SILICATES, TITAN ATES, ETC. 471 



prism angle free on the surface. Often globular or botryoidal 

 with very small crystal faces. The nodules when broken show a 

 radiated structure. The color is nearly always light green or 

 yellmvish. the color fading on exposure. 



Prehnite is a secondary mineral formed from solution in the cavi- 

 ties and veins of the basic igneous rocks, where it is associated with 

 the /eolites, datolite, calcite, and quartz. It is a common mineral 

 in the cracks of the traps of Massachusetts, Connecticut, and New 

 Jersey; also in the Lake Superior copper regions. 



It forms pseudomorphs after analcite, natrolite, and the plagio- 

 clases, and decomposes, forming chlorite. J 



When fused it breaks down and on cooling yields wollastonite. 



CHONDRODITE 



Chondrodite. [Mg^. OH)] 2 Mg-,(SiO 4 ) 2 ; Monoclinic ; Type, 

 Digonal Equatorial ; a : b : c = 1.0863 : 1 : 3.1447 ; p = 90 = 100 A 

 001; 100 A 110 = 47 22'; 001 A 101 = 70 57'; 001,011 = 72 22'; 

 Common forms, c(001), b (010), r 2 (125), r 3 (123) ; Twinning 

 plane, 105 ; Cleavage, basal ; Brittle ; Fracture, conchoidal ; 

 H. = 6-6.5 ; G. = 3.1-3.2 ; Color, shades of yellow, brown, and red ; 

 Streak, white or pale yellow; Luster, vitreous; Transparent to 

 translucent; a = 1.607; (3 = 1.619; -y = 1.639; y - a = .032; 

 Optically ( + ) axial plane perpendicular to 010 ; Bx A 6 = 25 30' ; 

 2V = 79 40'. 



B.B. Infusible, but whitens. Gelatinizes with HC1. Yields 

 a fluorine reaction in the closed tube. Often reacts for iron with 

 the fluxes. 



General description. Crystals are equidimensional ; at times 

 flattened parallel to 010, with striations on the orthodome parallel 

 to the orthoaxis ; crystals are complicated and very rich in forms ; 

 also granular and massive. 



Chondrodite is a member of the humite group; the other 

 members are humite, [Mg(F . OH)]2Mg6(SiO 4 ) 2 ; clinohumite 

 [Mg(F.OH)], M g7 (Si0 4 ) 4 . 



In composition they differ from each other by MgSiO 4 . Humite 

 is orthorhombic, while the others are monoclinic. They agree very 

 closely in their occurrences, associations, and physical properties. 

 They occur in metamorphic limestones containing considerable 

 magnesia. All three are found in the ejected blocks of limestone 

 on Monte Somma, Vesuvius, and at the Tilly Foster mine, Brew- 



