抄録
Afwillite and jennite occur as fissure-filling veins up to 3 mm wide in the spurrite zone in skarns at Fuka. Jennite occurs also as alteration products of spurrite. The occurrences suggest that afwillite was formed from fluids in fractured spurrite and that jennite was formed by alteration of afwillite and by hydration decomposition of spurrite.
Afwillite crystals are tabular up to 0.3 mm in width or radial aggregates of fibers up to 4 mm in length. The mineral is white in color with a vitreous luster and colorless in thin section. The mineral is monoclinic with unit cell parameters of a=16.276(4), b=5.6336(4), c=13.236(3)Å and β=134.89(1)°. EPMA and wet chemical analyses gave an empirical formula Ca2.98(Si2.00B0.04) Σ2.04O4.06 {(OH)5.90F0.04} Σ5.94 based on O+OH+F=10. Optically, the mineral is biaxial positive with refractive indices α=1.617, β=1.621 and γ=1.632, 2V=56°, and elongation positive or negative. Vickers microhardness is 351∼383 kg/mm2 (25g load) and density is 2.63g/cm3.
Jennite in the vein occurs as parallel or radial aggregates of acicular crystals with length up to 0.2 mm. The mineral is white in color with a vitreous luster and colorless in thin section. The mineral is triclinic with unit cell parameters of a=10.577(4), b=7.271(5), c=10.828(4)Å, α=99.60(4), β=97.62(3) and γ=110.18(4)°. EPMA and wet chemical analyses gave an empirical formula Ca9.00H2.00(Si5.95Al0.02) Σ5.97O17.93(OH)8.00·6.07H2O based on O=32. Optically, the mineral is biaxial negative with refractive indices α=1.548, β=1.562 and γ=1.570, and elongation positive. Vickers microhardness is 145∼155 kg/mm2 (25g load) and density is 2.33 g/cm3.
