CALCAREOUS MINERALS 23 



vescence. When pure, as in Iceland spar, the mineral is colour- 

 less, very transparent and lustrous, and displays the phenomenon 

 of double refraction strongly ; but more commonly it is cloudy or 

 white, or stained red or yellow by iron. The decomposition of 

 silicated minerals containing lime gives rise to calcite, and as this 

 is soluble in water holding COo, nearly all natural waters have 

 more or less of it in solution. It is widely diffused among the 

 rocks, and in a state of varying purity forms great masses of 

 limestone. 



2. Aragonite (CaC0 3 ) is a somewhat harder and heavier form 

 of calcite, with a specific gravity of 2.93 and a hardness of 3.5-4, 

 and crystallizes in compound prismatic forms which belong to the 

 orthorhombic system. It has not the marked cleavage of calcite 

 and is very unstable ; when heated it is converted into calcite and 

 falls into tiny rhombohedrons of that mineral. 



3. Dolomite is a carbonate of lime and magnesia, (Ca, Mg)C0 3 , 

 but with variable proportions of the two bases ; it resembles cal- 

 cite in appearance, and crystallizes in rhombohedrons which often 

 have curved faces. Sp. gr. = 2.8-2.9 ; 11 = 3.5-4. Dolomite may 

 be readily distinguished from calcite by the fact that cold acids 

 affect it but little. 



4. Selenite, hydrated sulphate of lime, CaS0 4 , 2H 2 0. Sp. gr. 

 = 2.31-2.33; H= 1.5-2. It crystallizes in right rhomboidal 

 prisms, belonging to the monoclinic system, and cleaves into thin 

 non-elastic leaves. When pure, selenite is transparent and colour- 

 less, but is often stained by iron. This mineral occurs largely in 

 granular masses, called gypsum, from which plaster of Paris is 

 made by calcining the gypsum and so driving off the water of 

 crystallization. Gypsum is slightly soluble and is present in most 

 natural waters. Alabaster is a gypsum of especially fine grain, 

 mottled in pale colours, or white. 



5. Anhydrite, CaS0 4 , is sulphate of lime without water ; it is 

 harder and heavier than gypsum (Sp. gr. = 2.9-2.98 ; H = 3-3.5), 

 and crystallizes in a different system, the orthorhombic. The 

 crystals have three sets of cleavage planes, which intersect each 

 other at right angles. 



