J. Japan, Assoc. Min. Petr. Econ. Geol. 76, 21-25, 1981. SYNTHESIS OF DUMORTIERITE IN THE SYSTEM A1203-Si02-B203-H2O AKIRA ONO National Institute for Researches in Inorganic Materials 1-1 Namiki, Sakura-mura, Ibaraki 305 Japan Synthesis of dumortierite indicates that stoichiometric dumortierite is unstable . Compositional variation of synthetic dumortierite is almost absent regardless of wide range of synthetic temperature and difference in coexisting minerals. Substitutions of Al-0•¬ A1-OH and A1O6•¬BO6 in dumortierite are revealed by chemical analyses and infrared spectroscopic study. INTRODUCTION CRYSTAL STRUCTURE Crystallochemical formula of dumorti- Dumoriterite is a nesosubsilicate con erite is considered as A17O3 (B03)(SiO4)3. sisting of isolated SiO4 tetrahedra, with BO3 However, chemical analyses of natural triangles as additional anions. The SiO4 dumortierite show small amounts of H2O tetrahedra are held together by Al ions in (Claringbull and Hey, 1958). Sum of weight octahedral site (Fig. 1). Viewing down percentages of A1203 and Si02 of any the [001] direction, the crystal can be dumortierite from the Ryoke pelitic gneisses considered as consisting of channels with are considerably small as compared with A103 octahedra and BO3 triangles (Wyckoff, ideal composition, A1703(BO3)(Si04)3(Ono, 1968). 1977). Synthesis of dumortierite was car Table 1. Chemical compositions of the starting ried out to elucidate these problems. materials Fig. I Structure of dumorierite. METHOD OF SYNTHESIS Starting materials were prepared by mixing mechanically SiO2, Al2O3, and H3BO3. (Manuscript received June 10, 1980) 22 Akira Ono SiO2 and A12O3 were prepared from H2SiO3 cal procedures are as follows: 1) determ and Al(OH)3 at 1500•Ž. Table 1 shows the ination of apparent concentrations of Si02 chemical•@ compositions of the starting and A1203. 2) ZAF correction procedure materials. Chemical composition "a" in was carried out, Dumortierite was treated as Table 1 corresponds to the stoichiometric the ternary system Al203-Si02-B203. Ap formula A1703(BO3)(SiO4)3. "d" in Table parent boron content was assumed so as to 1 is B2O3 excess as compared with synthetic attain 100 per centage in sum of oxide com dumortierite. Mixture of "d" and Si02 ponents after the ZAF correction. 3) Chemi are also used as a starting material in a few cal compositions obtained by this procedure experimental runs. are different from the ideal composition of The synthesis of dumortierite was car A1703(B03)(Si04)3. Boron atom per form ried out by Boyd-England type single ula unit on the basis of 18 oxygen always stage piston sylinder apparatus of 12.75mm exceeds 1.5, Occupany of Al atom in the in diameter designed by O. Fukunaga. NaCl tetrahedral Si positions is significant. Thus, with 5% MoS2, pyrex glass and Alsimag charge neutrality is only achieved by ex ceramics were used as the pressure medium. ceeding the seven positions of Al octahedral Pressure values are uncorrected data of sites available in the crystal structure. 4) the nominal pressure. Chromelalumel We propose, therefore, the following chemi thermocouples were used in all runs for the cal formula for dumortierite crystal. temperature measurements. The duration Al6(BR'Al1-x)03(B03)[(Si3-y,Aly)012-y](0H)y of experimental runs was 1 to 95 hours. Chemical compositions of dumortierite were Starting materials were sealed in platinum recalculated so as to support the above tube with no water added. The products chemical formula. In this recalculation, of the runs were identified by X-ray diffrac the A1203and Si02 contents were not chang tion patterns, optical properties and chemical ed, and the B203 contents were subtracted compositions analyzed by an electron probe and the H20 contents were added. Because microanalyzer. the amount of H20 content is very small, this procedure may be correct. New chemi ANALYTICAL METHOD cal formula is based on supposing sub Synthetic dumortierite were analyzed by stitution of oxygen (0(1) and 0(2) after an electron probe microanalyzer. Analyti Wyckoff (1968)) by OH. Fig. 2 Infrared spectrum of dumortierite. Synthesis of dumortierite 23 INFRARED SPECTROSCOPY Synthetic dumortierite were purified by dissolving out impurities with HF-HC1 acid. KBr pellets containing small amount of acid treated dumortierite were prepared by the ordinary method, and a vibrational spectrum of OH near 3600 cm-1 was measured by an IR spectroscopy. Fig. 2 shows a result of the measure ments. A weak absorption peak is present at 3500cm-1. The peak width is about 25cm-1. This is an evidence of OH in dumortierite crystal. Same peak is present in two natural dumortierite examined. The peak is present after heat treatment at 860•‹ C in air. - Fig. 3 P-T range of synthesis of dumortierite Dumortierite-out line is shown in the figure, EXPERIMENTAL RESULTS Results of experimental runs are shown ditions. Vapor phase is present