Clinotobermorite, CaSSi6(O,OH)lSoSH20, a new mineral from Fuka, Okayama Prefecture, Japan C. HENMI Department of Earth Sciences, Faculty of Science, Okayama University, Okayama 700 Japan AND 1. KUSACHI Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700 Japan Abstract Clinotobermorite, CasSi6(0,OHhs.5H20, has been found as a vein-forming mineral in gehlenite- spurrite skarns at Fuka, Okayama Prefecture. It is associated with tobermorite, piombierite, apophyllite, and calcite. The cIinotobermorite is colourless or white and occurs as tabular or acicular crystals. It is monoclinic with the space group Ce or C2le. The unit cell dimensions are a 11.331, b 7.353, c 22.67 A, ~ 96.59°. Microtwinning and stacking disorder on (001) are observed. On heating the cIinotobermorite at 300°C, the 002 spacing is reduced from 11.3 to 9.3 A. Its refractive indices are IX1.575, ~1.580, y 1.585, and the density 2.58 g/cm3 (meas.), 2.69 g/cm3 (calc). The Moh's hardness is 4.5. Calculation of the analytical data on the basis of six tetrahedral cations shows that this mineral has a simplified chemical formula Cas.3Si6(0,OH,F)1s.5H20. The chemical composition and the unit cell are closely related to those of tobermorite. It is most likely that clinotobermorite is a low-temperature polymorph of tobermorite. KEYWORDS:clinotobermorite, new mineral, tobermorite, Okayama Prefecture, Japan. Introduction morite. The mineral species and its name have TOBERMORITE (11.3 A tobermorite), plombier- been approved by the Commission on New ite (14 A tobermorite), tacharanite (12.6 A Minerals and Mineral Names, International tobermorite), riversideite (9.3 A tobermorite) Mineralogical Association, in May 1990. The type and oyelite (10 A tobermorite) have been known material of cIinotobermorite is deposited at the as tobermorite group minerals (McConnell, 1954; National Science Museum, Tokyo, Japan. Heller and Taylor, 1956; and Kusachi et al., The present paper deals with the mineralogical 1984). properties and mode of occurrence of the clinoto- Several calcium silicate hydrate minerals occur bermorite found at Fuka. This mineral has been in gehlentie-spurrite skarns at Fuka, Okayama. previously reported as 'monoclinic tobermorite' Three members of the tobermorite group min- by the authors in an article in Japanese (Henmi erals have been reported for this locality; these and Kusachi, 1989). are tobermorite and plombierite (Mitsuda et al., 1972) and oyelite (Kusachi et al., 1984). During a Occurrence mineralogical study of tobermorite, we found that some samples which were considered to consist of Skarns were formed on both sides of the quartz tobermorite crystals show an X-ray powder dif- monzonite dykes that penetrate limestones at fraction pattern distinctly different from that of Fuka. The skarns consist mainly of gehlenite and usual tobermorite. This mineral is not orthorhom- spurrite. Some parts of the skarns were retrogres- bic but monoclinic, and has a chemical compo- sively altered and were cut by numerous veins sition almost identical to that oftobermorite. Our consisting of such minerals as tobermorite study revealed that it is a monoclinic polymorph (Mitsuda et al., 1972), oyelite, bultfonteinite, of tobermorite. scawtite, xonotlite (Kusachi et al., 1984), afwillite We have named this new mineral clinotober- and jennite (Kusachi et al., 1989). Mineralogical Magazine, September /992, Vol. 56, pp. 353-358 @ Copyright the Mineralogical Society 354 C. HENMIANDI. KUSACHI centre of the vein in the following order: calcite, tobermorite, c1inotobermorite, plombierite and apophyllite. Fig. 1 shows the occurrence of c1inotobermorite in one of these veins. Physical and optical properties Clinotobermorite occurs as tabular crystals parallel to the c-face with up to 5 mm width andas aggregates of acicular crystals parallel to the b- axis up to 2 mm in length. The mineral is colourless or white with a vitreous luster in handspecimen and colourless in thin section. FIG. 1. Photomicrograph of cIinotobermorite. Crossed Cleavage is perfect on (001) and poor on (100). nicols. Abbreviations: T, tobermorite; CT, cIinotober- The mineral is optically