clakke.] MICAS AND CHLORITES. 47 
magnesian micas, with some excei)tions to be noted farther on, are 
representable as mixtures of the two types shown in the formula as 
given. The iron mica siderophyllite agrees closely in composition 
with a normal ferrous biotite, Al 2 (Si0 4 ) 3 Fe 2 KH. Aspidolite appears 
to be a sodium phlogopite, but needs reanalysis. In exceptional cases, 
muscovitic types seem to exist among these micas, and certain lepido- 
melanes approximate to the general formula Al 3 (Si0 4 ) 3 Fe"K+Fe /// 3 
(Si0 4 ) 3 Fe"K. Another unusual mica is Breithaupt's alurgite, which, 
as analyzed by Penfield,* corresponds to a mixture of molecules 
2. Al 3 (Si 3 8 ) 3 KH 2 
3. Al 2 (Si0 4 ) 3 K 2 H 4 
3. Al 2 (Si0 4 ) 3 Mg 2 KH, 
with a slight excess of H over E in the last compound. The second of 
these molecules, an alkaline biotite, is the characteristic feature of 
alurgite. Similar compounds, parallel to phlogopite, seem also to 
exist, having the general formula A^SiO^R'g; but all of these bodies 
conform sharply to the general theory of the micas, and are substitu- 
tion derivatives of the normal aluminum salt. 
In many of the magnesian micas fluorine is found, and the iron micas 
frequently contain oxygen in excess of the amount necessary to convert 
all of the silicon into the radicle Si0 4 . When this excess is real, that 
is, not ascribable to defective analysis, it may be due either to alteration 
or to admixtures of micas belonging to the clintonite group. This 
group, the members of which resemble biotite morphologically, are 
extremely basic, and are representable by the general formula 
Al— O 
Nsi'O^B'a 
in which R' 3 may be replaced by Al, or by R"R', R" being magnesium, 
calcium, or ferrous iron. The univalent group AlC^R" may also 
appear in R', giving as the extreme end of the clintonite series the 
compound (A10 2 R // ) 4 Si0 4 . In seybertite and brandisite we have nearly 
/°>Mg /°>Oa 
2 Al— O b +3 Al— O 
\si0 4 =(A10 2 Mg) 3 \si0 4 E:H 3 
while xanthophyllite f is 
/°>Oa /°>Mg /°>Oa 
5A1— O +5 Al— O +1 Al— O 
^Si0 4 ^H 2 .AlH 2 2 \si0 4 =(A10 2 Mg) :5 \si0 4 _I(A10 2 Ca) 3 
The xanthophyllite, upon ignition, decomposes into a portion soluble 
and a portion insoluble in hydrochloric acid. The insoluble portion 
has the composition of spinel, a mineral which is generally found 
'Am. Jour. Sci., (3) XLVI, p. 289. 1893. 
t See Clarke and Schneider, U. S. G. S. Bulletin No. 113, p. 27. 
