clarke.] GARNET-BIOTITE TYPE. 25 
In the first expression there is still a replaceable atom of aluminum, m 
the second expression none; in the first, at least, one calcium atom 
must link two Si(X groups, while in the other no such linkage occurs; 
and these facts may be connected, with others. For example, garnet 
alters into mica; and the mica group, as will be seen later, contains 
members in which the third aluminum atom is replaced. This points 
at once to the first type of formula as preferable, and the alterability 
of garnet into epidote brings the latter mineral into the same category. 
Zunyite and sodalite, being isometric, should also follow garnet; 
but derivatives of the second type are theoretically possible and may 
exist. Even under the first type alone, isomerism is conceivable, and 
the orthorhombic zoisite may be contrasted with the nionoclinic lime 
epidote as follows : 
Si0 4 =Ca Si0 4 = Al - OH 
/ >Al-OH / >Ca 
Al - SiO 4 =0a Al - Si0 4 =Ca 
X Si0 4 ZAl \SiG 4 Al 
even though we can not assign either species to either formula defi- 
nitely. My object here is«merely to show that the formula3 have prop- 
erties by virtue of which they are able to express known differences. 
Additional evidence for the formula assigned to garnet is furnished 
by the composition of vesuvianite, which is most simply represented as 
formed by the coalescence of two garnet molecules, with elimination of 
silica. The following formuhe express this relation, and also a rela- 
tion to epidote, quite perfectly : 
Garnet. Vesuvianite. 
Al-Si0 4 = 5 Al-Si(V:E ) 
\si0 4 ~Al \siG 4 =Al-OH 
Al-Si0 4 — ) 
X Si() 4 - ) 
If in vesuvianite It" 6 =Ca 5 Mg, this formula gives for the composition 
of the typical mineral : 
SiO. : 38.38 
A1 2 3 19. 55 
CaO 35. 81 
MgO 5.12 
H,0 1.14 
100. 00 
This agrees well with the published analyses, if we remember that 
ferric iron may partly replace aluminum, and that fiuorine sometimes 
replaces hydroxy!-. It is also sustained by the facts that garnet and 
vesuvianite often occur in contact limestones under similar conditions 
