CRYSTAL SYSTEMS 
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crystal form. Such replacement forms are met with in minerals 
and in alloys and are known as pseudomorphs. 
When a crystalline substance is found with its own crystal 
outlines it is said to be idiomorphic. 
When a crystal has opposite ends different, due to dissimilar 
faces it is termed hemimorphic. 
Two crystals may unite to form a double or twin crystal. 
Unions in threes or fours are less frequent. 
Many chemical compounds are known, however, which form 
more than one kind of crystal. Such substances are said to be 
dimorphous, irimorphous or polymorphous, according to the num¬ 
ber of observed kinds of crystals which they form. 
The six crystal systems are as follows: 
System. 
Called Also: 
1. Isometric. Regular, cubic, or tesseral. 
2. Tetragonal. Quadratic. 
3. Hexagonal. Rhombohedral. 
4. Orthorhombic. Rhombic, or trimetric. 
5. Monoclinic. Clinorhombic, monosymmetric, or 
oblique. 
6. Triclinic. Anorthic, or asymmetric. 
A crystal is said to be holohedral when all its planes are present. 
When one-half the planes are present (in accordance with an 
established law) the crystal is hemihedral; and if only one- 
quarter the possible planes, the crystal is called tetratohedral. 
Crystal aggregates uniting in such a manner as to yield branch¬ 
ing, fern-like, moss-like or tree-like forms are called dendrites, 
and the mass is termed a dendritic mass. If the aggregate con¬ 
sists of more or less long hair-like twisted, curved or bent crys¬ 
tals, it is said to have a trichiten structure, and the individual 
hair-like bodies are called trichites. But when the tiny long 
narrow crystals are straight and resemble needles, the crystals 
are said to be acicular. Tiny globular masses of radiating, 
acicular crystals are called spherulites or sphero-crystals. When 
these radiating aggregates consist of anisotropic crystals they 
