82 ISOMORPHISM AND THERMAL PROPERTIES OF FELDSPARS. 



lamellar section without evidence of zonal structure, proving that the 

 crystals are chemically homogeneous. 



Fracture lines cross the crystal irregularly and follow possible cleav- 

 age planes to only a limited extent. In some cases lamellae have 

 broken apart along the composition plane, which is also the second 

 cleavage plane (oio). 



In several places the albite twins have been cut at right angles to 

 the twinning plane and also at right angles to one of the optic axes of 

 the crystal. In one lamella it is almost exactly normal to the plane 

 of section, in the other very slightly inclined. The plane of the optic 

 axes in one lamella stands at 63 to the trace of the twinning 

 plane (010); in the other lamella it is 62 30' approximately. One 

 of the optic axes of the crystal lies almost parallel to the pinacoid 

 (010). This is the position given it in Michel-Levy's diagram* for 

 anorthite (An). 



The crystals contain numerous inclusions of colorless, apparently 

 amorphous substance, with a refraction higher than anorthite. It 

 appears to be isotropic. The outline is very irregular and rounded. 

 The shapes are curved and elongated. They contain gas bubbles. 

 These inclusions are distributed in planes, lines, and swarms, having 

 various directions with respect to the feldspar crystals. The arrange- 

 ment in some cases suggests skeleton forms. In some places the 

 inclusions are mostly gas. But the suggested feather structure bears 

 no fixed relation to the crystal orientation or the lamellar structure 

 of the anorthite. It appears like the structure of something obliter- 

 ated by the crystallization of the anorthite, or in some cases as though 

 the skeleton form were completely filled up by anorthite in perfect 

 orientation. The distribution of the inclusions in some instances is 

 such as to suggest changes in the rate of crystallization of different 

 parts of the crystal. In some places the crowding of minute inclusions 

 suggests very rapid crystallization. It appears as faint cross-banding 

 in the crystal shown in Plate I, from another preparation of anor- 

 thite (An). 



The glass is probably composed of material in excess of the propor- 

 tions necessary for the anorthite (CaAl 2 Si20 8 = An). It is not Si0 2 

 alone, for this would have an index of refraction lower than anorthite. 

 It may be a silicate of Al or Ca. 



(5 a ~ b )- This preparation is similar to (19). The crystals are 

 tabular parallel to the second pinacoid (010). The larger plates are 

 1.5 mm. thick. There is multiple twinning according to the albite 



* iltude sur la Determination des Feldspaths, etc. Paris, 1894. Plate vn. 



