OXIDES 371 



liy the waters of hot springs and geysers. Large quantities of 

 geyserite are being deposited by the geysers and hot springs of 

 the Yello\\>tone Park, and in New Zealand and Iceland. 



Tri polite, or infusorial earth, is an opal-like silica secreted by 

 organisms, as the shells of diatoms. They may form beds of con- 

 siderable extent ; such deposits are found in Maryland, Virginia, 

 Alabama, Missouri, and California. 



Tripolite is used commercially as a wood filler; as an abrasive; 

 as a filter stone ; in scouring soap ; as a polishing powder ; as a non- 

 conductor of heat for packing steam pipes and boilers ; in the manu- 

 facture of dynamite, as a holder for the nitroglycerine; and in 

 California as a building stone. 



Artificially opal is formed by the drying of gelatinous silica. 



SPINEL GROUP 



The spinels form a small, well-defined group of isometric minerals, 

 crystallizing in the ditesseral central type with an octahedral habit, 

 or in combinations of the octahedron, rhombic dodecahedron, and 

 the cube, while the cube with rare exceptions is a subordinate form. 

 Chemically they are salts of the general acid HR'"O 2 , in which R'" 

 may be Al, Fe"', Cr, Mn'", or mixtures of these, and the H may be 

 replaced by Mg, Fe", Zn, Mn", or Be ; in case of the latter in com- 

 bination with Al the mineral chrysoberyl is formed, the only 

 mineral of the series which is not isometric, but orthorhombic. 

 They are aluminites, ferrites, manganites, or chromites, and have 

 usually been classified with the oxides. They may be primary con- 

 stituents of the igneous rocks, in which, when they occur, they are 

 among the first minerals to crystallize. They may also be formed 

 as the results of thermal metamorphism or be derived from the 

 decomposition of other minerals as precipitates. 



Spinel, MgAl 2 4 . Hercynite, FeAl 2 4 . 



Gahnite, ZnAl 2 O 4 . Magnetite, FeFe 2 O 4 . 



Manganoferrite, MgFe 2 O 4 . 

 Franklinite (Fe . Zn . Mn) (Fe, Mn) 2 O 4 . 

 Jacobsite, (Mn . Mg) (Fe . Mn) 2 O 4 . 

 Chromite, FeCr 2 O 4 . 



SPINEL 



Spinel. Magnesium metaluminite, MgAl 2 O 4 ; MgO = 28.2, 

 A1 2 O 3 = 71.8; Crystal forms, o(lll), d(110), other forms, (001), 



