Clay Minerals (1986) 21, 225-230 225 NOTE TOSUDITE CRYSTALLIZATION IN THE KAOLINIZED GRANITIC CUPOLA OF MONTEBRAS, CREUSE, FRANCE Albite, muscovite granite and greisens of the Montebras cupola, Creuse, France, are strongly altered to a white, friable kaolinite-rich material. A dioctahedral phyllosilicate which is a regularly interstratified chlorite-smectite very similar to tosudite also occurs in some of the alteration products. This mineral, although frequently described in hydrothermally altered rocks of Japan (Sudo et al., 1954; Shimoda, 1969; Nishiyama et al., 1975; Ichikawa & Shimoda, 1976) is only rarely observed in Europe (Brown et al., 1974). Tosudite was identified by X-ray diffraction and infrared absorption spectrometry, located in thin sections, and chemical analysis performed by microprobe. The significance of the presence of this mineral in altered parts of the granite cupola will be discussed. Experimental X-ray diffraction was performed on powders and oriented samples on a Philips PW 1730 diffractometer (40 ma, 40 kv, Co-Kcc radiation). Several treatments on oriented samples were used: (i) saturation with ethylene glycol to test the expandability of the interstratified mineral; (ii) heating to 110 ~ 220 ~ 300 ~ 400 ~ and 500~ (1 h). Infrared spectra were recorded using KBr disks (0.5 mg specimen in 300 mg KBr) on a Beckman IR 4240 spectrophotometer in the 4000-250 cm -1 frequency region. Electron microprobe analysis were obtained with a Cameca MS 46 microprobe equipped with an energy-dispersive system (V = 15 kv, I = 1.5 na, counting time = 120 s). These conditions prevented loss of alkalis and the breakdown of the clay minerals (Velde, 1984). Lithium was analysed by flame emission with a Perkin-Elmer 2380 atomic absorption spectrophotometer. Geological setting The altered area of the Montebras granitic cupola is observable in a quarry (Fig. 1), where most of the granite is strongly kaolinized. High-temperature events can be seen in the greisens where lithium-rich micas (lepidolite and protolithionite) crystallize (Dudoignon, 1983) and in levels near the greisens. These crystallizations are affected by a lowtemperature alteration characterized by the assemblage illite-k-beidellite-kaolinite. Tosudite was identified only in some samples in the southern part of the quarry. Petrography of tosudite-rich specimens In thin sections, tosudite appears similar to kaolinite with low birefringence (grey first-order colours). It crystallizes in large flakes or spherolites (~20/tm long) and replaces indifferently all the minerals of the granite (albite, muscovite and quartz) (Fig. 2). 1986 The Mineralogical Society
Note 226 N t BIOTITE GRANITE ALBITE GRANITE + GREISENS WORKING FACE FIG, 1. Sketch of q u a r r y at M o n t e b r a s showing area where tosudite is found. FtG. 2. M o n t e b r a s tosudite in thin section. T = tosudite; Q = quartz; M = muscovite; A = albite.
Note 227 = 994 //~ 400~ 1,490 1282 7~ 7b 4.79 7.15. A 9 8,47 4.46 886 527 110~C 5.15 G 3'44 3:57 4.42 5.15 7.78. 1000 334 495 ;o 1"o 02o Co K~ FIG. 3. X-ray powder diffraction patterns of Montebras specimen. N = natural; G = ethylene glycol saturated. Mineralogical data X-ray powder diffraction data. Fig. 3 shows the XRD traces of an oriented sample. In the natural state, a series of XRD reflections at 30.15 (001), 14.77 (002), 9.08 (003), 7.45 (004) and 4.95 A (006) is observed, with traces of kaolinite (7.16 and 3.57/~) and illite (10 A). Saturation with ethylene glycol gives reflections at 30.87 (001), 15.26 (002), 9.98 (003), 7-78 (004), 5.15 (006), 4.42 (007) and 3.44 A (009). After heating at 110~ the first-order reflection moves back to 30.15 ~,, then to 28.47/~ at 220~ and disappears above 300~ At 500~ the structure collapses to 9.94 A. This behaviour at high temperatures is not usual for tosudites as, after heating at 500~ a regular series of basal reflections normally remains (Shimoda, 1969; Shimoda et al., 1975; Ichikawa & Shimoda, 1976; Sudo & Shimoda, 1978). The (060) value measured on randomly oriented material is 1.490/~.
