clarke.] ORTHOSILICATES OF DYAD BASES. 69 
which we have the empirical formula? established by Penfield and Howe.* 
Structurally written these become 
CUnohumite. 
Humite. 
Chondrodite. 
Mg 
Mg 
Mg 
ii 
II 
II 
Mg/ >Mg 
>Si0 4 / 
Jig/ >JIg 
>SiO/ 
Mg< >Mg 
/Si0 4X 
Mg< \Mg 
>Si0 4 < 
Mg< >Mg 
II 
(MgP) 3 
/ 8i °*\ 
Mg/ \Mg 
II 
(MgP) 8 . 
(MgF) 2 . 
or derivatives respectively of the salts Mg 8 (Si0 4 ) 4 , Mg 6 (Sr0 4 ) 3 , and 
Mg 4 (Si0 4 ) 2 , with one atom of magnesium in each case replaced by the 
two univalent — Mg — F groups. The clinohumite occurs in association 
with forsterite, and the two species have nearly the same specific grav- 
ity. Hence forsterite may be Mg 8 (Si0 4 ) 4 , and this is the only datum 
available from which to infer its molecular magnitude. The synthetic 
transformation of forsterite into clinohumite, if it could be effected. 
would go far toward settling the question. 
In the case of phenakite the triple formula Gl 6 (Si0 4 ) 3 is rendered 
probable by the existence of triinerite, Gl 3 Mn 2 Ca(Si0 4 ) 3 . It is also 
emphasized by the species helvite and danalite, which contain sulphur, 
probably combined in the dyad group — E — S — R — . Both of these 
species, in all of their known occurrences, agree with the general 
formula 
Gl 
II 
/SiO 
Gl< >G1 
r/ Nr 
\sio 4 
R 
<: s -\ 
in which R maybe either Fe", Mn, or Zn. The R is variable, but the 
other constituents are constant. In helvite, Mn and Fe occur, and in 
danalite zinc appears. In the Colorado danalite Zn predominates 
largely over Fe, and there is very little Mn. The Rockport danalite 
has Fe in excess of Zn, and rather more Mn. The Cornish danalite 
is very low in Zn, and the Fe largely exceeds the Mn. The ratio 
Gl:Si0 4 ::3:3, however, holds for all. 
If phenakite is Gl 6 (Si0 4 ) 3 , then willemite. which is morphologically 
similar, is probably Zn ti (Si0 4 ) 3 , with zinc partly replaced by manganese 
in the variety known as troostite. 
* Am. Jour. Sci. fS). XLVII. p. 188, 1894. 
