A Key for the Determination of Rock-Forming Minerals in Thin Sections

This historic book may have numerous typos and missing text. Purchasers can usually download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1908 edition. Excerpt: ...crystals in drying gelatine from HCl. Sodalite has less perfect cleavage, and seldom has borders. (Si0,)3Al2AlS0,NaCaNa2, H=5.5, G =2.27-2.50. Isometric. Cleavage: Cross-sections parallel to (100) or (110) show two systems at right angles. Habit: Usually occurs in crystals (110), crystal fragments, or rounded grains. Colorless, blue, gray, brownish, red, yellow, and green. Color often irregularly distributed or zonal. Index: ??? = 1.496-1.504. Sometimes shows optical anomalies either about inclusions or as a dark cross. Haüynite usually contains inclusions at the center or around periphery. It is often altered. Gelatinizes easily with HCl, and gypsum crystals result from the drying gelatine. Noselite gives few gypsum crystals. Sodalite has less perfect cleavage and seldom has borders. KAlSi2Oe, Isometric. Cleavage: Not noticeable, irregular cracks. Habit: Six-or eight-sided to rounded grains. Very often con-tains inclusions in regular zones. Colorless to gray. 1ü = 1.508, £ = 1.509. Birefringence: Very low, =001. Optical anomalies; shows poly-synthetic twinning. HCl attacks leucite slightly in thin sections, but the powdered mineral is dissolved with a separation of silica. All minerals but analcite differ from leucite in having higher indices and higher double refraction. The twinning and crystal form of leucite also distinguish it. Analcite and sodalite group may be confused with isotropic leucite. Either may be distinguished by the greater re-sistance of leucite to HCl and by its micro-chemical reactions, giving isometric potassium fluosilicate crys-tals, while analcite gives hexagonal sodium fluosilicate crystals. The mineral is isotropic. Colored. Index of refraction is greater than that of Canada...