American Mineralogist, Volume 80, pages 638-640, 1995

LETTERS

First synthesis of the hydroxyl end-member of , Mg7Si3012(OH)2

BERND WUNDER, OLAF MEDENBACH, PETER DANIELS, WERNER SCHREYER Research Group on High-Pressure , lnstitut fUr Mineralogie, Ruhr-Universitat Bochum, D-44780 Bochum,

ABSTRACT The hydroxyl end-member ofhumite, Mg7Si3012(OH)2, was synthesized at 30 kbar and 940°C in the system MgO-SiOrH20 (MSH). Optical properties are nx = 1.6348(10), ny = 1.6397(10), nz = 1.6612(10), 2Vz = 56.1(2)°, -~n = 0.026. Results of single- X-ray diffraction gave a = 4.78(2), b = 10.22(3), c = 21.02(9) A, V = 1027(7) A3, space group = Pbnm. The morphology of the synthetic humite crystal is comparable to that of natural F-rich humite. Microprobe analyses confirmed a nearly ideal formula, M&'.97(3)Si3.o2(IJ- °dOH)2.00(Z)'

INTRODUCTION SYNTHESIS The general chemical formula of humite group min- The successful high-pressure experiment was per- erals is M(OH,F)2 x nM2SiO., where M = Mg, Fe2+, formed in a piston-cylinder press constructed after Boyd Mn2+, Ti, etc., and n = 1 for norbergite, n = 2 for chon- and England (1960). The pressure cell consisted of rock drodite, n = 3 for humite, and n = 4 for clinohumite. salt, fired pyrophyllite, and a carbon cylinder as a resis- of this series occur in metamorphosed and tance furnace. The 2u uncertainty in P is about 1%; tem- metasomatic and dolomites. Additionally, Mn- perature was monitored using a chromel-alumel ther- rich humite has been found in skarns (Moore, 1978), and mocouple with an accuracy of about 10 K. Ti-rich and clinohumite have been found in Conditions of synthesis were 30 kbar, 940°C, and 24 kimberlitic (e.g., McGetchin et aI., 1970; Aoki h using a mixture of phases to determine the equilibrium et aI., 1976). With the exception ofthe Mn- and Ti-rich reaction chondrodite = clinohumite + brucite (see Fig. phases, which are nearly F-free, the humite group phases 1). The three phases were previously synthesized as pure contain both OH and F. For example, the humite-group phases in the MSH-system and then mixed in the MglSi- phases from limestones have the following (OH)/(OH + proportions of chondrodite. About 10 mg of the solid F) values: 0.19-0.30 norbergite, 0.22-0.37 chondrodite, material plus 20 wt% of water were filled into an Au-tube 0.38-0.51 humite, and 0.47-0.54 clinohumite (Ranka- of 8 mm length. After the experiment, independent of the ma, 1938; Sahama, 1953; Jones, 1968). The replacement products of the equilibrium experiment, a few relatively of OH by F seems to decrease in the series from norber- large of an unknown phase were observed in the gite to clinohumite. upper part of the Au tube. For further characterization, In the system MgO-Si02-HzO (MSH, Fig. 1),until now one single crystal of 100 x 100 x 40 ~m (Fig. 2) was only synthetic OH end-members of chondrodite (phase selected and mounted on a glass fiber. D, Yamamoto and Akimoto, 1974) and of clinohumite (Yamamoto and Akimoto, 1977) have been known. Both were synthesized in the P- T-range 29-77 kbar, -700- MORPHOLOGY AND OPTICS 1000 °C, and are possible containers of H20 in the Earth's The crystal forms short orthorhombic bipyramids (Fig. (e.g., Thompson, 1992). 2). This morphology is typical of natural F-rich humite Here we report the synthesis and some properties of (Trager, 1982). the OH phase with n = 3, Mg7Si30dOH)2' As in the case The optical data ofthe single crystal selected were mea- ofthe synthetic hydroxyl end-member of the topaz series sured using a microrefractometer spindle stage as de- (Wunder et aI., 1993), we informally call this phase scribed by Medenbach (1985), which also allows the pre- "humite-OH," and the known monoclinic hydroxyl end- cise determination of 2 V by direct conoscopic members are labeled "chondrodite-OH" and "clinohum- investigation. The data (Table 1) are, in principle, similar ite-OH" in Figure 1. to those known for all natural humite group phases (e.g.. 0003-004 X/9 5/0506-0638$02.00 638 WUNDER ET AL.: SYNTHESIS OF OH END-MEMBER OF HUMITE 639

