268 HAROLD L. ALLING 
8. “Orthoclase,” Sanidine Porphyry, Drachenfels, Siebenebirge, Rhenish p 
Prussia. (Specimen 986.) 
Microscopic examination, thin section: The slide slows phenocrysts of 
sanidine, usually zonally grown, the central] portion of which is slightly 
more potassic than the margins. 
Extinction angles, crushed fragments, the average of a large number. 
(o10) O23) 
(oor) 6.0° 
Inferred composition: ; . 
Kfeldsparn Wages oes 62 
Na-teldspareauscjcri tre 34 
Cazteldspan em eemer 4 
CHEMICAL AND MICROSCOPIC ANALYSES 
SiO. Al,03 CaO K.0 Na.O | MgO Ign Total 
65.87 | 18.53 -95 | 10.32 | 3.02 -39 -44 | 99.92 ° * 
K-feldspar Na-feldspar Ca-feldspar 
Chemicaleeeseseeeae 61.6 33-7 4.7 
Microscopic.......: 62.0 34.0 4.0 
Classification: Anorthoclase, potentially hypoperthite. 
Analysis. James D. Dana, System of Mineralogy. Orthoclase No.6. (Rg. Min. 
Ch. 1003, 1860.) 
g. “Sunstone,” Delaware County, Pennsylvania. (Specimen F 3-974.) 
Microscopic examination, thin section: The slide reveals that the specimen 
is microclinic perthite of two periods of development. The microcline 
intergrown with the albitic feldspar of the first generation is holding blebs 
of soda-rich feldspar that are clearly the result of a later development. 
It is believed that the albite phase of the second generation and the small 
flakes of hematite are due to the decrease in solubility of these constituents 
of the solid phase—they are due to exsolution. 
Extinction angles, crushed fragments: 
Potash phase........ (Gio) Se ee ee SS? 
(Com) scr. T7107 
SOG dup haschrr 3 see (GIO) een 21.2” 
(Gord anes 4.0° 
Inferred Composition Potash Phase Soda Phase 
K-feldspar........... 80 2 
Na-feldspar.......... 17 97 
Ca-feldspar.......... 3 I 
