OXIDES 



361 



regular it is onyx. At times the impurities are dendritic and ap- 

 pt.ii like pieces of moss inclosed within the specimen; these are 

 When dark in color and associated with limestones 

 in nodules it forms flint. Lydian or touchstone is also a very 

 dark, almct hhick, variety, used by the goldsmiths to test the 

 purity of their gold alloys, by means of the color of the streak 

 made by the metal in drawing it across the stone. 



,la>per is an opaque variety of impure SiO 2 of a red, brown ocher. 

 gray, green, or black color. 



In addition there are pseudomorphs of SiO 2 after shells, wood 

 (silicified wood), or bones ; and various minerals, as carbonates and 

 sulphates, may be replaced by SiO 2 from solution. 



TRIDYMITE 



Tridymite. Silicon dioxide, SiO 2 ; Hexagonal, Hexagonal 

 above 130 C., lelo\v probably Orthorhombic ; t = 1.653; 0001 A 

 1011 =62 21'; Common forms, c (0001), m (1010)_, a (1120), 

 p (1011) ; Twinning plane 1016 and 3034 ; Cleavage, 1010 distinct ; 

 Brittle, fracture conchoidal ; H. = 7 ; G. = 2.28-2.33 ; Colorless 

 to white; Luster, vitreous ; Streak, white ; = 1.477. 



B.B. Like quartz, but soluble in boiling alkaline carbonates. 

 Fuses at 1625 C. 



General Description. Crystals are small hexagonal tablets, 

 combinations of the base, prism, and pyramid, or aggregations of 

 these small scaly crystals. At ordinary temperatures tridymite is 

 pseudo-hexagonal ; above 130 it becomes truly uniaxial, and below 

 it shows low double refraction (.0018), is optically (+), and is prob- 

 ably orthorhombic in symmetry. It is not a common mineral, but 

 found in acid volcanic rocks and lavas, as in the lavas of Vesuvius 

 and Krakatau, Obsidian Cliff, Yellowstone Park, and in some 

 meteorites. It was discovered at Pachuca, Mexico, where it oc- 

 curs in aggregates of twins ; in the cavities of an andesine rock. It 

 has since been discovered in many localities connected especially 

 with, and in cavities of, the more recent acid volcanic rocks. 



Tridymite and quartz are polymers of SiO 2 . The transition 

 temperature between the two is near 800 C., quartz being the more 

 stable phase below, and tridymite the more stable phase above that 

 temperature. Artificially, tridymite has been formed both in 

 solution and in dry fusion. When a silicate is dissolved in the salt 