biaxial with refractive morite; P, plombierite; and A, apophyllite. indices ex= 1.575, ~ = 1.580 and y = 1.585. We were unable to obtain its optical axial angles (2V) Clinotobermorite occurs in veins filling fissures because of well-developed microtwinning. The of the contaminated rock which consists mainly of 2V angle calculated from the refractive indices is feldspar and pyroxene and may be considered to 89.8°. Vickers microhardness is 251-174 kglmm2 be a kind of endoskarn. The thickness of the vein (25 g load) and the Mohs hardness is 4.5. The varies from one to two centimetres. In these density measured with heavy liquids is 2.58 veins, minerals develop from the wall to the glcm3. The calculated density is 2.69 glcm3. o L!) ..;:t (V) L!) 00 (j) 4000 2000 1500 1000 650 cm-1 FIG. 2. Infrared spectra for (a) cIinotobermorite and (b) tobermorite from Fuka. CLINOTOBERMORITE, A NEW MINERAL 355 Table]. Chemical analyses of tobermorite group mlncraJs from Fuka. e I ino- tobcrmor i tc tobcrmor i lc plombicritc oyclitc :3 Si02 46. 55 48. 98 4S.12 46. 43.34 35 .:J Ti0;2 .01 .03 .0 .0 A 12°:3 O. :H:i .O:J 3.31 .2 0.39 .3 132°:3 o. 2:3 .03 O. 4. FCZO:J O. 01 O. 03 0.23 O. .0 ,I\1nO O. 06 O. 03 O. O. y1g0 O. 11 O. 04 .04 . o. 00 .0 ° CaQ 39 .04 37. 90 :15 .24 37. :J 34. 64 41.2 :-Ja20 O. 02 O. 01 O. 24 O. 05 O. K20 O. 10 O. 02 O. za O. O. 32 o. 112° 13 .75 11 .46 1S .52 13. 19. fj9 16.7 O. 18 .00 -0=1' 0. 08 0. 00 .. Total 100.34 99. S6 99 93 99.6 99.51 99.5** 1. Present s t u dy . ioe1 udes H20! -) .08 %. 2. Mitsuda and TaYlor ! 1978). locI udes H20! -) .7% and CO2 0.4%. ( 3. Kusach i et. al. 1980 and 1984) Clinotobermorite loses molecular water on ous bands below 1200 cm -I are characteristic of heating at 300°C in air, resulting in shrinkage of both tobermorite (Mitsuda and Taylor, 1978) and thedoo2 reflection from 11.3 to 9.3 A. clinotobermorite. The IR spectra of clinotobermorite are measured using the conventional KBr method for the region 4000 to 650 cm -1. Fig. 2 shows the Chemical composition spectra of clinotobermorite and tobermorite for comparison. They are very similar. The absorp- The chemical composition of clinotobermorite tionbands at 3450 cm-1 are attributed to the OH was obtained using an electron microprobe ana- stretching vibration and the weak bands at lyser except for F, B and H20. The concen- 1630cm-1 to the OH bending vibration. Numer- trations of F, B and H20 were determined by Table 2. Unit cell dimensions of tobermorite group minerals from Fuka. clinotobermori te tobermori te plombieri te oyelite* alA) 11. 331 (9) 11.233(3) 11.250(6) 11.25(2) ) 2) b(A) 7. 353(7) .372(3) 7. 344 (4 .25 I ) ciA) 22 . 67( 2) 22. 56(1 27.99(1) 20.46(3) (7) f3(') 96 .59 *Kusachi et al. (1980 and 1984). 356 C. HENMI AND I. KUSACHI Table 3. X-ray powder diffraction data for cl1notobcrmol te flnd tobermor 1 te from Fuka. The calculated values are based on 8=11.331. b"7.353, c=22.67 A and After Henmi and f' "96.59°. Kusachi (1989) . c 1 inotobcrmor i te tobermor ite 1 d(abs. d(calc. d(obs. 2 11 .25 100 11.262 11 .3 100 0 2 5.631 5 .63 9 ( g 4 5. 60 20 5.628 0 -3) 5.550 .45 38 1 0 5. 27 20 5.283 1 -p 5.264 0 2 .782 10 4.818 a 3.770 0 .745 36 3.754 3.754 a 1 J) 3.749 3.642 8 1 1 5 :~. 5] 4 30 2 0 5 2 3. 304 51 3.30:J 3.309 16 0 2 3 0 -31 3.302 2 -1 3. 068 45 3.079 3.080 82 2 2 2 1 3. 034 60 3.024 3 -4 3. 012 37 3.012 2 ) 2. 950 25 2.970 .972 70 3 3 2.944 2 -3 .910 23 2.913 0 8 2.816 .806 88b ( 0 4 .811 41 2.814 4 4 -g) 2.807 2 ~) .794 60 2.787 I 2.781 .723 4 ) 2.641 27 2 .642 =~ 2 .641 .513 20 1 1 8 2. 495 11 2.502 0 2 2.421 .424 9 2 0 ~) 2. 414 12 2.410 4 0 4 2.409 1 3 2.395 1 3 -y) 2. 394 20 2.388 4 a -6 2.387 3 1 6 2. 380 15 2.378 2.282 18b ( 1 3 1 10 0 0 10 ) .248 27 2. 252 2.250 18 :1 r; 4 2 -1 2. 244 4 2 -l) .190 13 2. 197 1 3 2. 185 2. 141 18 ( ~5 2. 117 2. 121 4 -§! 2. 115 4 2.079 .079 13 ( ~I 18) .075 20b 2.074 .071 13 5 2 2.073 2 I ~2.005 10h 2. 002 24 1. 917 16b 1 .875 13 . 84:~ 18 1. 837 14 .836 22b 1 748 14b 1 .712 10 ,. 670 17 .669 21 1 .647 12b 1 .604 10 .618 1 .593 17b means of wet chemical analyses for the specimen X-ray studies purified through heavy liquids and hand picking separation.