152 Ford and Bradley — Study of a Labradorite. 



series as given by Day and Allen,* (Albite =: 2"605 ; Anorthite 

 = 2-7(55) was fonnd to be 2'71(). 



Tlie mineral was water-clear and almost colorless, showinsj 

 only in the larger pieces a faint tinge of yellow. In some 

 cleavage fragments, when viewed perpendicular to the poorer 

 cleavage face &(010), a copper-like renection was seen, similar 

 to that in the sun-stone from Tvedestrand, Norway. It was 

 evidently due to minute tabular inclusions of some iron 

 oxide, probable hematite, which lay in planes parallel to 5 (010). 

 On examination under the microscope with a high-power lens 

 these inclusions were seen to be very minute and without 

 crystal outline, many of them possessing a curved shape similar 

 to that of a comma. One fragment showed larger inclusions 

 which in this case were arranged parallel to both cleavage 

 directions. These larger inclusions were more regular in 

 shape, Ijeing in general like very thin wafers with a circular or 

 slightly elliptical outline and having a lens-shaped cross-section. 

 A thin section of this specimen ground parallel to one of its 

 cleavage surfaces showed under the microscope, first a series of 

 nearly circular inclusions of a copper color scattered irregularly 

 over the field and which were lying parallel to the plane of the 

 section, and second, a series of thin, lens-shaped inclusions 

 arranged in parallel lines, these latter being the cross sections 

 of the inclusions lying in the cleavage plane perpendicular to 

 the section. 



Of the two cleavages, the one parallel to the base, c (001), 

 was excellent, while, as is usually the case with the feldspars 

 near the anorthite end of the plagioclase series, the cleavage 

 parallel to h (010) was imperfect and difficult to obtain. 

 Several measurements of the cleavage angle were made on 

 different fragments, the two best giving the following closely 

 agreeing angles, 85° 49' and 85° 50'. 



Sections were ground parallel to each cleavage face and the 

 angles of extinction were measured in sodium light with the 

 trace of the cleavage of the other face. On account of ther 

 imperfect cleavage parallel to h (010), the two sections that 

 were made parallel to this face were carefully adjusted so that 

 in each case they made with the basal cleavage an angle 

 having a value within a few minutes of theory. Sections 

 parallel to c (001) were easil}' made, but because of the imper- 

 fect nature of the cleavage parallel to h (010) it was not always 

 possible to be certain of this direction in the section. The 

 method adopted was to find a cleavage fragment that showed 

 both cleavages present, as proven by the measured angle 

 between them, and then to grind the section parallel to 



* The Isomorphism and Thermal Properties of the Feldspars ; Carnegie 

 Institution P-ablication, No. 31, p. 74, 1905. 



