538 MINERALOGY 



until one molecule of the water of crystallization is driven off, form 

 a cement known as " plaster of Paris," named from the Mont- 

 martre deposits near Paris where this cement was first made. This 

 calcined product when moistened absorbs water, forming a network 

 of fibrous crystals, and solidifies as a whole. When all the water 

 of crystallization is driven off, it forms anhydrite; the product 

 loses its power to absorb water or absorbs it very slowly, and its 

 setting or crystallizing power is lost. 



Satin spar and alabaster are polished as ornamental stones and 

 for inexpensive jewelry. 



EPSOMITE 



Epsomite. Epsom salts, MgSO 4 . 7 H 2 O ; MgO = 16.3, S0 3 

 = 32.5, H 2 O = 51.2; Orthorhombic ; Type, Digonal Holoaxial; 

 a:b:c = .9902: 1: .5709; 100 A 110 = 44 43'; 001 A 101 = 

 29 58'; OOlA011=29 43'; 11^111=52 38'; Common 

 forms, a (100), b (010), c (001), z(lll), n (101) ; Cleavage, 

 b perfect, Oil less so ; Brittle ; Fracture, conchoidal ; H. = 2-2.5 ; 

 G. = 1.68-1.75; Color and streak, white; Transparent to trans- 

 lucent; c^= 1.432; p = 1.455; y = 1.461; y - a = .029; 

 Optically (-); Axial plane = 001; Bx a = b; 2E = 78 20'. 



B.B. Boils and yields an infusible white alkaline residue 

 which becomes flesh-pink when treated with cobalt solution (Mg) . 

 Soluble in water ; the solution yields a white precipitate with barium 

 chloride (BaSO 4 ). It has a very bitter taste. 



General description. Crystals are prismatic in habit, combi- 

 nations of the sphenoids and unit prism ; they are interesting as 

 examples of the holoaxial type ; also in fine silky acicular crys- 

 tals. The mineral is named from the locality of Epsom Springs, 

 England, where it was first known. 



Magnesium sulphate is easily soluble in water ; all springs and 

 percolating ground waters contain both magnesium and calcium 

 sulphates in considerable quantity. They cause the permanent 

 hardness of natural waters. Where large amounts of water are 

 evaporating, as on the face of cliffs or on the surface of the soil 

 in very dry seasons, epsomite is left as white crusts. The white 

 crusts formed on fresh brick walls are in part epsomite. Such 

 residual crusts occur on the floors of the caves of Tennessee and 

 Kentucky and on many of the alkaline plains of California, Utah, 

 and Nevada. 



