Silky fracture

Dawsonite [Named after the Canadian geologist John William Dawson (1820-1899), principal of McGill University, Montreal, Canada] (ICSD 100140 and PDF 42-1346) NaAl(C03)(OH), M= 144.00 15.97 wt.% Na 8.34 wt.% C 18.74 wt.% Al 1.40 wt.%H 55.56 wt.%0 Orthorhombic a = 673 pm bs 1036 pm c 558 pm P.G. 2/m2/m2/m S.G. Imam (Z= 4) Barentsite type Biaxial (-) a= 1.462 P= 1.542 Y= 1-596 5=0.130 27=76.75" Dispersion weak <3 2420 (2434) Habit thin encrustations, bladed, needle-hke or radial crystals. Color colorless to white. Diaphaneity transparent. Luster vitreous to silky. Fracture uneven. Cleavage [110] Perfect. Streak colorless. Other fluorescent under short-wavelength UV with dull white. Occurrence low-temperature hydrothermal mineral. 

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