NO. 1801. MISCELLANEOUS MINERALOOWAL PAPERS— POOUE. 



577 



scope alongside of similarly prepared specimens of known marcasite 

 and pyrite. The pseudomorphs are compact and homogeneous, with 

 no perceptible core of pyrrhotite. The structure is similar to that of 

 the marcasite examined, but the color was found to be intermediate 

 between the two standard specimens, but on the whole inclining more 

 to white than to yellow, and distinctly white in the case of the smaller 

 crystals. The comparisons were, of course, made after cleaning with 

 hydrochloric acid. The color, therefore, is the only doubtful point 

 in the sure determination of the mineral as marcasite; but a careful 

 weighing of the evidence and consideration of the work of Julien," 

 Stokes,^ and others renders the identification as marcasite reason- 

 ably conclusive. 



The determination of the crystal form as that of pyrrhotite is based 

 upon the identification of the faces c(OOOl) and 0(2021) (see figs. 5 

 and 6, pi. 63) according to the following data: 



c : z. 



z : z. 



Average of 8 meas- 

 urements. Re- 

 flection goniom- 

 eter. 



63° 52' 



Average of 5 meas- 

 urements. Re- 

 flection goniom- 

 eter. 



52° 46' 



Average of 7 meas-'Average of 5 meas- 

 urements. Con- urements. Con- 

 tact, i tact. 



64° 



53° 



Theoretical for 

 pyrrliotite. 



63° 32' 

 53° 11' 



The measurements were obtained with difficulty, as the crystals 

 were poorly adapted for giving reflections. The signals for the 

 smoothest faces were blurred and the centers had to be approximated. 

 In some instances the reflection was aided by the application of a thin 

 coat of shellac to the faces to be measured; in other instances signals 

 were obtained from minute pieces of glass cemented to the faces; but 

 such measurements were only used when checked around the entire 

 crystal. A general coincidence from the several methods and agree- 

 ment with results obtained by contact offset the unreliability of any 

 individual measurement. The possibility that the crystal form is 

 orthorhombic, with faces new for marcasite, was considered. A com- 

 bination of a new brachydome (021), with a new pyramid (433), gives 

 an approximate hexagonal girdle; but the calculated angles'^ for such 

 forms are not in close agreement with the measured values. 



The specimens were obtained from the oxidized zone of an iron 

 mine in the Carboniferous Mountains of the Teutoburger Wald, near 



a A. A. Julien. On the variation of decomposition in the iron pyrites; its cause and 

 its relation to density. Annals N. Y. Acad. Sci., vol. 3, 1884, pp. 365-404; also, vol. 

 4, 1888, pp. 133-223. 



& H. N. Stokes. On pyrite and marcasite. Bull. 186, U. S. Geol. Surv., 1901, 



c(001):(021)=67° 57^ 



(001):(433)=68°20^ 



(021):(433)=55° 19^ 



(433):(433)=55° V. 



Proc. N. M. vol. 39—10 39 



