74 CONTRIBUTIONS TO MINERALOGY. " Lbull.262. 
A careful study of over 40 analyses of vesuvianite leads us to sus- 
pect that it may be a mixture of several similar molecules, which we 
can represent by the subjoined expressions: 
Al 2 Ca 7 (SiOJ 6 (A10H) 2 , 
Al,Ca 7 (Si0 4 ) 6 (A10 2 R,) 4 , 
Al 2 Ca 7 (Si0 4 ) 6 H 4 , 
Al,Ca 7 (Si0 4 ) 6 Ca a ; 
the first and third compounds usually predominating-. Structurally, 
and with due regard to the analogy between vesuvianite and other 
species, these formulae may be written as follows: 
/SiO,= j c /SiO = 1 (( 
Al— SiO,= i uaa Al— SiO,= i L 
R". \Si0 4 = K', 
1 
Ca Ca 
I I 
/Si0 4 = R". /Si0 4 =R' 2 
Al— SiO,= | n Al— SiO = | n 
\Si0 4 = j ^ \SiO = \ ^ 
About one-seventh of the calcium is replaced by magnesium, and in 
the variety wiluite the group BOII, analogous to AlOH, probably 
occurs. 
Vesuvianite, then, is to be regarded as a basic orthosilicate, belong- 
ing to a group of compounds of which garnet is the normal salt. 
Epidote and the scapolites are other members of the same class, and 
their formulae are all strikingly similar to one another. As product 
of contact metamorphism they originate under like conditions, anc 
they all undergo alteration in much the same way, yielding similar oi 
even identical derivatives. Garnet, vesuvianite, epidote, and tin 
scapolites must be considered together if the problem of their const! 
tut ion is to be properly solved. 
