clarke.] MISCELLANEOUS SPECIES, 63 
VII. MISCELLANEOUS SPECIES. 
Among the ortho- and trisilicates of aluminum, ferric iron, and other 
triad elements, there are a considerable number which do not fall con- 
veniently into any of the preceding groups of minerals, or which are 
doubtful as regards their genetic affinities. Some of them have obvious 
relationships to other species, and some are quite obscure in character, 
but all seem to be conformable to the theory of substitution. 
First in order of importance is the mineral staurolite, a highly basic 
silicate, which is evidently akin to andalusite, dumortierite, and silli- 
manite, and which, like them, is orthorhombie. Like andalusite, fur- 
thermore, staurolite alters into muscovite, an entire crystal becoming 
transformed throughout into an aggregate of mica scales. 
By far the best evidence as to the composition of staurolite is that 
furnished by the analyses of Penfield and Pratt,* who adopt Groth's 
formula HAl 5 Fe // Si 2 Oi 3 . This, structurally, is best written 
/O— H 
Al— Si0 4 =(A10) 2 
Sl ° 4 =(A10) 2 
which expresses a partial relation to the micas, andalusite, and so on. 
The theoretical percentage composition calculated from this formula 
agrees well with the results of analysis, except that it gives the silica 
nearly one per cent too low, a discrepancy which Penfield and Pratt 
attribute to inclusions of silica in the minerals analyzed. 
By means of a slightly different formula the relations of staurolite 
to the other species can be much more clearly shown, but it assumes 
that the ideal staurolite is not yet known. The expressions proposed 
are as follows: 
Andalusite. Dumortierite. Staurolite. 
.Si0 4 =(A10) 3 ,Si0 4 =(A10) 3 ,Si0 4 =(A10) 3 
Al— Si0 4 =Al Al— Si0 4 =(A10) 3 Al— Si0 4 ~(A10) 3 
^SiO.ZAl \si0 4 =Al \si0 4 =Fe 
I 
Fe 
I 
.Si0 4 =Fe 
Al— Si0 4 =(AlO) 3 
\si0 4 =(A10) 8 
* Am. Jour. Sci., (3) XLVIT, p. 81, 1894. 
