologie de la Surface, URA 72, 40 avenue du Recteur Pineau, 86022 Poitiers Cedex, France

Abstract: Authigenic euhedral kaolins of nano-to-millimetre size were found in dolomite cavities in the Cambrian basement of the Lodève Basin, France. X-ray diffraction, DTA and infrared-spectroscopy data demonstrate that nacrite is the only kaolin phase present in the cavities. Nacrite is associated with authigenic quartz, barite, dolomite and calcite. Petrographic data indicate a contemporaneous growth of nacrite and quartz after dolomite. In some cavities, barite and nacrite crystallized together. The crystallization of quartz, nacrite and barite could be the result of fluid influx and trapping in a karstified basement during the Late Permian extensional tectonic regime that affected the Lodève Basin.

Key-words: nacrite, X-ray diffraction, infrared spectroscopy, DTA, Lodève basin, France, fluid-rock interaction.

Introduction

The kaolin group comprises three polytypes with the chemical composition AUSi4Oio(OH)8. Kaolinite has a one-layer stacking sequence and triclinic symmetry, whereas dickite and nacrite have a two-layer stacking sequence and mono-clinic symmetry (Bailey, 1980). Nacrite is the rarest kaolin polymorph, with about 40 occur­rences reported in the literature. According to the more recent structure refinement (Zheng & Bai­ley, 1994), nacrite crystallizes in the space group Cc with the parameters a = 8.906 Å, b = 5.146 Å and c = 15.664 Å, ß = 113.58°. The stacking of layers is based on -a/3 interlayer shifts along the a axis, the vacant octahedral site alternate be­tween A and B sites in successive layers, and alternate layers are rotated by 180°.

Identification of the kaolin polytypes can be achieved by X-ray diffraction and more easily by infrared spectroscopy (Prost et al., 1989). By using these methods, natural occurrences of nacrite have been identified in various environments.

Nacrite is generally considered as a high-

temperature polymorph and most occurrences support a hydrothermal origin (i.e., Marumo, 1989). Its formation in advanced diagenetic rocks has also been reported by Kisch (1983) and Ruiz-Cruz (1995). However, Biihman (1988) postu­lated that nacrite could be formed within a few weeks at low temperature. Only one synthesis ex­periment of nacrite has been performed by Per-myakov at 300-335°C and 285-300 atm (in Zheng & Bailey, 1994). Hanson et al. (1981) re­ported hydrothermal kaolin deposits in Mexico in which nacrite coexists with dickite and kaolinite. A high-temperature hydrothermal origin is also suspected by Marumo (1989) for nacrite iden­tified in Kuroko deposits (Japan). Another occur­rence of nacrite has been reported recently by Shen et al. (1994) in the Jiangshan-Shaoxing deep fault in Zhejiang province (China). Nacrite in kaolin deposits in the Southern Upland fault of Scotland is also related to syntectonic fluid circulation within fault (Russel & Kirkpatrick, 1992). This paper focuses on nacrite precipitated in dolomite cavities from the Cambrian basement of the Lodève Permian basin. The size of the

* email : Martine.Buatier@univ-lillel.fr

0935-1221/96/0008-0847 $ 1.50 ) 1996 E. Schweizerbart'sche Verlagsbuchhandlung. D-70176 Stuttgart

848 M. D. Buatier, J.-L. Potdevin, M. Lopez, S. Petit

(A)

o u c β n L O o JQ <

3BOO 3 7 0 0 3 6 0 0 βoo 3BOO 120O lOOO

Wavenumbβr (cm-1)

Fig. 1. Nacrite characterization, a. Powder XRD pattern, b. IR spectrum.

nacrite crystals can reach two millimetres and southern part of the Massif Central (France). It nacrite is the only kaolin phase present in the corresponds to a southward dipping half-graben cavities. filled by pelite-dominated continental deposits

and sealed by a thick horizontal Mesozoic cover (Lopez, 1992). This cover is widely eroded, al-Geological setting

The Lodève Permian basin is located in the lowing the basement to be exposed in the north-

Occurrence of nacrite in the Lodève Permian basin (France) 849

10

-10

-20 200 400 800 1000 600

Temperature / °C

Fig. 1 - c. DTA curves of (1) separated nacrite crystals and (2) nacrite powder.

ern part of the basin. The basement is composed principally of moderately folded non-metamor-phic Cambrian carbonates and epimetamorphic Precambrian schists with vertical to moderate south dip. Barite mineralizations and associated nacrite occur in the Cambrian dolomite near the basal Permian unconformity in connected pock­ets of centimetre to tens of metres size, or in brecciated dolomites. Barite also occurs as spaced veins or horizontal crack-filling in con­glomerates and shales of the overlying Permian, related to incremental filling of extrados normal faults (Lopez, 1992).