228 Note TABLE 1. IR absorption data of Montebras tosudite and some related minerals. Hokuno = Ichikawa et al. (1976). Takatama = Shimoda (1975). Kamikita = Sudo et al. (1954). Gibbsite = Farmer (1974). S = strong; M = medium; W = weak; Inf = inflection; Sh = shoulder; B = broad. Montebras Hokuno Takatama Kamikita Gibbsite 3670 Inf 3670 Inf 3670 Inf 3660 Inf 3630 S 3630 S 3640 S 3630 S 3617 3540 S 3535 S 3545 S 3540 S 3520 3428 3400SB 3375M 3390 SB 3380 SB 3380 1625M 1640M 1404W 1055 Sh 1040 SB 1030 S 1035 SB 1030 SB 1020 1005 S 940 Sh 940W 950 Inf 940 Inf 958 914 825W 830W 820W 827W 802 750W 750 S 750 S 743 708 S 705 S 630 Sh 629M 630 Sh 532M 533 S 523 Sh 510 Sh 470 S 465 S 420 M 425 M 340 S 704 s 478 S 478 S These XRD data suggest that the specimen is a regular interstratification of dioctahedral chlorite and dioctahedral smectite (Shimoda, 1969; Shimoda et al, 1975; Ichikawa & Shlmoda, 1976; Sudo & Shimoda, 1978). Infrared absorption data. IR absorption data of tosudite from the Montebras and Japanese occurrences are given in Table 1 and appear very similar. The absorption bands at 3630 and 3540 cm -1 have been attributed to OH stretching in the silicate structure and in the 'gibbsite' sheet, respectively. There are similarities between the Montebras tosudite and gibbsite bands (Table 1) but they are not exactly identical. These shifts can be explained by the presence of an impure gibbsite layer in tosudite. Thus the band at 708 cm -~ can be related to the Mg(AI)-OH vibration in the gibbsite sheet (Sudo & Shimoda, 1978; Ichikawa & Shlmoda, 1976). The Si-O vibration bands appear at 1040 and 510 cm -~. Chemical analyses. Electron microprobe analyses are reported in Table 2. The lithium content was estimated by flame spectrometry performed on 50 mg of purified sample and added to the oxide percentages obtained using the electron microprobe. The cations per 50 oxygen atoms were then calculated as reported in the literature (Shimoda, 1969; Shimoda et al., 1975; Ichikawa et al., 1976) (Table 3). The sum of the oxides is less than 100 /6 because the H20 content of the mineral cannot be measured with the electron microprobe (the literature indicates ~ 15% H20 in tosudites).
Note TABLE 2. Electron microprobe analyses of Montebras tosudite (wt%). 229 1 2 3 4 5 Average SiO 2 45.48 43.45 45.92 40.55 45.99 44.28 AI203 36.41 39.13 37.63 32.43 34.62 36.04 Fe20 a 0.32 0.37 0.52 0.47 0.10 0.36 MnO 0.07 -- -- 0.17 0.17 0.28 MgO -- 0.08 0.26 0.25 0.27 0.17 TiO 2 -- 0.03 0.31 -- -- 0.07 CaO 0.02 0.21 0.38 0.38 0.37 0.27 K20 1.35 1.07 2.49 2.23 2.55 1.94 Na20 0" 13 0.24 0.29 0"30 0"50 0.29 TABLE 3. Average chemical analysis and cations per 50 oxygens for Montebras tosudite. Oxides (average value) Cations per 50 O SiO 2 43.43 Si 14.13 AlzO a 35.35 AI 13.55 FezO 3 0-35 Fe 3+ 0-09 MnO 0.27 Mn 0.07 MgO 0.17 Mg 0.08 TiO 2 0.07 Ti 0.02 Li20 1.60 Li 1.05 CaO 0.27 Ca 0.09 K20 1.90 K 0.79 Na20 0.28 Na 0.18 Total 83.70 The structural formula based on the negative charge of 50 oxygens (= O4o(OH)20) is: Interlayer cations: K0.79 Na0.1s Cao.o9 + nh20 'Gibbsite' sheet: Li1.o5 Mgo.0s Mn0.07 Tlo.o2 9 Feo.o9 3+ A 13.6s (OH)12.oo Silicate layer: Als.oo [Si14.1a All.s7 ] 04o.oo (OH)s.oo Lithium is presumed to be present in the 'gibbsite' sheet (Ichikawa et al., 1976; Nishiyama et al, 1975). The octahedral sheet of the chlorite appears to be dioctahedral with dominant AI and Fe3+. This mineral contains a significant amount of lithium. Occurrences