Scheerer on Isomorphism. 383 
minute quantities of the protoxyd of iron are sufficient for im- 
parting to a silicate (when not ina pulverulent form) a decided 
green tinge, if not interfered with by the presence of other color- 
ing substances. Assuming, therefore, and we are doubtless bet- 
ter borne out in doing so, that the iron is in the condition of a 
peroxyd, the ratio of the’ oxygen becomes 
Si R R 
26:20 ; 15:64 ; 5-26, 
agreeing very closely with the ae R? Si2?+3R Si, which 
would require a ratio of 26°20 : 15°72 : 5-24," The formula cited 
above is however that of the sien hard fahlunite, whereas 
that of cordierite, according to Berzelius, has been hitherto held 
to be 3R? Si?+S8AlSi.* The ratio of the oxygen agreeing 
with this last formula, is 
Si Al R 
26:20 : 14-97 : 5°62, 
and is therefore an approximation to the first that was obtained, 
in which the iron in the Kragerée cordierite was held to be in 
the state of a protoxyd. As it does not however appear to me 
that this assumption is fairly borne out, I here give the preference 
to the more simple formula R? Si?+3R Si 
This leaves us to consider whether the cmianaes of cordier- 
ite from other localities may not be also expresse sl 
Pler formula. The ratio of the oxygen is, however, 
= In the Cordierite from nen according to Stromeyer, 25: 2 : 14-81 : 5-90 
Simiutak, 25°51] : 15-47: 
_ 15-47 ; 5-43 
‘ te Ori to : “ 25-22 : 14-82 : 5-83 
4. « rie TE ot a Bound 95-95 : 15-36 : 5-18 
5. &“ “ “« ‘Thom 25-21 : 14-71 : 6-23 
6. ‘s Connecticut, “ 25°78 : 13-41 : 6-39 
The mean of these six analyses gives * 
Si Al R 
25°47 ; 14:76 : 5:83, 
Whereas, peoding to the formula R* Si?+3AlSi, it should be 
25-47 : 15.27: 5-09. On the assumption, however, that, in these 
Sie oftvan sae a small portion of the iron exists in the form of a 
Protoxyd, the average ratio is convertible to 
Al R 
05: ‘47: 15°27 : 5: 37, 
oe eee 
* Or, more fully, Fe3 Siz +261 Si+2 [Mgs Si2-+3.Al Si). 