Nacrite and associated minerals were sampled near the village of Saumont in old mining-work. The mineralizations appear in the Cambrian dolomites as one-metre thick brecciated layer cemented with barite. Nacrite occurs in cen­timetre-size dolomite cavities and is intermingled with euhedral barite.

Methods

A petrographic study of the nacrite cavities was carried out combining observations by opti­cal microscopy, cathodoluminescence and back-scattered scanning electron microscopy. Micro-probe analyses of dolomite, nacrite and barite were performed using the Camebax SX50 elec­tron microprobe operated at 15 kV and 6 nA,

with a counting time of 10-20 s and a spot size of 1 µm. Nacrite crystals were hand picked, purified, and ground. Dolomite and calcite crys­tals were removed by reacting the mixture with HC1 5/N. After washing several times with dis­tilled water by centrifugation, the specimen was analyzed by powder X-ray diffraction (XRD) and infrared spectroscopy. XRD analyses were per­formed between 3 to 60°26 with a Philips dif-fractometer at 40 kV using CuKa radiation with a scan speed of 0.5°/min. Infrared spectra were recorded in the 4800-200 c m 1 range on a Nicolet 510 spectrometer using pressed pellets prepared by mixing 3 mg sample with 300 mg KBr. Differential thermal analyses were performed on nacrite with a Netzch STA 409 EP system. The curves were obtained with samples of 20 mg heated at 107mn in air.

Results

Nacrite characterization

The peak positions in the XRD patterns of a powder made with separated crystals are con­sistent with nacrite (Fig. la, Table 1). The strong intensity of the (00/) reflections is related to the platelet morphology of the particles that tend to be oriented along the (000 planes.

850 M. D. Buatier, J.-L. Potdevin, M. Lopez, S. Petit

Table 1. X-ray

d(hkl) ~

7 .214 (9 )

4 . 4 5 2 ( 5 )

4 . 3 6 4 ( 6 )

4 . 1 4 3 ( 5 )

3.940 (4)

3 .603(4 )

3 .480(5 )

3 .418(4 )

3 .067(4 )

2 .929

2 .527 (3 )

2 .431 (3 )

2 .397 (3 )

3 .319 (3 )

1 .930(2)

diffraction data for

intensity (%)

100

4.2

6.6

8

2

75

3.1

2.2

7

2

2

3.5

6.6

1.6

3.2

powdered nacrite.

(hkl) 1

0 0 2 ,

11T

202, 110

112, 200

1 11

113, 004

2 0 4

112

202

1 13

0 2 1 , 311-

022, 314

310, 006

1 16

225, 317, 025

In the whole frequency range, the IR spectrum obtained (Fig. lb) is similar to the nacrite refer­ence spectrum given by Farmer (1974). By this spectroscopic method, nacrite can be distin­guished easily from the other kaolin polymorphs by the location of its OH stretching bands. The doublet at 3629-3648 cm-1 is characteristic of nacrite. In the OH bending region, the small band situated near 950 cm-1 is also characteristic of this mineral (Prost et al., 1987).

The differential thermal analyses curves of separated nacrite crystals (1) and of nacrite powder (2) are shown on Fig. 1c. Curve (1) shows a main endothermic peack located at 680° and the characteristic sharp exothermic peak at 1000°C which is consistent with published DTA curve of nacrite (in Mackenzie, 1970). Curve (2) shows a dehydroxylation at lower temperature

Fig. 2. a. Macroscopic view of a cavity filled by nacrite. b. Photomicrograph of the cavities showing the sequence of phase precipitation. Euhedral dolomite is clearly distinguished from matrix dolomite, exsolution of iron oxides (in dark) differentiates the rim from the core of the dolomite rhombs. Acicular nacrite crystals grew on the dolomite edges and are completely cemented by a late calcite cement, c and d. Backscattered-electron images of the dolomite cavities. c. Nacrite-dolomite contact showing that nacrite grew after dolomite, d. Quartz-nacrite contact showing that nacrite nucleated as quartz was still growing, (na, nacrite; ba, barite; do, dolomite; ca, calcite ; ox, oxide; qtz, quartz).

