Danburite-Topaz Group – Danburite and Topaz Composition
Danburite
Composition. Calcium-boron silicate, CaB2(SiO4)2.
Crystallization. Orthorhombic. Prismatic crystals, closely related to those of topaz in habit
Structure. Commonly in crystals.
Physical Properties. H. = 7-7.25. G. = 2.97-3.02. Vitreous luster. Colorless or pale yellow. Transparent to translucent.
Tests. Fusible (3.5-4), giving a green flame. Insoluble in acids.
Occurrence. Found in crystals at Danbury, Conn.; Russell, New York; eastern Switzerland; Madagascar; Japan.
Topaz
Composition. (Al. F)2SiO4 with isomorphous (Al.OH)2 SIO4.
Crystallization. Orthorhombic. In prismatic crystals terminated by pyramids, domes and basal plane. Often highly modified. Prism faces often vertically striated.
Structure. In crystalline masses; also granular, coarse or fine.
Physical properties. Perfect basal cleavage. H = 8 (unusually high). G = 3.52-3.57. Vitreous luster. Coreless, yellow, yellow –brown, pink, blues, greenish. Transparent to translucent.
Test. Infusible. Insoluble. Recognized chiefly by its crystals, its basal cleavage, its crystals, its basal cleavage, its hardness (8) and high specific gravity.
Occurrence. A mineral formed through the agency of fluorine bearing vapors given off during the last stages of the solidification of igneous rock. Found in cavities in rhyolite lavas and granite; a characteristic mineral in pegmatite veins, especially in those carrying tin. Associated with other pneumatolytic minerals, as tourmaline, cassiterite, apatite fluorite, etc. ; also with quartz, mica, feldspar. Found at times as rolled pebbles in stream sands. Notable localities for its occurrence are the nerchinsk district in Siberia in large wind-yellow crystals; from Mursinsk, Ural Mts., in pale blue crystals: from various tin localities in Saxony; from Minas Geraes, Brazil; Omi and Minot Provinces, Japan; San Luis Potosi, Mexico; Pikes peak, near Florissant and Nathop, Colorado; Thomas Range, Utah; Streeter, Texas; San Diego Co., California; Stoneham. Maine.
Name. Derived from the name of an island in the Red Sea but originally probably applied to some other species.
Use. As a gem stone. A number of other inferior stones are also frequently called topaz. The color of the stones varies, being colorless, wine-yellow, golden brown, pale blue and pink. The pink color is usually artificial, being produced by gently hating the dark yellow stones; it is permanent, however.
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