Manganese mines of St. Marcel, in Italy. 289 



that of muscovite. Cleavage plates are flexible and somewhat 

 elastic. No distinct crystals have been observed but Breit- 

 haupt mentions plates with traces of hexagonal outline. The 

 mica occurs as an aggregate of plates, imbedded in a soda rich 

 pyroxene, which will be described later on. The largest 

 cleavage plates that have been observed are about 15 mm in 

 diameter. When examined in convergent polarized light some 

 show a uniaxial others a biaxial interference figure, the double 

 refraction being rather strong and negative. Breithaupt men- 

 tions a uniaxial interference figure, but in a footnote states 

 that at the same locality a second mica is found, somewhat 

 lighter in color, which is strongly biaxial. The uniaxial char- 

 acter is undoubtedly the result of twinning, for plates can be 

 found, which in some parts are uniaxial, in others biaxial, and 

 where the plane of the optical axes is in two different posi- 

 tions. Some, moreover, give a confused interference figure, as 

 would be expected from two or more biaxial plates, placed one 

 above the other in twin position. There is no difference in 

 specific gravity between the uniaxial and biaxial plates, they 

 are identical in behavior before the blowpipe and there is 

 every reason for believing that they are the same chemical 

 substance. Two measurements were made of the divergence 

 of the optical axes as follows : 



2E . y . (Na flame) = 5*7° 0' and 56° 5' 



The dispersion is distinct, /o>u. 



When treated with a mixture of strong hydrofluoric and 

 sulphuric acids the biaxial plates show monosymmetric etching 

 figures while those of the uniaxial plates are triangular or 

 hexagonal, similar to those of muscovite and biotite, described 

 by H. Baumhauer.* The percussion figure is a normal six 

 rayed star. The mica is of the first class according to Tscher- 

 mak, the plane of the optical axes being at right angles to 

 the symmetry plane, as shown by the etching or the percus- 

 sion figure. The color will serve as one of the best means of 

 identifying this mica. It is a deep brownish-copper red ; the 

 intensity of which in transmitted light, varying of course with 

 the thickness. Cleavage pieces have something the color of 

 clear chips of almandine garnet. The basal cleavage gives a 

 peculiar pearly luster and a sort of coppery metallic tone, like 

 some varieties of red foil. Breithaupt compares the color to 

 lamellar cinnabar and its resemblance to some of the foliated 

 zincite from Franklin, !N. J., is very close. The fine powder 

 is pale pink. What is rather remarkable for a colored mica is 

 that its ratio of absorption is so nearly alike in all directions 



*Sitzb. Akad. Munchen, 1874, p. 245 and 1875, p. 99. 



Am. Jour. Sci. — Third Series, Yol. XLYI, No. 274.— October, 1893. 

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