554 WATSON AND WHERRY: PYROLUSITE FROM VIRGINIA 



III. Pyrolusite crystals from Bikonhalli, Shimoga district, Mysore. 



C. S. Fawcitt, analj^st. (Memoirs Geol. Survey India, Pt. I, 

 p. 82. 1909.) 



IV. Pyrolusite crystals (longish, dull, prismatic crystals of nearly 



square form) from Ghatia in Banswd,ra State, Rijputdna. C. 

 S. Fawcitt, analyst. (Memoirs Geol. Survey India, Pt. I, p. 

 82. 1909.) 

 V. Pseudomanganite, bronzy luster, from Sandur Hills, India. C. S. 

 Fawcitt, analyst. (Memoirs Geol. Survey India, Pt. I, p. 84. 

 1909.) 



From the analyses it will be seen that the resemblance in com- 

 position of the crystals and crystalline mineral from Virginia is 

 remarkably close, and the correspondence of each to the Indian 

 pyrolusite is evident. The physical characters of each are alike 

 and, as shown above, are those of pyrolusite. Each shows the 

 presence of a small quantity of manganous oxide and about 1.5 

 per cent of combined water, which would usually be interpreted 

 as the remnants left in the change of the mineral from manganite 

 to pyrolusite, on the basis that the pyrolusite is pseudomorphous 

 after manganite. The water, on the other hand, might be ac- 

 counted for on the basis of hydration. Such alteration from man- 

 ganite to pyrolusite has been generally regarded as easily ac- 

 complished and involves, chemically, dehydration and oxidation, 

 accompanied by a change of physical characters, the more im- 

 portant of which are increased specific gravity, decreased hard- 

 ness,* and darker (black) streak. 



Judged from the few detailed chemical analyses of pyrolusite 

 which the writer has examined, such small percentages of MnO 

 and combined H2O, when taken alone, are of little value as 

 diagnostic features of the pseudomorphism of pyrolusite after 

 manganite. (See statement on Indian pyrolusite below.) Allien 

 combined, however, with accurate tests of physical properties, 

 including crystal form, the evidence should be conclusive. 



Attention is directed in the table to analyses III and IV of 

 picked specimens of pyrolusite crystals described by Fermor 



8 Alteration to polianite, which has the same composition as pyrolusite and is 

 sometimes pseudomorphous after manganite, would involve not only increased 

 specific gravity but also increased hardness. 



