art. 15. MINERALOGY OF GOUGE CLAYS—-SHANNON, % 
has heretofore been separated from these minerals in a qualitative 
way on the basis of its fusibility. Heated in a closed tube the min- 
eral first darkens and then becomes white again and yields water 
having an acid reaction. 
Composition and chemical properties.—Optical examination by Mr. 
Ross of the material which had been hand selected for analysis showed 
it to be essentially homogeneous except for minor amounts of two 
minerals, one certainly colorless zircon in minute crystals, the other 
an isotropic colorless mineral which was doubtless the light-colored 
sphalerite mentioned above. These were entirely removed by a 
method which deserves mention as it is applicable to numerous mica- 
ceous minerals. A screen was selected which had meshes smaller 
than a majority of the sericite flakes and the material was screened 
repeatedly, alternating the screening with rolling the powder, spread 
thinly on plate glass, with a hardened steel rod. The brittle impu- 
rities were thus crushed so as to pass the screen while the micaceous 
sericite remained behind. Even before purification the impurities 
amounted only to a fraction of 1 per cent. 
Upon analysis the purified sample gave the following results and 
ratios: 
Analysis and ratios of sericite from Idaho. 
Per cent. Ratios. 
1S OI ei Sena re beak Nip coke eat Dh 46. 58 0. 7725 Te 25 1.002 
PACH OE AMS 1008 Mt. De Tes Tee BS eh 37. 46 pie 
BesOstraec bletawk Ae. eb enet dro . 80 0050 cities Se 
ORY GW2 5 WARE) seis Aah ee meee ae trace. 
MG Ohs eis Se RIBS Deol L abe os 1.16 0288 
Oe Yep Pn RE i cs Ba i gag 6. 38 . 0677 44, 32 1.151 
Na Olea /S.catte ee ee ee . 64 0103 
HOrabove! 110340 shkwe Nae e 6. 06 3364 
HO below 110°. :..... cs eee 30 
Ota. 2 Sete sateen ae 99. 38 
The ratios yield the formula (H,K),O, Al,O,, 2SiO, with the univalent 
bases—potash and water—replaced to a slight extent by magnesia. 
The bases are somewhat high which probably indicates that a part of 
the water, about 1 per cent, is adsorbed. The remainder of the water 
(5.03 per cent) is essential to the mineral which agrees with the nor- 
mal muscovite formula. The percentage of alkalies is 7.02 while the 
water is 5.03, the former being slightly higher in weight. Moleculary, 
however, the ratio of water to alkalies is 3.58 : 1.00. Assuming the 
mineral to be an isomorphous mixture of a theoretical pure potash 
muscovite molecule (K-AISiO,) and a theoretical hydrogen muscovite 
end molecule (HAISi0,), the latter is greatly in excess in the present 
muscovite. The very low content of low temperature water is note- 
worthy. 
