By Peter G. Read
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Extra info for Beginner's Guide to Gemmology
2. Two of the planes of symmetry in an octahedron revolution. 1). 2). k c u b e has nine such planes. Centre of symmetry A crystal possesses a c e n t r e of s y m m e t r y w h e n identical faces and edges occur o n exactly o p p o s i t e sides of a central p o i n t . The seven crystal systems Crystals can be grouped i n t o seven basic crystal systems, all of which have different degrees of s y m m e t r y . These seven crystal systems are classified in t e r m s of imaginary lines of reference called crystal axes, which pass t h r o u g h t h e centres of t h e crystal faces, or edges, t o m e e t at a p o i n t inside t h e crystal called t h e origin.
Crystals in t h e t e t r a g o n a l , hexagonal and trigonal systems have o n e o p t i c axis a n d are called uniaxial, while o r t h o r h o m b i c , m o n o c l i n i c a n d triclinic crystals have t w o such axes a n d are called biaxial. In t h e same way t h a t a p o i n t in three-dimensional space can be defined b y m e a n s of t h r e e sets of c o o r d i n a t e s , so t h e various faces of a crystal can be identified b y w h a t are k n o w n as crystallographic indices. T h e m o s t c o m m o n l y used indices are t h o s e devised b y t h e n i n e t e e n t h c e n t u r y British crystallographer W.
The cubic system, with examples of pyrite (rear), spinel (left) and fluorspar crystals CRYSTALLOGRAPHY 41 ircon Fig. 4. 3). ) (2) The tetragonal system This has t w o equal-length axes at right-angles t o e a c h o t h e r , and a third (principal) axis which is either s h o r t e r or longer t h a n t h e o t h e r t w o , a n d at right-angles t o t h e m . T h e r e are five axes of s y m m e t r y (four two-fold a n d o n e four-fold), five planes of s y m m e t r y a n d a c e n t r e of s y m m e t r y .
Beginner's Guide to Gemmology by Peter G. Read