Manganese mines of St. Marcel, in Italy. 293 



been derived from the average. It will be seen from the ratio 

 that the MgO and CaO are present in the proportion 1 : 1 as 

 in normal diopside MgCa(Si0 3 ) 2 . Deducting from the total 

 ratio Mn0+MgO + CaO:SiO 2 == '264:264= 1:1, there re- 

 mains Si0 2 : A1 2 3 + Fe 2 3 + Mn 2 3 : JSTa 2 + K 2 G = -646 : 

 •177 : -153 = 400 : 1-09 : 0*94 or nearly 4:1:1, which is re- 

 quired by a metasilicate of the general formula EaR(Si0 3 ) 2 . 

 The mineral may therefore be considered as containing the 

 following molecular compounds : 



Diopside Mo;Ca(3i0 3 ) 2 28*8 per cent. 



" Jadeite NaAl(Si0 3 ) 2 __ 35-7 " 



Acraite NaFe(SiOJ 2 32-5 " 



? NaMn(SiO,) a 3-0 " 



100-0 " 



These results throw some light on the soda and sesquioxide 

 rich pyroxenes, showing that in this case, at least, they are 

 normal metasilicates. Moreover this mineral holds about the 

 same relation to the pyroxenes that glaucophane does to the 

 amphiboles, being also similar in the orientation of its pleo- 

 chroism. 



As far as can be learned, the pyroxene which agrees most 

 closely with this in composition is a jadeite or chloromelanite 

 from Mexico, analyzed by A. Damour* and containing 8*89 

 per cent of Fe 2 3 , equivalent to 25 '7 per cent of the acmite 

 molecule. The color is described as black, in thin section 

 deep green and this is in strong contrast to the Piedmont min- 

 eral, which with a larger percentage of Fe 2 3 is light in color. 

 It is a well known fact that glasses which are colored green by 

 iron can be decolorized by a higher oxide of manganese, and 

 probably in this case the presence of about 1 per cent of 

 Mn 2 3 has served to complement the green color which would 

 be expected from the large percentage of the acmite molecule. 



Violan. 



In 1833 Breithauptf described a mineral of rare occurrence 

 at Piedmont, to which, from its beautiful blue color, he gave 

 the above name. ~No chemical analysis was given but only a 

 description of its physical and pyrognostic characters. 



Des Cloizeaux^: has given a brief description of the crystals, 

 on which he was able to identify in the prismatic zone the 

 forms 100, 010, 110 and 120, as well as a prismatic cleavage, 



*Bull. Soc. Min. de France, 'iv, 1881, p. 157. Also Dana's System of Min., 

 Sixth edition, analysis 26, page 370. 



f Jour. prak. Chetn., xv, p. 329. % Manuel de Mineralogie, 1862, p. 66. 