Stishovite Coesile Periclose Quartz MgO Mg2SiO,- MgSiOJ Orthoenstatite ~-pha5e Clinoenstotite

Fig. 1. Selected phases ofthe system MgO-Si02-H20 (MSH) synthesized at pressures below 100 kbar. The molar ratios of MgO:Si02:H20 are given in parentheses. Fig. 2. Scanning electron micrograph showing the morphol- ogy of synthetic humite. Indexing of the faces according to Tro- ger (1982).

Deer et aI., 1992). However, significant differences were noted when the optical data of the new phase were com- CHEMICAL COMPOSITION pared with those given by Yamamoto and Akimoto (1974) for chondrodite and by Yamamoto and Akimoto (1977) The chemical composition of the crystal shown in Fig- for clinohumite (Table 1). Additionally, the new phase ure 2 was determined by a CAMECA SX 50 electron shows straight extinction, thus distinguishing it from the microprobe at Ruhr University Bochum. Synthetic py- monoclinic OH end-member homologues. Because the rope (Mg3A12Si30'2) served as a standard. mean Ii of the new phase is close to the The average of nine individual analyses gave (in weight value of a theoretical OH humite end-member as calcu- percent) MgO 58.48(0.21), Si02 37.77(0.16), and H20 lated by Sahama (1953), and because this value lies near- (determined by difference) 3.75(0.28). On the basis of 14 ly midway between those of the OH end-members of atoms and with the assumption that H20 is present in chondrodite and clinohumite (Table 1), it seems likely the form of OH groups, the structural formula the unknown phase is the OH end-member of humite. M&'.97(3)Si302(I)O,zCOH)2.oo(2)was calculated. Within the precision of measurements this corresponds nearly per- fectly to the formula of the OH end-member of humite ' X-RAY DATA Mg7Si30,2(OH)2' To assess the compatibility of the optical data, the X-ray Single-crystal photographs of the crystal shown in Fig- density (p, ure 2 were taken on a STOE precession camera using Zr- = 3.106 glcm3), and the chemical formula, the Gladstone-Dale relationship was tested. The Kp/ Kc-l pa- filtered MoKa radiation. The crystal was first oriented rameter was calculated as 0.001, which indicates an ideal with regard to its morphology using the spindle stage. consistency (the "superior" category of Mandarino, 1979). Photographs taken from the Okl, lkl, hkO, and hkllayers did not show any reflections that violate the orthorhom- bic space group Pbnm, which was chosen because of the structural relation of humite to forsterite (Jones, 1969). TABLE 1. Optical data of the OH end-members of humite, chcn- The values of d correspond closely to tho~e given on drodite, and clinohumite JCPDS card no. 12-755 for natural humite. Two cone- Chondrodite Clinohumite axis photographs gave 4.75 and 20.92 A, which corre- (Yamamoto and Humite (Yamamoto and spond to the known values of ao and Coof natural humite. Akimoto, 1974) (this work) Akimoto, 1977) Subsequently, the crystal was investigated on a four- nx 1.630(2) 1.6348(10) 1.638(2) circle single-crystal diffractometer (SIEMENS P3). An or- ny 1.642(2) 1.6397(10) 1.641(2) thorhombic unit cell was found with a = 4.78(2), b = nz 1.658(1) 1.6612(10) 1.669(1) 10.22(3), C = 21.02(9) A, and V = 1027(7) A3. The rela- n 1.643 1.645 1.649 tively high standard deviation is the result of the poor -!J.n 0.028 0.026 0.031 (0) quality of the crystal which yielded split reflections. 2Vz ? 56.1(2) ? 640 WUNDER ET AL.: SYNTHESIS OF OH END-MEMBER OF HUMITE

