CHAPTER VI. 
UmERALOGY— continued. 
Silicates (cow^cZ.)— Garnets. 
Manganese-garnets- — Spessartite — Spandite — Grandite- — Aplome — Cnlderile 
Man ganese-gar D ets. 
The garnets form a well-marked group of minerals belonging to the 
^, ^ , , holohedral section of the isometric system, the 
Characters ot garnets. . i 
commonest forms being the dodecahedron, trapezo- 
hedron (or icosatetrahedron), and hexoctahedron. Cleavage or parting 
parallel to the dodecahedron, d, is sometimes shown, but the mineral 
usually breaks with a sub-conchoidal to uneven fracture, H.=6"5 - 7 "5. 
G.=3'15 - 4"3. Lustre vitreous to resinous. Colour various shades 
of red, brown, yellow, green, and black. Streak white. Often show 
anomalous double refraction. The foregoing characters are taken from 
Dana's Mineralogy. 
In composition the garnets are orthosilicates conforming to the gene- 
ral formula 3R'''O.R'"203.3Si02. They are usually 
ganie?s^°^^*'°" °^ divided into six species according to the bases 
replacing the R" and R'". The R" may be replaced 
by Ca, Mg, Fe, and Mn, as main constituents ; whilst occasionally 
alkalies, nickel oxide, and baryta, in small amounts, may enter into the 
composition of this group. The R2O3 group may consist of AI2O3, Fe203, 
or Cr203 ; whilst in very rare cases Y2O3 may enter into this group in 
small amount. Dana does not give any analyses in which Mn enters 
into the R2O3 group ; but as will be seen from the analysis of a 
example of spessartite from Chargaon in the Central Provinces, it is 
necessary to assume that in this case at least some of the manganese 
is in the form Mn203. The silica is sometimes, though rarely, replaced 
by Ti02, and in one case by Zr02, to a very small extent. 
The names and formulae of the six recognised species of garnets^ are 
as follows : — 
Grossularite .... 3CaO. Al203.3Si02 
Pyrope . 
Almandite 
Spessartite 
Andradite 
Uvarovite 
3MgO.A!203.fiSi02 
3FeO.Al203.3Si02 
3MnO.Al2O3.3Si02 
3CaO.Fe203.3Si02 
3CaO.Cr203.3Si02 
Omitting sohorlomite, 3CaO.(Fe, Ti)203.3(Si, TijOg. 
M 
