CHAPTER XII 



SULPHATES, CHROMATES, TUNGSTATES, AND 

 MOLYBDATES 



THENARDITE 



Thenardite. Anhydrous sodium sulphate, NasSC^ ; Na^O = 

 56.3, SO 3 = 43.7; Orthorhombic ; Type, Didigonal Equatorial; 

 ft:b:c = .5976:1:1.2524; 100 A 110 = 30 52'; 001,101 = 64 

 30'; 001,011 = 5124'; Common forms, c (001), a (110), m (110), 

 o(lll), t(106), r(101), e(011); Twinning plane, 101, also Oil, 

 crossed twins common; Cleavage, basal distinct; Fracture, 

 uneven; Brittle; H. = 2-3; G. = 2.68-2.69 ; Color, white, gray 

 or brownish ; Streak, white ; Transparent to translucent ; Opti- 

 cally (+) ; Axial plane, 001 ; Bx a = b ; 2 V = 83 32'. 



B.B. Fuses easily and colors the flame yellow (Na). Easily 

 soluble in water, the solution yielding a heavy white precipitate 

 with barium chloride (BaSO 4 ). After ignition reacts alkaline 

 with turmeric paper. 



General description. Crystalline habit is pyramidal or short 

 prismatic, usually combinations of the unit pyramid, prism, base, 

 and dome. Owing to its great solubility in water its occurrence is 

 restricted to desert lake regions, as those of Siberia, Chili, Arizona, 

 Nevada, and California. 



Sodium sulphate is the last of the sulphates to crystallize from 

 solution upon evaporation. The anhydrous sulphate isseparatrd 

 above a temperature of 32 C. ; below this temperature the 

 dcca hydrate mirabilite, Na 2 SO 4 . 10 H 2 O, is in equilibrium and 

 separates from the saturated solution. In a dry atmosphere this 

 effloresces, losing its water and passing into the anhydrous salt 

 thenardite. 



Thenardite is associated with halite, trona, gypsum, hanksite, 

 I'orax, and other minerals characteristic of the desert lake de- 

 posits. 



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