46 
THE CONSTITUTION OF THE SILICATES. 
[BULL. 125. 
intelligible in the light of the formulae adopted ill this memoir. In its 
more typical occurrences muscovite agrees sharply with the formula 
given, but it varies in composition within well-defined limits. First, it 
ranges toward its sodium equivalent, paragonite, which has the cor- 
responding formula Al 3 (Si0 4 ) 3 NaH 2 . Secondly, in fuehsite, the chromic 
mica, a chromium salt partly replaces the aluminum compound, and 
similar ferric replacements are also known. In all of these variations, 
however, there is no change of ratios, and the typical formula remains 
undisturbed. In the barium mica, cellacherite, there is possibly a 
molecule of the type Al 3 (Si0 4 ) 3 R // R / , but this is uncertain. Other 
interpretations of the barium micas are admissible. 
The most important variation in muscovite is in the direction of 
increased silica. Normal muscovite contains 45.3 per cent of Si0 2 , but 
varieties exist in which the percentage rises to nearly 59. Muscovites of 
this class have been designated by Tschermak as phengites, and they are 
most easily explained upon the supposition of trisilicate admixtures. 
The molecule Al3(Si 3 8 ) 3 KH2 is identical in type with ordinary ortho- 
silicate muscovite, and its presence completely accounts for all excesses 
of silica over the normal amount. In Sandberger's lepidomorphite, for 
instance, the orthosilicate and trisilicate molecules occur in nearly 
equal proportions. All known muscovite may be represented by the 
general formula Al 3 (Si0 4 ) 3 E, / 3 +Al 3 (Si 3 8 ) 3 R / 3 , in which the latter mole- 
cule varies from zero to fifty per cent, and with ferric iron or chromium 
sometimes replacing aluminum. 
With the biotites and phlogopites the variability of composition is 
even greater than in the case of muscovite. For the typical molecules 
the composition would be 
I Al 2 (Si0 4 ) 3 Mg 2 KH. 
Al(Si0 4 ) 3 Mg 3 KH 2 . 
1 
Sid? . 
43.06 
24.40 
19. 14 
11.25 
43.27 
12.26 
28.85 
11.29 
4.33 
AI;0 3 
MgO 
K 3 
H ? 
2.15 
100. 00 
100.00 j 
To these types but few natural micas actually correspond, although 
intermediate mixtures are very common. Furthermore, ferrous salts 
frequently replace the magnesium compounds, and ferric molecules 
replace those of aluminum. Manganese, also, sometimes occurs among 
the dyad bases, and occasionally trisilicates in small amount are com- 
mingled with the ortho-salts. Because of these complications, the 
reduction of a given analysis to a specific formula may be very difficult, 
and this difficulty is often increased by uncertainty in the determination 
of water, or of the state of oxidation of the iron. In spite of these 
difficulties, however, the reduction is generally possible, and the ferro- 
