Abstract

Tourmaline (dravite) was synthesized from oxide-hydroxide mixtures in the presence of NaCl-bearing solutions in the system Na 2 O - MgO - Al 2 O 3 - SiO 2 - B 2 O 3 - H 2 O - HCl at 600 degrees C, 200 MPa. Different initial arrangements of the starting materials in the samples indicate that a chemical gradient in a system lowers the nucleation rate and produces large crystals of up to 130 mu m length or overgrowths on seed crystals of up to 400 mu m. The use of a chloride solution and seed crystals alone does not necessarily reduce the nucleation rate. Besides dravite, which forms overgrowths on the seed crystals and single independent crystals, small amounts of quartz, albite and chlorite are also formed. The dravites are chemically inhomogeneous and can be differentiated into three zones: inner core, outer core and rim. The beginning of growth is characterized by a wide scatter in composition and a Si deficiency (5.4-5.7 Si pfu). Some parts of the rims have Si-values exceeding 6.0 apfu (calculated for 24.5 O or 15 cations; excluding Na). Al decreases from 7.0 to 6.5 from core to rim, Mg shows an increase from 2.2 to 2.5 Mg pfu and Na has constant values in the inner core and rim of 0.83-0.86 pfu, whereas the outer core is characterized by a decrease in Na (0.73-0.78 Na pfu). Average lattice parameters are a(Aa) = 15.901(5), c(Aa) = 7.200(2) and V (Aa 3 ) = 1576.9(6). Changes in dravite composition results from fractional crystallization. The fluid composition changes as tourmaline is progressively isolated from the system by new growth. The outermost rims therefore reflect the tourmaline composition that is in equilibrium with the final fluid at 600 degrees C, 200 MPa. The preserved growth zoning indicates that the diffusion rate in tourmaline is very low, recording histories of changing fluid composition.

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