KOCKS : THEIR CONSTITUENTS AND KINDS. 



6T 



ZiRCOx, Beryl, Titanite (Sphene) are other anhydrous silicates. Zircon is a silicate 

 of zirconia ; beryl (aquamarine when pale green and transparent), a silicate of alumina 

 and beryllia ; titanite or sphene, a silicate of calcium and titanium. 



4. Silicates of Magnesium and Iron or Calcium, with Little or no Alumina and 



no Water. 



Chrysolite. — Occurs in green glassy grains or crystals in basalt and related rocks, 

 and also paler green in rock masses. Also called olivine, and in France pen'Jo^. H = 6-7. 

 Infusible. Composition of a common variety: Silica 41-4, magnesia 50-9, iron protoxide 

 7-7 = 100. A related mineral, /a?/a?(<e, contains iron withoiit magnesium, and is fusible. 

 The crystals often occur changed, partly or wholly, to serpentine. 



Chondrodite. — A yellow to brown magnesium silicate, containing fluorine, occurring 

 in crystalline limestones. A kind found in ejected masses of limestone at Vesuvius 

 is called luimite. 



Horxblexde (often called amphibole). — Occurs in prisms of 124° 30' (which is 

 also the cleavage angle). Colors various, from black to green and white. The most 

 common kind in rocks is an iron-bearing variety, in black cleavable grains or in oblong 

 black prisms. Figs. 47, 48, and 49 represent common crystals, and 50 tufts of crystals 

 as they often appear in some rocks. The kind in slender green crystals or fibers is 

 called aetinolite — a common form of its crystals is shown in Fig. 49 ; the white (a kind 

 common in crystalline limestones, and containing much lime), tremolite. The mineral 



47. 



48. 



50. 



49. 



51. 



52. 



^^ ^ 



Is common in fibrous masses ; and, when the fibers are as fine as flax, it is called asbestos. 



A common black hornblende consists of silica 48-8, alumina 7*5, magnesia 13-6, lime 



10-2, iron protoxide 18-8, manganese protoxide, 1-1 = 100. 



Pyroxene (including augite) . — Like hornblende in chemical composition and in 



most of its characters ; but the crystals, as in the annexed figures, 51, 52, instead of 



being prisms of 124° 30', are prisms of 87° 5' or nearly (angle / 

 on 7), and are often eight-sided from the truncation of the four 

 edges, as in Fig. 52. Pyroxene and hornblende are hence para- 

 morphs, being different in crystallization, but alike in composition. 

 Black and dark green pyroxene in short crystals is called augite ; 

 it is an iron-bearing kind, and is common in igneous rocks. 



Enstatite. — Near pyroxene in cleavage angle, but prisms 

 orthorhombic. Infusible or nearly so. It is in part a silicate 



of magnesium. "When a silicate of magnesium and iron, it is often called bronzite ; and, 



if containing much iron, hypersthene. 



5. Silicates of Magnesium, etc., with Water. 



Talc. — Very soft, H = l. Crystallizes in flexible folia like mica, but inelastic; also 

 massive-granular (soapstone or steatite); white and very fine-grained {French chalk). 

 Feels very greasy. Consists of silica 62-8, magnesia 33-5, water 3-7 = 100. Infusible. 



