228 PHYSIOGRAPHY. 



Gilbertite. 



GlESECKlTE. 



Crystallized in six-sided prisms. 



Cleavage not visible. Fracture uneven, splintery. 



Lustre resinous, faint. Color olive-green, grey, brown. 



Streak uncolored. Feebly translucent on the edges . . . opake. 



Hardness == 2-5 ... 3-0. Sp. gr. = 2-832. 



1. Analysis. 

 By STROMEYER. 



Silica .... 46-07 



Alumina .... 33-82 



Magnesia .... 1-20 



Black oxide of iron . . . 3-35 



Oxide of manganese . . . 1-15 



Potash * . . . . 6-20 



Water .... 4-88 



2. It occurs in Greenland, with Feldspar. 



3. The above description probably applies to a variety of Mica, similar 

 to Finite. 



GILBERTITE. 



Massive ; foliated. 



Lustre pearly. Color white, with a shade of yellow. Trans- 

 lucent. 



Sectile. Hardness = 4-00 ? Sp. gr. = 2-648. 

 1. Analysis. 

 By THOMSON. 



Silica .... 45-155 



Alumina ' . . . . 40-110 

 Lime .... 4-170 



Magnesia .... 1-900 



Protoxide of iron . . . 2-430 



Water .... 4-250 



2. It occurs at St. Austle in Cornwall, and contains through its mass 

 particles of Fluor, and what appears to be Apatite. 



3. It is quite probable, that the above mineral, which is well known 

 as Cornish Talc, is an aggregate of Talc, Mica and several other mine- 

 ral species. 



GIOBERTITHV (See Magnetite.) 



