452 PROCEEDINGS OF THE NATIONAL MUSEUM. 



in order to learn if the two occurrences were the same the New Jersey 

 material was examined in some detail. The specimens consist of 

 flat slabs of altered coarse-grained diabase, coated with a layer of 

 soft, finely micaceous material of a golden brown color and sub- 

 metallic luster. Other specimens are veins up to 2 cm. wide, bounded 

 by slickensided surfaces and filled with a very light porous aggre- 

 gate of minute scales of a dirty greenish brown color. Some speci- 

 mens are so light as to float upon water. The mineral is soft, barely 

 hard enough to scratch gypsum. Owing to its extremely porous 

 nature the specific gravity could not be determined. Before the 

 blowpipe it fuses with difficulty to a black magnetic slag without 

 exfoliation. It is readily soluble in hydrochloric and sulphuric 

 acids. Boiled in concentrated nitric acid it does not dissolve, but 

 becomes rust brown in color. Under the microscope the material 

 is for the most part in shreds and irregular flakes of minute size with 

 only rarely a suggestion of hexagonal outline. Basal plates are sen- 

 sibly isotropic. Sections perpendicular to the basal cleavage show 

 birefringence colors reaching first order yellow. The mineral ex- 

 hibits marked pleochroism in tones of light and dark brownish 

 green. The mean index of refraction is about 1.63. The larger 

 flakes are almost opaque, a condition due to a dark pigment, probably 

 limonite. The material analyzed was from one of the greenish 

 brown veins. While it was essentially pure, there were left behind, 

 when the mineral was dissolved in acid, a few grains of a greenish 

 insoluble mineral probably augite. The results of the analysis are 

 given below together with Steiger's analysis and the mean of two 

 analyses of stilpnomelane from Westfield. Comparison of the 

 analysis of this mineral with that from Westfield will show that the 

 two are identical, except that the New Jersey specimen has suffered 

 more oxidation than that from Massachusetts. Extreme suscepti- 

 bility to oxidizing influences is one of the most marked characteris- 

 tics of stilpnomelane, the mineral readily passing over into an in- 

 definite, wholly ferric pseudomorph, known as chalcodite, which is 

 of uncertain composition, a part at least of the iron being present as 

 limonite. The change is accompanied by a considerable increase 

 in the index of refraction, and also by a decrease in the percentage 

 of constitutional water with a greater increase in hygroscopic water. 

 The low water content of the present material may be due to loss of 

 hygroscopic water during the years it has been preserved in a drawer 

 in the collections. 



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