7. “Microcline,” San Diego County, California. 
THE MINERALOGRAPHY OF THE FELDSPARS 267 
(Specimen 958.) 
Microscopic examination, thin section: Soda microcline, showing but 
faint microclinic twinning, and in a few areas, entirely clear. Slivers 
removed from the specimen with the gentle application of the knife blade 
show no twinning. The suggestion is strong that in grinding pieces for 
thin sections sufficient pressure was present to hasten the inversion of the 
soda orthoclase to soda microcline. To test this theory crushed frag- 
ments were heated in a quartz crucible over a Scimatco burner for one 
hour, three hours, and five hours. The percentage of the twinned speci- 
mens was measured in each case after the fragments had cooled. The 
results are tabulated below: 
. . Number of Number of 
Time of Heating . = Percentage of 
No. in Hours Pernned Er gments Te ecaen Twinned Fragments 
Ths Gg Ghar eee eee ° 28 64 19.6 
Do ccc CTE eae I 53 go 36.8 
festivals ces css 3 56 48 54.0 
ee EA eh icy efesieile''s & « 5 81 34 65.0 
Extinction angles, crushed fragments: 
Twinned 
(or0) 
(oor) 17.0 
6.5° 
° 
Untwinned (010) 7.0° 
(oor) 
TS 
These observations point to the fact that the specimen is in the act of 
inverting from the one modification to the other. 
QUANTITATIVE MICROSCOPIC ANALYSIS 
Phase Percentage K-feldspar Na-feldspar Ca-feldspar Silica 
Soda microcline.... 80 80 18 2 
Wligoclase......... 17 2 85 TQM Taegan ioe ccensis 
QUWEICA cauee ae GH le ta ees encarta lan Me mt ence 100 
SOG@asmicrocline, »..|.....2....- 64.0 14.4 Beatie ot Pa OI cee 
MT ROGIASE Ee oreotsetheoni|lcctols-a ies e ans 0.3 I4.5 Dy facing al eas a ea 
(QUERIED 3 5 ww 5 icy es orl ess Be rere yo aN enc Uday ls na POR eg a 3 
Mota... ke al ee a eet 64.3 28.9 3.8 3.0 
Classification: 
Popular: Microcline microperthite 
Technical: Microcline hypoperthite. 
