Reéxamination of American Minerals. A5 
position is that of an oligoclase, as will be seen by the following 
ana 
s 2. 
Silica, 63:87 : 64°64 
Alumina, 21°82 : 21°98 
me, 2°14 = 1 
Magnesia, trace ; trace 
oaa, 10°18 : 9°80 
Potash, “BO ‘ 
Ignition, ; 29 ‘ 29 
98°80 99°38 
The specimens examined were received from Prof. Dana 
We have examined the moonstone feldspar, from Mineral Hill, 
Delaware Co., Pa., which is also oligoclase. 
15. Greenwood Mica—Biotite. 
The chemical constitution of only a very few American bio- 
tites has ex — In fact, von Kobell’s* analysis of a mica 
Si X1 fe Mg “ti i 
4000 1616 750 2154 1083 058 020  3800=99°76 
The results of our analyses are: 
1 
: 2. 
erate ‘ 39°88 : . 39°51 
Alum ; 14-99 . ; 1511 
Peroxyd of iron, 7°68 
agnesia, 23°69 23°40 
Potash, 9°11) 10-20 
a, 1125 
ater, , 1°35 
Fluorine, 95 “95 
orine, “44 
9°16 98°95 
These give the oxygen ratio: - 
; R # Si 
3 - - - 11:31 : 931 : 20°72 
2. - - - 11:20 : 9-45 : 20°53 
or very nearly 1: 1: 2, which carrer to the formula 
R 
A small at oe the onygons in the ee is replaced by fiuo- 
tine and chlor 
speci sean examined were received from Messrs. Jenkins 
& Horton, of Monroe, N. Y. 
* J. £. pr. Chem., xxxvi, 309, and Dana’s Mineralogy, $d edit., page 360. 
