516 Wright — Three Contact Minerals. 



resinous to greasy. Translucent to transparent in thin flakes ; 

 in large masses, sub-transparent to opaque. Streak white to 

 pale gray, the gray probably due to fine magnetite inclusions. 



In the thin section, the gehlenite appears weakly birefract- 

 ing with comparatively high refractive index. Magnetite 

 inclusions are abundant and often show crystal outline. The 

 magnetite also occurs, tilling cleavage and fracture cracks in 

 the gehlenite and evidently was precipitated both before, dur- 

 ing and after the crystallization of the gehlenite. In certain 

 of the sections the magnetite crystals showed a distinct tend- 

 ency to an arrangement parallel with the first and second order 

 prism faces. Round earthy spots also occur filled with earthy 

 matter and are apparently of secondary origin, although they 

 may possibly be weathered original sphernlites of some mineral 

 earlier than the gehlenite. 



In the thin section the basal cleavage is well marked and 

 after it the crystals are often developed in thick tabular form. 

 In thick slides the interference color becomes intensely yellow, 

 reminding one somewhat of the peculiar yellow interference 

 tints of certain epidotes. 



In convergent polarized light the interference cross is wide 

 and uniaxial. Optically negative. On one section the bire- 

 fringence was measured at eo — e — 0*0055. The refractive 

 indices were measured directly on an Abbe Pulfrich total 

 refractometer in Na light and found to be a> = 1*666 ± '003 and 

 e = 1*661 ± *003. These values were obtained by using a 

 polished plate of the granular material, and the refractive indices 

 could not be determined under such conditions with an accuracy 

 greater than ± "003. 



The specific gravity at 25° was determined on two samples 

 by pycnometric methods at 3'029 and 3*049 with an aver- 

 age =- 3*039. Part of this variability is probably the result of 

 differences in relative amounts of inclusions. 



On uncovering a thin section and treating the exposed sur- 

 face with weak hydrochloric acid and then, after thorough 

 rinsing, with a solution of fuchsin, the gehlenite was found 

 to have gelatinized slightly. This fact was corroborated by 

 a chemical test with powdered material, which was found to 

 gelatinize readily. 



The following chemical analysis does not agree with any 

 simple formula and a comparison of other gehlenite analyses 

 indicates that under the term gehlenite a solid solution series 

 between several different end members is probably included. 



Compared with the other analyses, the Velardena gehlenite 

 is somewhat lower in silica and magnesia, and higher in 

 alumina and lime, but otherwise very similar, and is essentially 

 a calcium aluminum silicate. 



