316 A TREATISE ON METAMORPH1SM. 



ALTTMINUM-SILICATE GROUP. 

 TOPAZ, ANDALUSITE. SILLDIAMTE, AXD CYAXITE. 



The aluminum-silicate group includes — 



Topaz: 



Al„F,8i0 4 or Al,(F,OH) 2 Si0 4 . 



Orthorhornbic. 



Sp. gr. 3.4-3.6. 



Andalusite: 

 Al 2 Si0 5 . 

 Orthorhornbic. 

 Sp. gr. 3.16-3.20. 



SiUimaniti : 

 Al,Si0 5 . 

 Orthorhornbic. 



Sp. gr. 3.23-3.24. 



Cyanite (disthene): 

 Al,Si0 5 . 

 Triclinia 

 Sp. gr. 3.56-3.67. 



occurrence. — Topaz is a much less common mineral than andalusite. 

 sillimanite, and cyanite. Like them, it occurs in the schists and gneisses 

 of sedimentary origin, especially those in which other fluorine minerals are 

 found, such as tourmaline and beryl. Unlike andalusite, sillimanite, and 

 cyanite, it is sometimes found in cavities in fresh volcanic rocks, as, for 

 instance, rhyolite. 



Andalusite is a frequent constituent of the metamorphosed sedimentary 

 rocks, especially of the argillaceous kinds. It often occurs in crystals, 

 including many other minerals in the partly metamorphosed sedimentary 

 rocks; but is also found in large, well-formed crystals in the schists. 

 Frequently in the metamorphosed sedimentary rocks its development has 

 been promoted by the contact effect of igneous rocks, especially the 

 granitic rocks. Its most characteristic associates are sillimanite and cyanite. 

 With the former it frequently has parallel intergrowths. Also it is fre- 

 quently associated with garnet and staurolite, and not infrequently with 

 tourmaline. Andalusite is rare, if indeed not altogether absent in the 

 metamorphosed igneous rocks. 



Sillimanite is a common mineral in the strongly metamorphosed sedi- 

 metary rocks, such as schists and gneisses, where it frequently replaces 