in all ex in Fig. 3. Representative runs are listed perimental runs, therefore vapor blew out in Table 2 together with experimental con from platinum tube when Pt-capsules were Table 2 Results of representative experimental runs Chemistries of the starting materials "a" and "d" are given in Table 1. All experiments used by starting material "a" are listed in this Table. d: dumortierite, cor: corundum, qtz: quartz, sill: sillimanite, b: brown material, v: vapor 24 AkiraOno cut out after experimental runs. Dumorti Table 3 Chemical compositions of synthetic dumortierite. erite could not be produced below 10 kb, 870•Ž. Corundum, quartz and brown material are present instead of dumortierite. Brown material is partly soluble in water, and tiny crystals of H3B03 were formed after evaporation of water. Brown materi al may be rich in B203 component. Brown color is due to translucent nature of fine aggregate of crystals. Absence of dumor tierite in low pressure experimental runs does not mean unstability of dumortierite in low pressure. Corundum, quartz and brown material may be a metastable mineral •@assemblage. Coexistence of corundum and quartz is surely metastable with respect to sillimanite. Dumortierite synthesized below 870•Ž and above 10kb coexist with small amount of corundum and brown material. Dumort ierite coexisting with quartz were syn thesized from the starting material added by considerable amount of Si02. Above 900•Ž transparent glasses are present together with corundum and dumortierite. Amount of glass increase with increasing tem perature. Dumortierite is absent in high temperature side of line shown in Fig. 3 where corundum, sillimanite and glass occur. At high pressure, dumorierite was formed after one hour. The rapid reac Fig. 4 Phase relations of the examined system at 850•Ž tion rate suggests that large dumortierite a, d: starting materials (see Table 1) , A: syn crystals of longer run durations synthesized thetic dumortierite, b: brown material (fine- at high pressure may be in equilibrium grained undetermined material). with other phases. Dumortierite could not be produced blage of corundum--quartz occurs at low from the starting material of "a" at 840•Ž, pressure and/or low temperature region, and 11.6 kb. At higher temperature and pres synthesis of dumortierite depends on bulk sure, dumortierite were synthesized to composition of starting materials. gether with considerable amount of co Analytical results of dumortierite are rundum and quartz. given in Table 3. Chemistry of dumorierite It is notewotrhy that metastable assem is similar with each other regardless of wide Synthesis of dumortierite 25 range of synthetic temperature and difference ortierite is similar to that of synthetic one. of coexisting minerals. Fig. 4 shows rela 3) Appreciable amount of Mg replaces Al. tionships between chemical compositions of These data suggest substitutions of Al-O•¬ starting materials and products in sub Al-OH and A1O6•¬BO6. solidus temperature region. Corundum, X-ray structural analysis based on A17 dumortierite and brown material are produc 03(BO3)(Si04)3 composition indicates that ed from the starting material "d" Brown temperature factors of Al(l), O(1) and 0(2) material may be SiO2-rich and A1203-poor are very large (Okamura and Ono, 1978). in chemical composition as shown in Fig. This supports presence of the substitutions 4. Starting material "a" is on a line con described above. necting sillimanite and dumortierite. REFERENCES NATURAL DUMORTIERITE Claringbull, G. F. and Hey, M. H. (1958), New data Dumortierite occur commonly in the for dumortierite. Min. Mag., 31, 901-907. Okamura, F. P. and Ono, A. (1978), Crystal chemical Ryoke pelitic gneisses. Chemical compo peculiarity of dumortierite from Takato. sitions of these dumortierite may be dif Annual Meeting Abs. Mineral. Soc. Japan. ferent from those of synthetic ones, because 145 (in Japanese). Ono, A. (1977), Petrological study of the Ryoke condition of formation is greatly different metamorphic rocks in the Takato-Shiojiri from each other. Examination of natural area, central Japan. J. Japan Assoc. Min. dumortierite revealed the following facts. Petrol. Econ. Geol. 72, 453-468 (in Japanese). Wyckoff, R. W. G. (1968), Crystal Structures, second 1) Infrared absorption peak of OH vibra edition vol. 4, Interscience Publishers. tion is present. 2) B203 content of dum A1203-SiO2-B203-H20系 に お け る デ ュ モ テ ィ エ ラ イト の 合 成 小 野 晃 ヂ ュモ テ ィエ ラ ィFの 高 圧 合 成 と分 析 結 果 に よ る と,従 来 考 え られ て きた 理 想 化学 式 の もの は 合 成 で きな か っ た 。 広 い 温 度 範 囲 に 渡 っ て,ま た 共 存 す る鉱 物(石 英 や コ ラ ン ダ ム)の 種類 に 関 係 な く,合 成 結 晶 の 固 溶 範 囲 は 小 き い 。 分 析 値 はB203が9%を 超 え,こ れ はAlの サ イ トにBが 置 換 し て い る こ と を 示 す 。 ま たSiO4〓AlO4 の置換 もみ とめ られ る。 赤 外 吸 収 ス ペ ク トルに よ る と,3500cm-1にOHの 振 動 に よ る吸 収 ピー クが 認 め られ た 。 こ れ はSiと6配 位 のAlを 結 び つ け て い る酸 素 がOHに 置 換 きれ て い る こ と を 示 す 。 天 然 の デ ュ モ テ ィェ ラ ィ トにお い て も同 様 の 事 柄 が 認 め られ た 。.