Fluoborite in magnesian skarns from Baita Bihor (Bihor Massif, Apuseni Mountains, Romania)

摘要

Fluoborite from Baita Bihor (Bihor Massif, Apuseni Mountains, Romania), one of fewer than 20 areas in the world where this mineral has been positively identified, has been submitted to a minute study. The mineral occurs in boron-bearing magnesian skarns and is particularly located in the outer zones of metasomatic columns. It is closely associated with calcite, fluorite, chondrodite, norbergite, in a paragenesis that also includes dolomite, magnesite, kotoite, suanite and ludwigite. Secondary parageneses, including szaibelyite, magnetite, brucite, chrysotile, lizardite, lepidocrocite and goethite superpose on the skarn areas. Fluoborite occurs as #mu#m-sized acicular crystals in sheaf-like aggregates generally surrounded by calcite. The mineral is fluorescent under short-wave UV radiation (#lambda#=256 nm), the response colour being yellowish cream. Indices of refraction, determined using spindle-stage techniques, are #omega#=1.548(2) and c = 1.522(3). The measured density (D_m = 2.83(2) g/cm~3) agrees perfectly with the calculated one (D_x = 2.832 g/cm~3). Unit-cell dimensions, calculated from least-squares refinement of X-ray powder data, are a = 8.8935(18)A and c = 3.1103(12) A. The formula derived from electron-micro-probe analysis, calculated on the basis of 1 atom of boron and 3 atoms of oxygen per formula unit (with OH deduced for charge compensation) is (Mg_(2.967)Mn_(0.001) Fe_(0.001)C_(0.004))(BO_3)(OH_(1.317)F_(1.629)). Based on carbonate geothermometry and on experimental syntheses reported in literature temperatures between 380 deg C and 450 deg C at pressures between 0.6 and 3 kbar can be estimated for the fluoborite crystallisation.