of lithium-bearing tosudite have already been described (Nishiyama et al., 1975; Ichikawa et al., 1976; Brown et al., 1974). The interlayer cations are composed mainly of potassium with lesser amounts of Na and Ca. This would explain the slight expansion of the structure after ethylene glycol treatment. Discussion and conclusions In spite of the collapse of the structure after heating at 500~ XRD, IR and chemical investigations suggest that the Montebras specimen is an ordered interstratification of dioctahedral chlorite and dioctahedral smectite. This tosudite contains a significant amount
230 Note of lithium assumed to be in the 'gibbsite' sheet. Potassium is the main interlayer cation in the smectite layers. Several experiments have been made to define the stability field of tosudites (Matsuda & Henmi, 1973; Ichikawa & Shimoda, 1976). From all these experiments it appears that the tosudite domain is between that for mica for higher temperatures, and that for interstratified mica-montmorillonite for low temperatures. At Montebras, tosudite occurs in some samples of the albite-muscovite-granite. In other specimens, randomly interstratified illite-smectite was found. Tosudite could be a hydrothermal alteration product of muscovites and could transform to interstratified illite-smectite at lower (<100~ temperatures (Velde, 1985). The Montebras tosudite could therefore be an intermediate phase between muscovite and interstratified iuitesmectite. A fuller account of its mineralogical properties will be given in a subsequent paper. Laboratoire de P&rologie des Alt&ations Hydrothermales, UA 721 du CNRS, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France. M. CREACH A. MEUNIER D. BEAUFORT Received 16 December 1985. REFERENCES BROWN G., BOURGUIGNON P. & THOREZ J. (1974) A lithium bearing aluminium regular mixed montmoriltonite-chlorite from Huy, Belgium. Clay Miner. 10, 135-144. DUDOIGNON P. (1963) A ltdrations hydrothermale et superg~ne des granites. Etude des gisements de Montebras (Creuse), de Sourches (Deux-S~vres) et des ardnes granitiques (Massif de Parthenay). Thtse 3e Cycle, 941, Univ. Poitiers, France. FARMER V.C. (1974) The Infrared Spectra of Minerals, p. 149. Mineralogical Society, London. ICHIKAWA A. t~ SHIMODA S. (1976) Tosudite from the Hokuno mine, Hokuno, Gifu prefecture, Japan. Clays Clay Miner. 24, 142-148. MATSUDA T. & HENMI K. (1973) Hydrothermal behaviour of an interstratified mineral from the mine of Ebara, Hyogo Prefecture, Japan. (An example of changes from randomly interstratified clay mineral to regular one). J. Clay Science Soc., Japan, 13, 87-94. NISHIYAMA T., SHIMODA S., SHIMOSAKA K. & KANAOKA S. (1975) Lithium-bearing tosudite. Clays Clay Miner. 23, 337-342. SH1MODA S. (1969) New data for tosudite. Clays ClayMiner. 17, 179-184. SHIMODA S. (1975) X-ray and I.R. studies of sudoite and tosudite. Contrib. to Clay Min. in Honor of Professor Toshio Sudo, 92-96. Mineralogical Society of Japan. SUDO T., TAKAHASHI H. & MATSUI H. (1954) Long spacing of 30 A from a fire clay. Nature 173, 161. SuDo T. & KODAMA H. (1957) An aluminium mixed-layer mineral of montmorillonite-chlorite. Zeit. Krist., 190, 379-387. SUDO T. SHIMODA S. (t978) Clays and Clay Minerals of Japan. Elsevier, Amsterdam, 325 pp. VELDE B. (1984) Electron microprobe analysis of clay minerals. Clay Miner. 19, 243-247. VELDE B. (1985) Clay Minerals: A Physico-Chemical Explanation of their Occurrence. Elsevier, Amsterdam, 427 pp.