Melanterite and Chalcanthite Groups. 



227 



is optically positive, 2 V is near 90°, and the dispersion of 

 the optic axis is slight. The fibers are imperfect and 

 measurements of extinction angles gave inconsistent 

 results. Several sections nearly normal to X and per- 

 haps lying on a cleavage face gave extinction angles, Y to 

 the elongation, of about 34°. The mineral is probably 

 monoclinic and Y to the elongation is large. 



The principal optical properties of zinc-copper melan- 

 terite, goslarite, pisanite, and bieberite are given in Table 

 I for comparison. In optical properties zinc-copper 

 melanterite is very similar to pisanite, but it has a some- 

 what lower birefringence ; it has a different color and a 

 somewhat lower birefringence than melanterite and bie- 

 berite. Goslarite is distinctly different from the others 

 and obviously belongs to a different group. The mono- 

 clinic form corresponding in composition to goslarite has 

 not been described. 



Table I. Optical properties of members of the melanterite group and of 



goslarite. 



Zinc-copper 

 melanterite 



Pisanite 8 



Melanterite 



Bieberite a 



Goslarite 



Crystal System 



Mon.? 



Mon. 



Mon. 



Mon. 



Orth. 



Color 



pale green- 

 ish blue 



pale blue 



pale green 



Carmine 



colorless 



Opt. character 



+ 



+ 



+ 



_ 



— 



2V 



near 90° 



very large 



86 



near 90° 



46 



Dispersion 



p > v perc ? 



p > uperc? 



p > v perc. 



slight 



p < v slight 



a 



1-479 



1-472 



1-471 



1.477 



1 457 



P 



1-483 



1-479 



1-478 



1-483 



1-480 



7 



1-488 



1-487 



1.486 



1-489 



.1484 



7- a 



•009 



•015 



•015 



•012 



•020 



a Unpublished data on artificial mineral by E. S. Larsen. 



Chemical analysis. — The zinc-copper melanterite se- 

 lected for analysis showed under the microscope a little 

 admixed chlorite and sulphides and about 1 or 2 per cent 

 of zinc-copper chalcanthite. The result of an analysis of 

 the water soluble portion of this sample by M. L. Glenn 

 is shown in Table II. The formula derived from this 

 analysis is (Zn,Cu,Fe)O.S0 3 . 7H 2 in which Zn : Cu : 

 Fe : : 100 : 98 : 19. The analysis fits the formula very 

 well except for the water and it is probable, as will be 

 shown later, that the mineral had partly dehydrated to 

 zinc-copper chalcanthite between the time it was examined 

 microscopically and the time it was analyzed. 



