THE MINERALOGRAPHY OF THE FELDSPARS 22y7 
labradorite, or even bytownite. In such rocks the feldspars are 
undoubtedly approaching, as a limit, the potash-lime binary sys- 
tem. The infrequency of intergrowths of these feldspars is the 
cause of the failure to recognize the system in nature. 
Referring to the triangular plot of the feldspar analyses (see Fig. 
19), it will be seen that a few specimens called ‘‘labradorite” and 
“anorthite”? are approaching the side of the triangle occupied by 
the potash-lime feldspars. 
The lack of specimens of this binary system prevents any 
accurate attempts being made to outline or to plot the physical 
and optical properties. They may, however, be inferred. The 
specific-gravity curves are steeply inclined, extending from 2.58 for 
orthoclase to 2.765 for anorthite. The gravity line for microcline 
and soda-microcline probably does not reach the high lime ranges, 
and ends somewhere between the two limits. The indices of 
refraction likewise are more inclined. In the center of the diagram, 
_ these lines have no practical significance, as undercooled meta- 
stable crystals of analogous to anorthoclase are probably unknown 
or very rare in normal rocks. 
When we come to the nomenclature and classification of the 
binary, we enter virgin fields. What name shall be applied to 
the intergrowths described above as occurring in the Adirondacks ? 
Shall perthite be used? There is considerable objection to such 
a practice; it would be extending the meaning of a well-established 
term and the modern tendency is in the opposite direction. Rogers' 
has proposed a slight extension of the term perthite to include 
intergrowths of two alkali feldspars when the orientation of the 
blebs differs from the customary relation. This implies that a 
certain element of textural habit is associated with the composi- 
tional significance. All of this leads to the obvious conclusion 
that a new word is required. The writer proposes, for intergrowths 
of potassic and high lime plagioclase feldspars, either eutectics or 
due to exsolution, the term ‘“‘oranite.” Its derivation tells the 
composition: orthoclase-anorthite—ite, the mineralogical ending. 
The same name can include intergrowths of soda microcline and 
potash anorthite. 
tA. F. Rogers, Jour. Geol., XXI (1913), pp. 202-7. 
