lb m\y \<u;k STATE MUSEUM 



mineralogic divisions, such as carbonates, oxids, silicates, etc, 

 also presenl a strikingly close similarity in the arrangemenl 

 of their molecules as shown by their crystallization, cleavage 

 and optical properties; minerals so related are said to form an 

 isomorphons group. 



in some instances a combination of elements occurs crystal- 

 lized in two or more series o( crystal forms which arc notably 

 separate and distinct ami frequently presenl the symmetry of 

 different systems; iliis gives vise 1 to two (sometimes three) 

 species of identical chemical composition and is known as dimor- 

 phism (or trimorphism where three species are concerned). A 

 very good example of dimorphism is presented by the carbonate 

 of calcium, which crystallizes in the rhombohedral group o( the 

 hexagonal system as the mineral calcite and in orthorhombic 

 forms as the mineral aragonite. Calcite stands ai iho head of 

 an isomorphous group o\' carbonates which all crystallize in very 

 closely related rhombohedral forms, similarly aragonite repre- 

 sents an isomorphous group o( orthorhombic carbonates which 

 agree very closely in axial ratios and crystal habit. 



Ps&udoniorphs 



Not uncommonly the composition of a crystallized mineral will 

 undergo some change by reason of the addition, loss or replace 

 ment of one or more elements. Thus pyrite, which is a sulfld 

 of iron, may, under certain conditions, undergo a change of 

 composition, the sulfur being replaced by oxygen and some water 

 ami the resulting mineral will have the composition of 

 linumite. This change is //<»/ accompanied by a corresponding 

 change in external form, therefore the altered substance will 

 present the crystalli alien and structure of the Original mineral 

 but the composition, COlbr, luster and hardness of the mineral 

 to which it has altered. Such a product of alteration is called 

 a pseudomorph, in the above instance limonite is said to 

 form a pseudomorph after pyrite. 