Occurrence of nacrite in the Lodève Permian basin (France) 851

but the major endothermic peak is at about 650-700°C. This behaviour is probably an effect of the particle size (Mackenzie, 1970).

Petrographic study

The cavities in which nacrite was identified are completely or partially filled by dolomite, quartz, nacrite and calcite. Barite is also present in some cavities (Fig. 2a). A detailed petrographic analy­sis ascertained the chronology of the phase crys­tallization (Table 2).

Dolomite is the first formed mineral, it pre­sents a rhombohedral morphology easily distin­guished from the anhedral dolomite in the rock matrix. Dolomite rhombs are zoned with a core

Table 2. Chronology of the mineral crystallization in dolomite cavities (based on the petrographic observa­tions).

Mineralogical event Dolomite core Dolomite rim | Euhedral quartz Nacrite Barite Dolomite exsolution Calcite cementation

-time-

rich in inclusions and a 20 to 60 µm thick rim. This zonation is clearly visible in optical and cathodoluminescence microscopy. However, mi-croprobe analyses show that the core and the edges present a similar chemistry with about 2-3 wt % FeO and 0.10-0.20 wt. % MnO. When dolomite crystals are located in open cavities or in contact with the late calcite cement, exsolution of iron oxides are present in the rims (Fig. 2b and c). The exsolution is absent and dolomite rims are limpid when barite seals the cavities. These observations suggest that dolomite exsolu-tions formed after barite precipitation and before calcite cementation.

Euhedral quartz crystals, 30 to 100 µm in diameter, occur between nacrite layers and are partially included in dolomite rhombs. They are dark in cathodoluminescence microscopy. Their euhedral shape suggests that they grew in open spaces after dolomite (Fig. 2d).

Most of the nacrite crystals are thin but milli­metre-size platelets, as shown by the elongated sections perpendicular to the (001) layers. Their chemical compositions are very close to the ideal formula AU[Si4Oio](OH)8. Analyses (Table 3) show traces of various ions (e.g., Mg, Ca, Ba, Fe) that may be related to carbonate or barite con­tamination. In cathodoluminescence nacrite

Table 3. Representative microprobe analyses of the minerals filling the dolomite cavities.

Analysis S i 0 2

T i 0 2

AI2O3 FeO MnO MgO BaO ,SrO ;CaO |Na20 |K2Q Total Basis (O) iSi jTi |AI |Fe2+ Mn Mg Ba Sr Ca Na

