THE CONSTITUENTS OF METEORITES 525 



attacked by hot or cold hydrochloric acid, but is completely 

 dissolved by nitric acid without the separation of free sulphur. 

 This solubility distinguishes it from chromite. Its system of 

 crystallization is not known though it exhibits rectangular and 

 triangular partings which indicate one of the systems of high 

 symmetry. Meunier obtained the mineral artificially by treat- 

 ing an alloy of iron and chromium at a red heat with hydrogen 

 sulphide. 



Tridymite. — This mineral has been positively identified in 

 only one meteorite (Steinbach), but it probably also occurs in 

 the Vaca Muerta and Crab Orchard Mountains meteorites. These 

 are all ironstone meteorites. In the Steinbach meteorite it 

 forms from 8.5 to 33 per cent, of the non-metallic constituents 

 and occurs intergrown with bronzite. 



Maskelyne, who first described the mineral, considered it on 

 account of its optically biaxial character a new orthorhombic 

 form of silica and gave it the name of asmanite. Since tridy- 

 mite is now known, however, to exhibit biaxial characters and 

 the minerals agree in most other respects, they are generally 

 considered identical. 



Tridymite occurs in meteorites in the form of rounded grains 

 or plates, some of which reach a length of 3™™. They are 

 colorless to white to rusty brown in color. 



Well developed crystals are rare but from facets on rounded 

 grains a total of twelve forms has been determined. 



Analyses show a composition of practically pure silica, with 

 iron oxide and magnesia present as impurities. 



Lawrencite. — This is a solid ferrous chloride which has been 

 described from the iron meteorites of Tazewell, Smith Mountain, 

 and Laurens county. Formula Fe CI2. Color green to brown. 

 While described in the solid form from only the few meteorites 

 mentioned, the presence of lawrencite in many other iron meteor- 

 ites is generally believed to be indicated by the greenish drops 

 which exude on their surfaces. These drops are ferric chloride 

 or mixtures of ferric and nickel chloride, while occasionally 

 pure nickel chloride occurs. 



