298 Schaller — Ch<i,iic<il Composition of Mohjbdic Ocher. 



be scratched off the quartz was not quite pure and contained 



a trace of limonite The sample examined gave 24 - 3 



per cent of sesquioxide of iron, some of which was certainly 

 mechanically mixed with it.' 1 Yet it would necessitate a mix- 

 ture of nearly one-third limonite to give 24'3 per cent Fe.,0 3 , 

 while according to Genth, the sample only " contained a trace 

 of limonite." The analytical result was correct, but the inter- 

 pretation wrong. The sample doubtless contained a trace of 

 limonite, as the 24*3 per cent is slightly higher than the figure 

 required for the formula Fe 2 3 .3Mo0 3 .7iH„0, which gives 22 

 per cent Fe„0 3 . 



Physical Properties. — The mineral has a fibrous structure 

 and also forms radiating groups. Its color is yellow and the 

 lustre often silky. All of the fibers examined gave parallel 

 extinction. The double refraction is strong and the direction 

 of elongation is always an axis of minimum elasticity. The 

 pleochroism is strong but is masked by the very strong absorp- 

 tion. Normal to the elongation of the fibers, the transmitted 

 light is pale yellow, parallel to the elongation the color is a 

 much stronger yellow. The absorption parallel to the elonga- 

 tion is so strong that thick fibers frequently appear almost 

 black and opaque. These same properties are mentioned by 

 Lacroix* for molybdite from Corsica. 



Crystals of Mo0 3 , prepared by roasting molybdenite in an 

 open crucible, show very decided differences in their physical 

 properties from those of the natural mineral. They are not 

 fibrous but platy and are colorless, non-pleochroic and show no 

 difference in absorption in different directions. 



Chemical Analyses. — The largest sample obtained comes 

 from Westmoreland, New Hampshire, and was very kindly 

 furnished from the Brush Collection of Yale University by 

 Prof. ¥m. E. Ford. Nearly a gram of material was obtained 

 and this was divided into portions of a quarter gram weight. 

 Macroscopically, the specimen looks more earthy and not so 

 finely crystallized as those from some of the other localities, 

 but under the microscope, the material, with its characteristic 

 optical properties, was seen to be homogeneous, free from 

 limonite, and to contain a small quantity of molybdenite scales. 

 The first figure given for the water content, 16*98 per cent, 

 was obtained by weighing the water direct, using the method 

 of glass tubes as advocated by Penfield. The other two figures 

 represent the loss up to 200°, at which temperature all of the 

 water of the mineral is given off. The residue was dissolved in 

 HC1, and after filtering off the insoluble matter, the iron was 

 precipitated by ammonia, filtered off, dissolved in HC1, and 

 reprecipitated and weighed, while hydrogen sulphide was passed 



*Min. de France, iii, 8. 



