claeke.] GARNET-BIOTITE TYPE. 21 
Possibly the tetragonal sarcolite, which has the general formula of a 
garnet with the lime partly replaced by soda, may fall here also ; but the 
analyses of this mineral are unsatisfactory, and its relations are still 
uncertain. Biotite will be more fully considered in the special chapter 
devoted to the mica group. Prehnite, according to Doelter,* decom- 
poses on fusion, yielding the same products as garnet. 
Under the third subtype of this series we find the garnet group itself, 
together with epidote and several related species. The sodalite group 
is also akin to garnet and to the second subtype, and will be considered 
in this connection a little later. Under the generic term garnet several 
species are included, all being isometric and strictly isomorphous, in 
which magnesium, calcium, and ferrous iron replace one another, while 
chromium, aluminum, and ferric iron are also equivalent terms. Thus 
we have: 
Grossularite Al 2 (Si0 4 ) 3 Ca 3 
Pyrope Al 2 (Si0 4 ) 3 Mg3 
Almandite Al 2 (Si0 4 ') 3 Fe // 3 
Spessartite Al 2 (Si0 4 ) 3 Mn 3 
Andradite •. Fe 2 (SiO 4 ) 3 0a 3 
Ouvarovite 0r 2 (SiO 4 ) 3 Ca 3 
To these maybe added schorlomite, a garnet in which titanium occurs 
both as part of the acid, that is, with Ti0 4 replacing Si0 4 , and also as 
Ti"' among the triad bases, equivalent to aluminum. The monoclinic 
partschinite, isomeric with spessartite, also falls into this group. 
In the epidote group, at least four species appear, one, zoisite, being 
orthorhombic, whiie the other three are monoclinic. These species are 
characterized by the bivalent group of atoms =A1 — OH, thus: 
Zoisite Al 2 (SiG 4 ) 3 Ca 2 (A10H) 
Epidote, a ( Al 2 (Si0 4 ) 3 Ca 2 (A10H) 
Epidote, b ( Fe 2 (Si0 4 ) 3 Ca 2 (FeOH) 
Pieclmontite (AlMn) 2 (Si0 4 ) 3 Ca 2 (A10H) 
Allanite ( AlCeFe) 2 (Si0 4 ) 3 (CaFe) 2 ( AlOH) 
or, in general, as compared with garnet, 
Garnet R" / 2 (Si0 4 ) 3 R"3 
Epidote R" / 2 (Si0 4 ) 3 R" 2 (R" / OH) 
The facts that garnet alters into epidote and that the two minerals 
are often associated give emphasis to the formulae. For epidote, 
however, an alternative formula is possible, in which the hydroxyl is 
supposed to be united with calcium rather than with aluminum. But 
epidote is a peculiarly stable mineral, while the group — Ca— OH would 
imply instability and easy alteration. The alternative formula, there- 
fore, is improbable, although it would ally epidote more easily with 
anorthite, which is a product of its fusion. 
* Allgem. Chem. Mineralotfie, p. 183. 
