GUIDE TO THE MIXERALOGIC COLLECTIONS 



79 



ite, at Richmond Maes., with the bauxite of Georgia and Ala- 

 bama, and in Dutchess and Orange counties, X. Y. 



It is an ore of aluminium but has not-been found in sufficient 

 quantities to render it of much importance. 



Psilomelane (black hematite) 



Psilomelane is a hydrous manganese manganate of somewhat 

 doubtful composition and usually very impure. 



Psilomelane does not occur crystallized. It is found in black 

 or dark gray, botryoidal, reniform or stalactitic masses with a 

 submetallic or dull luster. 



It is commonly associated with pyrolusite in alternate layers 

 and is found in many localities given for the latter mineral. It 

 is applied to the same uses as pyrolusite. 



OXYGEN SALTS 

 CARBONATES 



The anhydrous carbonates form two distinct isomorphous 

 groups, one of which is distinguished by rhombohedral crystal- 

 line forms of singularly close relation in the various species 

 which form the group; this is named from its most prominent 

 member the calcite group. Similarly the orthorhombic forms of 

 aragonite are closely related crystallographically to those of the 

 other carbonates in the aragonite group. 



Calcite (calcareous spar, limestone) CaCo 3 



Calcite is the carbonate of calcium and contains 44$ carbon 

 dioxid and 56$ lime. 



Calcite crystallizes in the rhombohedral. class of the hex- 

 agonal system, the great variety and beauty of its crystals mak- 

 ing it an object of interesting study to the novice as w 7 ell as to 

 the trained mineralogist. The unit rhombohedron shown in fig. 

 194 is an important form both because it is prominent in many 

 varieties and because the perfect and strongly marked cleavage 

 of calcite takes place in planes parallel to the faces of the unit 

 rhombohedron. Many varieties are scalenohedral in habit, 

 crystallizing in forms similar to those shown in fig. 195-97. The 

 name " dog tooth spar " is given to this type, a specimen of 

 which is shown in pi. 24 2 . A flat rhombohedron (fig. 198) is 

 prominent in the variety known as " nail head spar." 



