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X-site vacant Al-tourmaline

Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität, D-44780 Bochum, Germany

View larger version (11K): [in this window] [in a new window]   Fig. 1. Ternary projection of the system Al2O3-B2O3-SiO2-H2O (ABSH) from water showing relevant known crystalline phases as well as theoretical compositions and starting materials for tourmaline synthesis. T1 represents the theoretical formula Al3Al6 [Si6O18](BO3)3O2(OH)2 as in Table 1, T2 = Al3Al6[Si4Al2O18](BO3)3 (OH)4. SM1 and SM2 are the starting materials for the compositions T1 and T2, however each with a 100 mol % excess of B2O3. Synth. Al-T is the projection point of the synthetic Al-tourmaline as analyzed here (Table 4). Dumortierite solid solution after Werding & Schreyer (1990). The mineral boralsilite (Grew et al., 1998) was not encountered. Hydrous solids are underlined.

View larger version (122K): [in this window] [in a new window]   Fig. 2. a, b. SEM images of portions of the run product for tourmaline synthesis at 30 kbar, 600 °C (see Table 2, C). In both pictures relatively stout prisms of the tourmaline phase can readily be distinguished from extremely thin needles of dumortierite. a) Note typical striations along a prism face of tourmaline on the right. Massive portion on the far right is the embedding medium. b) Largest crystal shown is tourmaline exhibiting trigonal morphology.

View larger version (143K): [in this window] [in a new window]   Fig. 3. Thin-section photograph of a rectangular fragment of a polycrystalline single-phase aggregate of tourmaline (Tu) embedded in a matrix of a fine-grained phase mixture. Product of synthesis run at 20 kbar, 600 °C (see Table 2, F).

View larger version (8K): [in this window] [in a new window]   Fig. 4. Plot of tourmaline cell dimensions c versus a for common end-members (see Table 1) as taken from Deer et al. (1986), compared to those of the new Al-tourmaline synthesized here as well as of the synthetic excess-boron olenite of Schreyer et al. (2000).

View larger version (15K): [in this window] [in a new window]   Fig. 5. Plot of cell volumes V of Alrich tourmalines taken from the literature as well as of the new Altourmaline synthesized here against their contents of tetrahedral boron replacing silicon as derived from chemical analyses. For the natural olenite type-specimen of Sokolov et al. (1986) as well as for the synthetic NaAl-tourmaline of Rosenberg et al. (1986) boron was assumed to be present in stoichiometric amounts only, without any [4]B.

View larger version (12K): [in this window] [in a new window]   Fig. 6. Projection of the ABSH-system as in Fig. 1 showing possible phase relations of the new Al-tourmaline (Al-T) and other phases as indicated for the condition 20 kbar, 600 °C (see Table 2) derived from the behaviour of the two different starting materials SM1 and SM2. For further discussion see text. Fluid compositions and dashed tie lines are hypothetical and do — for reasons of clarity — not depict the coexistences of fluids with dumortierite solid solutions (Du).