4G • NEW YORK STATE MUSEUM 



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

 also present a strikingly close similarity in the arrangement 

 of their molecules as shown by their crystallization, cleavage 

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

 isomorphous group. 



In some instances a combination of elements occurs crystal- 

 lized in two or more series of crystal forms which are notably 

 separate and distinct and frequently present the symmetry of 

 different systems; this gives rise 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 of the 

 hexagonal system as the mineral calcite and in orthorhombic 

 forms as the mineral aragonite. Calcite stands at the head of 

 an isomorphous group of carbonates which all crystallize in very 

 closely related rhombohedral forms. Similarly aragonite repre- 

 sents an isomorphous group of orthorhombic carbonates which 

 agree very closely in axial ratios and crystal habit. 



Pseudomorphs 



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 sulfid 

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

 composition, the sulfur being replaced by oxygen and some water 

 and the resulting mineral will have the composition of 

 limonite. This change is not accompanied by a corresponding 

 change in external form, therefore the altered substance will 

 present the crystallization and structure of the original mineral 

 but the composition, color, 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. 



