THE MINERALOGRAPHY OF THE FELDSPARS Del 
Furthermore in only a very few cases where the three oxides, 
Na,O, K.O, and especially CaO, had been looked for, was any one 
of the three components entirely wanting. Such a revelation may 
not be surprising to the petrographer for he knows that it is usually 
impossible to determine with a microscope the composition of the 
alkali feldspars with anything like the accuracy obtainable in case 
of the plagioclase series. 
The writer has secured some sixty specimens of so-called ortho- 
clase from many world-famous localities and has satisfied himself 
after careful petrographic examination of them in thin sections 
and in crushed fragments that in the majority of cases the mineral 
is not orthoclase at all, but that it is a microcline relatively high 
in soda and more frequently a microclinic perthite. It is but a 
reasonable assumption, therefore, that the specimens which fur- 
nished the material from which the analyses have been made had 
not been examined petrographically, for if such examinations had 
been made, the name orthoclase would not have been applied to 
them in such a careless manner. The names given in Figure 19 
are the original ones published in connection with the chemical 
analyses which, as already stated, have been secured from many 
sources. The works on mineralogy by Dana, Hintze, Bayley, 
etc., have contributed many. Various bulletins of the state and 
federal geological surveys have been consulted. The volume of 
the Asches on The Silicates in Chemistry and Commerce has fur- 
nished a considerable number of analyses of the plagioclase series. 
The analyses themselves are not here reproduced but the references 
to the literature are given in the bibliography. 
Hintze does not distinguish the analyses of orthoclase from 
those of microcline, grouping them together. ‘This necessitates 
the symbol for ‘‘orthoclase and microcline.” The purpose of this 
triangle diagram did not warrant an extreme degree of accuracy 
and consequently the circles have been located as close to the 
actual recast figures as possible without any overlapping, which 
would cause undue confusion in recognizing the different species 
there represented. 
In many ways the diagram (Fig. 19) speaks for itself. It 
clearly indicates that most if not nearly all feldspar specimens are 
three-component systems. 
