392 Penfield and Sperry — Triclinic Feldspars with 



percentage of CaO. All satisfy Tschermak's formula very well 

 as mixtures of Na 2 Al 2 Si 8 16 and Ca 2 Al 4 Si 4 ]6 . In VI there 

 is an unusually large percentage of K 2 due to an admixture 

 of microcline, which could be distinctly seen with the micro- 

 scope on a section parallel to the basal cleavage. Below the 

 analyses the proportion of anorthite to albite is given, K 2 

 being left out of the calculation, together with the per cent of 

 anorthite, the direction of the rhombic section and the extinc- 

 tion angle upon i-i measured with the polarizing microscope. 



I. II. III. IV. V. VI. 



Branchv'le, Hitteroe, Haddam, Miner'l Hill, Danbury, Pierrepcrat, 

 Conn. Norway. Conn. Delaware Conn. St. Law. Co., 

 Co., Pa. N.T. 



No. in Brush collection, 1790 1758 1757 1782 1764 1791 



SiO„ 66-58 66-83 66-06 66-34 65-73 63-76 



A1„0 3 21-26 20-88 21-57 20-72 21-32 22 ! 67 



Fe 2 3 -07 '25 -18 -12 -41 



CaO 1-18 1-46 1-80 1-85 -1-95 3*05 



Na 2 10-26 10-36 9-57 9'44 9*66 6*89 



K 2 0_ -76 -70 1-01 -98 -95 3-60 



Ign. -16 -27 -38 -19 '40 



100-27 100-75 100-19 99-71 99"92 100-78 



Specific gravity 2-610 2-632 2*633 2-627 2-628 2-622 



Ratio An : Ab 1:16-0 1:13-0 1:9-6 1:9-2 1:9-0 1:4-1 



Per cent of anorthite, 5-87 7-25 8-94 9-20 9-70 ]5-l7 



Rhombic section +12° 14° 13° 12° 10° 10° 



Extinction on *-£ (010), 15° 15° 16° 12° 15° 6° 



All of the above feldspars, with the exception of No. 6, are 

 very similar and could be classified as albites, as they contain 

 more than nine parts of the albite molecule to one of anorthite. 

 Their extinction angle, 12°-16°, is near to that of pure albite 

 + 19°. In No. 6 we notice no very perceptible change in the 

 direction of the rhombic section, as we should expect for so 

 large an addition of the anorthite molecule; there is quite a 

 perceptible change, however, in the direction of the extinction 

 angle. Judging from the above, however, it will be safe to 

 predict that where the striations on i-i make an angle of about 

 + 12°, the feldspar will be a mixture of albite with from five to 

 ten per cent of anorthite. 



Feldspars more basic than oligoclase. — On the material at our 

 disposal we have not been able to find many examples with 

 striations on i-%. One fine specimen, labeled labradorite from 

 Labrador (No. 1732 in Professor Brush's collection), polished 

 on the brachypinacoid to show the beautiful colors so common 

 in that species, shows three cleavage surfaces: two basal cleav- 

 ages covered with very distinct wide striations and one i-i, 



