The hydroxyl end-member of humite, Mg7Si3O12(OH)2, was synthesized at 30 kbar and 940 °C in the system MgO-SiO2-H2O (MSH). Optical properties are nx = 1.6348(10), ny = 1.6397(10), nz = 1.6612(10), 2VZ = 56.1(2)°, −Δn = 0.026. Results of single-crystal X-ray diffraction gave a = 4.78(2), b = 10.22(3), c = 21.02(9) Å, V = 1027(7) Å3, 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, Mg6.97(3)Si3.02(1)-O12(OH)2.00(2).

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