FIRST SYNTHESIS OF THE HYDROXYL END-MEMBER OF HUMITE, MG7SI3O12(OH)(2)

The hydroxyl end-member of humite, Mg7Si3O12(OH)(2), was synthesized a t 30 kbar and 940 degrees C in the system MgO-SiO2-H2O (MSH). Optical properties are n(x) = 1.6348(10), n(y) = 1.6397(10), n(z) = 1.6612(10) , 2V(z) = 56.1(2)degrees, -Delta(n) = 0.026. Results of single-crystal X-ray diffraction gave a = 4.78(2), b = 10.22(3), c = 21.02(9) Angstr om, V = 1027(7) Angstrom(3), space group = Pbnm. The morphology of the synthetic humite crystal is comparable to that of natural F-rich humi te. Microprobe analyses confirmed a nearly ideal formula, Mg-6.97(3)Si -3.02(1)O-12(OH)(2.00(2)).