Penfield and Kreider — Mineralogical Notes. 143 



off at 100° C. and nearly all of it by heating the fine powder 

 for two hours at 200° C. The water is therefore water of crys- 

 tallization and the mineral is undoubtedly to be regarded as a 

 hydra ted salt of the acid H 2 Mn 2 5 . The acid in question is 

 analogous to di-silicic acid, H„Si0 5 , a normal salt of which is 

 represented by petatite, LiAl(Si0 5 ) 2 . From a more careful 

 examination of the crystals of the mineral it has been found 

 that they are not isometric as stated by 

 Roepper, but exhibit a combination of Flg< 1- 



rhombohedron and base as shown in ^-^~"~cf\\ 

 figure 1, and yield a perfect basal cleav- \~ / \ A 



age. In reality the crystals look much ^ / X \ 



more like octahedrons than would be \\ T I / \\ 

 inferred from the figure, since the faces \ \ / / \ \ 

 are vicinal and the rhombohedral planes \\ / / M 

 curve and flatten somewhat as they ap- \ \ / T \ 



proach the base. No satisfactory meas- \'\ / \ 



urements could be made, the angle \\~/ ~--^£ 



Cs^r, 0001^1011, varying several cle- \\Z— - — " ~~ 



grees, but averaging near that given by 



Moore for chalcophanite. The crystals described by him were 

 scales and rarely plates, showing the combination of g and r. 



We take pleasure in expressing to Prof. B. W. Frazier of 

 South Bethlehem, Pa., our thanks for the loan of a specimen 

 from the collection of Lehigh University which was labeled 

 hydrofranklinite in Professor Rcepper's own handwriting and 

 which was in every respect similar to the material analyzed 

 by us. 



2. On the Separation of Minerals of High Specific Gravity by 

 means of the fused Double Nitrate of Silver and Thallium. 



Mineralogists and petrographers are certainly indebted to 

 Dr. J. W. Petgers for the fortunate discovery* that the nitrates 

 of silver and thallium, when brought together in the molecu- 

 lar proportion of 1 : 1, yield a double salt, which fuses at 75° C. 

 to a clear, mobile liquid, having a specific gravity of about 5 

 and miscible with water in all proportions at temperatures be- 

 tween its melting point and 100° C. The melting point also 

 diminishes rapidly as water is added, going down to 50° or 60° 

 C, and fusion and solubility pass uninterruptedly into one 

 another. We have thus at our command for the separation of 

 mineral particles a liquid far exceeding in specific gravity any 

 of the previously described heavy solutions, and which has the 

 advantage of being practically colorless, neutral, soluble in 

 water and of being readily recovered from the aqueous solu- 

 tion by simple evaporation on the water bath. As suggested 

 by Petgers, separations may be made in test tubes heated in a 



* Loc. cit. 



Am. Jour. Sci — Third Series, Yol. XLVIII, No. 284.— August, 1894. 

 10 



