THE MINERALS OF IDAHO 301 



combination of rhombic dodecahedron and trigonal trisoctahedron. 

 A light-brown massive variety which was associated with bornite and 

 powellite was analyzed with the following results: 



Analysis of massive garnet from the Peacock >ninc 



(W. H. Melville, analyst) 



Per cent 



Silica (Si0 2 ) 38. 67 



Alumina (A1 2 3 ) 10. 08 



Ferric oxide (Fe 2 3 ) 16. 00 



Ferrous oxide (FeO) .91 



Lime (CaO) 33. 35 



Magnesia (MgO) . 77 



Copper oxide (CuO) Trace. 



Ignition (H 2 0) . 06 



Total 99. 84 



Palache has studied garnet from this region and his descriptions 

 are here reproduced. 23 Grossularite occurs at the White Monument 

 mine in cinnamon-brown crystals showing only the trapezohedron 

 (211) associated with a little epidote in an impure limestone. These 

 often include layers of crystalline calcite, the exterior layer being 

 garnet. 



Andradite occurs in the Peacock mine in light to dark brown 

 completely developed and very symmetrical crystals showing the 

 forms (101), (211), and sometimes (321), embedded in glassy copper- 

 stained quartz from which they easily separate, leaving beautifully 

 sharp casts. A second type occurs in rough crystals an inch or 

 less in diameter with dodecahedral form, edges slightly truncated 

 by the icositetrahedron (211). The centers of these crystals are of 

 a dark-brown garnet substance while the exteriors are uniformly 

 coated with a shell of clear quartz Y2 mm. thick conforming in 

 minutest detail to the form of the garnet crystal. In thin section 

 one of these shells formed a single crystal individual, apparently an 

 original deposition controlled in its position by the garnet crystal. 

 Outside the quartz is often a similar coating of iridescent epidote 

 and at times thin plates of hematite lie between the quartz and epi- 

 dote. Calcite has also formed similar layers in some crystals, and 

 where removed it leaves them cavernous. The surfaces of garnet 

 crystals coated with quartz and epidote are plane and look like 

 pseudomorphs of these minerals after garnet. 



Andradite crystals from the Copper Key mine are notable for the 

 etching they have undergone. The forms originally present were 

 (101) and (211) in almost equal development and perfectly sym- 

 metrical. The crystals have been rounded by deep corrosion (fig. 76) 



'■' Charles Palache. Amcr. Journ. Sci., vol. 8, p. .'(Oil, 1899. 



