Geochemistry and Main Uses of Minerals - Thanatia: The Destiny of the Earth's Mineral Resources - page 524

mula XY (Si;Al) 2 O 6 (where X represents Ca, Na, Fe +2 and Mg and more rarely
Zn, Mn and Li. Y represents ions smaller in size, such as Cr, Al, Fe +3 , Mg,
Mn, Sc, Ti, V and even Fe +2 ). On the basis of chemical composition and crystal
structure, the following species are recognised: enstatite and hyperstene (both or-
thorhombic), augite, clinoenstatite, clinohypersthene, aegirine, diopside, pegeonite,
jadeite, spodumene, pigeonite or hedenbergite (all monoclinic).
B.1.4 The amphibole group
The amphibole group comprises a number of species and although falling both in the
orthorhombic and monoclinic systems, they are all closely related in crystallographic
and other physical properties, as well as in chemical composition. They form isomor-
phous series and extensive replacement of one ion by others of a similar size can take
place, giving rise to very complex chemical compositions. The difference in chemical
composition between compounds of the amphiboles and corresponding compounds
of the pyroxenes is not significant. A general formula for all members of the for-
mer can be written in the following manner: (W;X;Y ) 78 (Z 4 O 11 ) 2 (O;OH;F) 2 ,
in which the symbols W, X, Y, Z indicate elements with similar ionic radii and
thus can replace one another. W stands for Ca, Na and K; X represents Mg and
Fe +3 (sometimes Mn); Y is indicative of Ti, Al and Fe +3 ; and Z of Si and Al.
The main amphibole minerals are tremolite, actinolite, cummingtonite, hornblende,
glaucophane, arfvedsonite or riebeckite.
B.1.5 The olivine group
The minerals of the olivine group are silicates of bivalent metals and crystallise in the
orthorhombic system. The composition of olivine generally corresponds closely to
(Mg;Fe) 2 SiO 4 , with limited substitution. Typical olivines are: forsterite, fayalite,
olivine, tephroite, monticellite, glaucochroite and larsenite.
B.1.6 The mica group
The minerals of the mica group have the easily recognisable perfect basal cleavage
in common. The composition of individual specimens may be very complex but
a general formula of the type W(X;Y ) 23 Z 4 O 10 (OH;F) 2 can be written for the
group as a whole. In this formula W is generally K or Na, X and Y represent
respectively Al, Li, Mg, Fe 2+ , and Fe 3+ ; Z represents Si and Al, the Si:Al ratio
being generally about 3:1. Some of the main mica minerals are biotite, muscovite,
paragonite, phlogopite and lepidolite.