Hijdrous Silicate of Manganese and Calcium. 501 



Physical properties.— Single crystals of orientite are 

 transparent except where clouded by disseminated oxides. 

 The crvstalline druses are bright and sparkling and are 

 light brown in color when the crystals are free from 

 impurities. In the majority of specimens, however,^ the 

 crvstals themselves contain more or less finely divided 

 black impuritv and consequently range from dark brown 

 to almost black in color. The crystalline-granular mas- 

 sive material is dark browm in color with pitchy to dull 

 luster. The luster of the crystals is more or less resinous. 

 Occasionallv they are iridescent to submetallic or metallic 

 in luster due to very thin exterior coatings of a steel-gray 

 manganese oxide. '^ The powder is light hair-brown and 

 the streak slightly darker. The mineral shows a very 

 imperfect cleavage parallel to m{110) and probably also 

 a still less perfect cleavage parallel to c(OOl). It is brittle 

 with a hardness of 4.5-5. The mean of _ four closely 

 agreeing determinations of the specific gravity is 3.05. ^ 



Optical properties: Orientite is biaxial positive with 

 the optic plane parallel to c(OOl). The optical orienta- 

 tion is : 



X = G Y = c Z = & 



Crystals of the tabular or type 2 habit lie on the face of 

 6(010) and yield a symmetrical interference figure indi- 

 cating that the obtuse bisectrix is perpendicular to this 

 plane or that the acute bisectrix is perpendicular to 

 a (100). Type 1 crystals often rest on a face of m(llO) 

 and then yield a figure showing a perfectly centered optic 

 axis indicating that the optic axes coincide with the 

 normals to the prism faces; therefore 2V = 67°; 2E 

 (calculated) = 156°. The pleochroism is marked : X, red 

 brown; Y, yellow; Z, brownish yellow. Absorption 

 X>Z>Y. The dispersion is pronounced but omng to the 

 large value of 2E the bars of the optic axis are so straight 

 that the direction of curvature is difficult to distinguish. 

 So far as could be determined the dispersion is p<v. The 

 indices of refraction are high while the birefringence is 

 moderate. The values for the indices of refraction and 

 the birefringence are as follows : 



a=1.758 ^1^1.776 7=1.795 y— a=.037 all ±1.005 

 Composition and chemical properties. — Crystallized 

 orientite is unattacked by cold dilute hydrochloric acid 

 but is readily soluble in hot hydrochloric acid with evolu- 



Am. Jour. Sci.— Fifth Series, Vol. I, No. 6.— June, 1921. 

 34 



