OE ee 
J. D. Dana on the Feldspar group of minerals. 399 
each 1:3. The group, therefore, is one in nature, and chem- 
must have some explanation of this unity. The follow- 
ing ee bear on the question of its origin. 
1. The oxygen ratios for the protoxyds, sesquioxyds, and 
llow: 
silica, in the different Feldspars, are as to 
R Al Si RX Si 
Anorthite 1:3:4 Oligoclase tere 
Labradorite Vrs76 Albite 176513 
Andesine 1:3:8 Orthoclase 1:3:12 
Hyalophane 
[The ratio 1 : 3 for the basic oxyds is expressed in the for- 
mula R+ Al, or (ik? tees and, for the basic metals, in jeg for- 
mula B+38Al, or (}R+3¢ Al) ; or if the pe erissads are dis- 
preits indicated, in the formula (R,R,)+3¢Al, or GR, G33 
Besides the ahoysspennenas ratios, some analyses — 5 
: 44, 1:3: 5,1: 3:5}, and other ratios between 1:3:4 and 
Bis 8; between 1:3: 8and 1:3: 9; and between 1:3 :9 
and 1:3:12. But these cases, (and also those in which there 
is some variation from the ratio 1 : 3,) are usually regarded as 
exceptional to the above as the true normal ratios ; and they 
have been attributed to the following causes : 
analyses, 
(6) Impurities: and often mixtures of different feldspars 
through inter-crystallization, some remarkable examples of 
ag have been detected by ‘microscopic and optical examina- 
rey Alteration; caused either (1) bed the infiltration of ordin- 
ted or filtered through se 
process aided by mineral ingredients in te water resulting in 
chan introduction of magnesia, oxyds of iron, € 
c 
regards the views here presented, it matters not whether 
these intermediate ratios are irregularities produced by the 
saeee methods, or are normal ratios. 
illus- 
group here including not only Mg, Fe, etc., but also Ks, nie Lis, ete., as 
trated int the olnenification-0b 204 of the former paper. Iu se beyond the ex- 
‘pressions of the old scarey! er See ca initigiie eto all chemical readers, wie 
rsed in the new system or 
