O. Andersen — Aventurine Feldspar. 375 



Some Observations on Hematite. 



For comparison with the reflecting lamellae of aventurine 

 feldspar several samples of hematite were examined as to trans- 

 parency, absorption colors and pleochroism. Fine powder of 

 the samples was imbedded in liquid Canada balsam and 

 examined under the microscope. The degree of transparency 

 could be estimated by selecting tabular grains lying on the flat 

 surface, noting the color and then tilting the grains on edge 

 (by moving the cover glass) for measurement of the thickness. 

 The transparency varies within wide limits, some hematites 

 being practically opaque even in the finest powder, others 

 being transparent. For a sample of medium transparency 

 (micaceous hematite from Montgomery County, Pa.) the fol- 

 lowing observations were made : 



Absorption color Thickness 



Perfectly opaque 0*01 mm 



Very dark blood red, almost opaque 0*003 mm 



Deep blood red 0-001 mm 



Grains of various orientation were seen in the sections. 

 Some were parallel to the base and were perfectly isotropic ; 

 others were oriented at angles with the base and showed a 

 strong double refraction with definite extinction directions. 



If the hematite possessed a strong pleochroism this should 

 be observed in the grains showing double refraction. In spite 

 of careful observation, however, no distinct change in color or 

 tint could be seen in any of the grains examined. We, there- 

 fore, conclude that the hematite has little or no pleochroism. 



The absorption colors of the transparent varieties of hematite 

 are very characteristic, blood red, brown, red, etc., according 

 to variety and thickness and are distinctly different from those 

 of goethite, which are much lighter and always a purer brown 

 or yellowish. 



It will be seen that these properties of hematite agree well 

 with the corresponding properties of the reflecting lamellae of 

 aventurine feldspars. 



Summary of the Properties of the Lamellce. 



We may now summarize the data bearing on the identifica- 

 tion of the reflecting lamellae. 



(1) The hexagonal outlines often shown by the lamellae, 

 taken in connection with the optical properties which agree 

 with uniaxial crystals in plates after the base, point to a hex- 

 agonal or trigonal symmetry. 



(2) The absorption colors are the same in the lamellae as in 

 hematite and the lack of distinct pleochroism is also character- 

 istic for both. 