k [Total

Nacrite Nacrite Nacrite Nacrite Dolomite 44 50 52 53 57

46.99 0.00

38.62 0.07 0.00 0.00 0.00 0.00 0.09 0.03 0.00 85.80

14 4.05 0.00 3.92 0.01 0.00 0.00 0.00 0.00 0.01 0.01 0.00 7.99

46.26 0.00 39.25 0.00 0.00 0.04 0.21 0.00 0.08 0.00 0.00 85.84

14 3.99 0.00 3.99 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 8.01

46.60 0.00 38.12 0.12 0.00 0.00 0.29 0.00 0.06 0.00 0.02 85.21

14 4.05 0.00 3.91 0.01 0.00 0.00 0.01 0.00 0.01 0.00 0.00 7.99

46.28 0.01 37.89 0.02 0.06 0.00 0.29 0.04 0.06 0.03 0.00

84.68 14

4.05 0.00 3.91 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 7.99

0.01 0.00 0.00 2.21 0.21

" 19.79 0.03 0.07

30.19 0.00 0.00

52.51 2

0.00 0.00 0.00 0.06 0.01 0.92 0.00 0.00 1.01 0.00 0.00 2.00

Barite 63

0.00 0.00 0.15 0.00 0.00 0.00 63.16 1.15 0.02 0.12 0.07 64.65

1 0.00 0.00 0.01 0.00 0.00 0.00 0.96 0.03 0.00 0.01 0.00 1.00

Barite 64

0.00 0.00 0.31 0.00 0.00 0.00 61.88 3.02 0.00 0.17 0.00

65.37 1

0.00 0.00 0.01 0.00 0.00 0.00 0.91 0.07 0.00 0.01 0.00 1.00

Calcite Calcitel 28 29

0.00 0.03 0.00 0.09 0.36 1.34 0.06 0.09 57.32 0.04 0.01 59.34

2 0.00 0.00 0.00 0.00 0.01 0.06 0.00 0.00 1.92 0.00 0.00 2.00

0.00 I 0.04 0.00 0.00 0.00 0.64 0.00 0.01 58.04 0.00 0.01 58.74

2 I 0.00 ! 0.00 0.00 0.00 0.00 I 0.03 ! 0.00 ' 0.00 1.97 0.00 0.00 2.00 !

852 M. D. Buatier, J.-L. Potdevin, M. Lopez, S. Petit

shows at first a subdued blue background as de­scribed by Hanson et al. (1981); after a few minutes, a bright deep blue colour is visible. When barite fills the cavities, nacrite can be ob­served as inclusions in barite but cavities par­tially filled with nacrite intermingled with elon­gated crystals of barite were also found suggesting that nacrite and barite grew contem­poraneously, even if nacrite started to crystallize before barite.

In some cavities, drusy calcite is the last ce­ment with an oscillatory zoning clearly visible under the cathodoluminescence microscope.

Conclusion

Nacrite is described for the first time in the Lodève basin, France. It consists of millimetre-size crystals and is the only kaolin phase present. Nacrite is generally considered as the high-temperature and high-pressure kaolin polytype (Shen et al., 1994). However, the conditions of nacrite crystallization in natural environments or in laboratory experiments are not well known and no relevant experimental data on this rare mineral exists. The remarquable occurrence reported here thefore offers the opportunity to obtain more data on the stability conditions of nacrite versus the other kaolin polymorphs. Petrographic investiga­tions suggest that nacrite formed together with quartz and barite. Lopez (1992) showed that barite is related to underbasinal fluid discharge and trapping in the karstified basement during the Late Permian extensional regime. Further geo-chemical and fluid-inclusion investigations are being carried out to constrain the P-T conditions of precipitation and the nature of the fluids from which nacrite crystallized.

Acknowledgements: We wish to thank P. Re-court for his technical assistance. We are grateful to A.-M. Fransolet and D. Beaufort for their criti­cal comments on the manuscript.

References

Bailey, S.W. (1980): Polytypism of 1:1 layer silicates. in "Hydrous Phyllosilicates" (S. W. Bailey ed.). Reviews in Mineralogy, 19, 9-26.

Biihman, D. (1988): An occurrence of authigenic nacrite. Clays Clay Minerals, 36, 137-140.

Farmer, V.C. (1974): The layer silicates, in "The Infrared spectra of Minerals" (V. C. Farmer ed.) Mineralogical Society, London, 331-365.

Hanson, R.F., Zamora, R., Keller, W.D. (1981): Nacrite, dickite, and kaolinite in one deposit in Nayarit, Mexico. Clays Clay Minerals, 29, 451-453.

Kisch, H.J. (1983): Mineralogy and petrology of burial diagenesis and incipient metamorphism in clastic rocks, in "Diagenesis in sediments and sedimentary rocks" (G. Larsen & G.V. Chilingar eds), Elsevier, 289-494.

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Prost, R., Damene, A., Huard, E., Driard, J. (1987): Infrared study of structural OH in kaolinite, dickite and nacrite at 300 to 5 K. Proceeding of the Inter­national Clay Conference, Denver, 1985, L.G. Schultz, H. Van Olphen and F.A. Mumpton eds, The Clay Mineral Society, Bloomington, Indiana, 17-23.

Prost, R., Damene, A., Huard, E., Driard, J., Leydecker, J.P. (1989): Infrared study of structural OH in kaolinite, dickite, nacrite, and poorly crystalline kaolinite at 5 to 600 K. Clays Clay Minerals, 37, 464-468.

Ruiz-Cruz, M.D. (1995): Nacrite from the Malaguide Paleozoic (Betic Cordilleras, Spain). Abstract Euroclay' 95, Leuven, 355-356.

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Received 20 June 1995 Modified version received 16 October 1995 Accepted 22 March 1996