ACKNOWLEDGMENTS Medenbach, O. (1985) A new microrefractometer spindle-stage and its applications. Fortschritte cler Mineralogie, 63, I I 1-133. Thanks are due to Deutsche Forschungsgemeinschaft for supporting our Moore, P.R (1978) Manganhumite, a new species. Mineralogical Maga- work under the auspices of a Research Group. The authors are grateful zine, 42, 133-136. to H.J. Bernhardt, who carried out the microprobe analyses, and to W. Rankama, K. (1938) On the mineralogy ofsomc members of the humite Gebert for performing the single-crystal X-ray data collection. group found in Finland. Bulletin de la Commision Geologique de Fin- lande, 123,81-93. Sahama, T.G. (1953) Mineralogy of the humite group. Annales Acade- miae Scientiarum Fennicae, A-III, Geologica-Geographica, 33, I-50. REFERENCES CITED Thompson, A.R (1992) Water in the Earth's upper mantle. Nature, 358, Aoki, K., Fujino, K., and Akaogi, M. (1976) Titanochondrodite and ti- 295- 302. tanoclinohumite derived from the upper mantle in the Buell Park kim- Tri\ger, W.E. (1982) Optische Bestimmung der gesteinsbildenden Miner- berlite, Arizona, U.S.A.. Contributions to Mineralogy and Petrology, ale; Teill Bestimmungstabellen: 5. Neubearbeitete Auflage von H.U. 56, 243-253. Bambauer, F. Taborszky und RD. Trochim. E., p. 53, Schweizerbart, Boyd, ER., and England, J.L. (1960) Apparatus for phase-equilibrium Stuttgart, Germany. measurements at pressures up to 50 kilobars and temperatures up to Wunder, R, Rubie, D.C., Ross, C.R., II, Medenbach, 0., Seifert, E, and 1750 "C. Journal of Geophysical Research, 65, 741-748. Schreyer, W. (1993) Synthesis, stability, and properties of AJ,Si04(OH),: Deer, W.A., Howie, R.A., and Zussman, J. (1992) An introduction to A fully hydrated analogue of topaz. American Mineralogist, 78, 285- rock-forming minerals (2nd edition), p. 16-21, Wiley, . 297. Jones, N.W. (1968) Crystal chemistry of the humite minerals. Ph.D. dis- Yamamoto, K., and Akimoto, S. (1974) High pressure and high temper- sertation, Virginia Polytechnic Institute, Blacksburg, Virginia. ature investigations in the system MgO-SiO,-H,O. Journal of Solid - (1969) Crystallographic nomenclature and twinning in the humite State Chemistry, 9, 187-194. minerals. American Mineralogist, 54, 309-313. - (1977) The system MgO-SiO,-H,O at high pressures and temper- Mandarino, J.A. (1979) The Gladstone-Dale relationship: III. Some gen- atures: Stability field of hydroxyl-chondrodite, hydroxyl-clinohumite eral applications. Canadian Mineralogist, 17, 71-76. and 10 A-phase. American Journal of Science, 277, 288-312. McGetchin, T.R., Silver, L.T., and Chodos, A.A. (1970) Titanoclinohum- ite: A possible mineralogical site for water in the upper mantle. Journal MANUSCRIPT RECEIVED FEBRUARY 21, 1995 of Geophysical Research, 75, 255-259. MANUSCRIPT ACCEPTED APRIL 3, 1995