260 HAROLD L. ALLING 
EXAMPLES OF PLAGIOCLASE FELDSPARS 
(Specimen 961.) 
Microscopic examination, thin section: Broad albite twinning. 
Extinction angles, crushed fragments: 
r. ‘“Albite,” Amelia Court House, Virginia. 
(o10) 18.3° 
(Con) iceOn 
Inferred composition: 
K-feldspar........ 1.0 
Na-feldspar! 2/5.) 97.0 
Ca-feldspar........ 250 
CHEMICAL AND MICROSCOPIC ANALYSES 
SiO. Al,03 CaO MgO Na.O K.0 H.0 Fe.0; Total 
Te Raresee cree O7OOM ete 7/2 I.50 .03 10.01 130 era there eee 100.80 
ON Ae Be pie) gee O8/544 iy LOSZ5 hos elles wena Il.67 343 ek oh eee 99.890 
BR ew varies: 68.22 | 19.06 TAOn 5 Sih II.47 220 .69 -15 | 100.19 
K-feldspar Na-feldspar Ca-feldspar 
iG Aloe orca a6 2.24 90.60 7.14 
DESPA pe eta ane 2.49 O7-52..\ «| (esl ene eee eee 
BONG ee rad ele? 96.9 I.9 
Microscopic 1.0 97.0 2.0 
Classification: Albite. 
Foote and Bradley, Amer. Jour. Sci. (4), XXXVI (1913), 47, 
Chem. News, XLVI (1882), 204. Dana, System of 
Analyses I and 2. 
after Robertson and Musgrave. 
Mineralogy. Albite 15. 
Analysis 3. Allen and Day, Carnegie Inst. Pub. 31, p. 48, G. P. Merrill. 
2. ‘‘Oligoclase,”’ Arendal, Norway. (Specimen 960.) 
Microscopic examination, thin section: Perfectly normal plagioclase with 
albite twinning. 
Extinction angles, crushed fragments: 
(010) 4.0° 
(001) 1.0° 
Index of refraction, beta: 1.547 
Inferred composition, ignoring potash component Ab,;An,;. 
Inferred composition, considering potash component: 
Kefeldsparw. ssn er 10.0 
Na-feldspar;...... 75.0 
Ca-feldspar....... 15.0 
