214 Hillebrand and Penfidd — Additions to the 



masses which maj easily be crushed by pressure between the 

 fingers. The crystals are very symmetrical, and are exactlj^ 

 like those of natrojarosite, figure 1, although generally only 

 one rhombohedron >• is present. On the average the crystals 

 are a trifle smaller and noticeably thinner than those of natro- 

 jarosite. A number of crystals were measured on the reflect- 

 ing goniometer, the chief difficulty arising rather from the 

 vicinal character of the faces than from their small size. One 

 unusually large crystal, 0*28'^'^ broad and 0'015™°^ thick, was 

 finally found, having the development shown in figure 2, which 

 2 is unusuaj, for generally r (1011) and 



not 5 (0221) is the prevailing rhom- 

 bohedron. Fortunately the crystal 

 was so taken up on a minute point 

 of wax that the measurement of 

 Sa^^s in three rhombohedral zones 

 was possible. The results of five measurements of S/^s over 

 the upper and lower pole edges varied between 109° W and 

 109° 30^, the average being 109° 16^ ; while six measurements 

 over the middle edges varied between 70° 10^ and 71° 00', the 

 average being 70° 36^ The average of the two supplemen- 

 tary values gives Sa^s^ 2201^0221 = 109° 20', which has been 

 assumed as fundamental, and from it the following axial ratio 

 has been calculated : 



c = 1-216. 



On the crystal from which the foregoing measurements were 

 obtained the basal plane was vicinal and hence no reliable 

 measurements oi c^^s could be had from it. On a number of 

 other crystals, however, the angle of C/\r was measured with 

 varying results, the variation resulting from the uncertainty of 

 the reflections from the basal planes. Four measurements of 

 G/^T^ which were recorded in the note-book as derived from 

 the best reflections, varied between 54° 15'_and 54° 44', the 

 average being 54° 30', while Ca^v^ 0001/^1011, by calculation 

 from the fundamental measurement, is 54° 32'. Hence it may 

 be assumed that the axial ratio as established is reasonably 

 exact. The calculated value of Ta^v, 1011/.1101, is 89° 42'. 



In polarized light the crystals exhibit normal optical prop- 

 erties and negative birefringence. Being on the average thin- 

 ner than crystals of natrojarosite, it is seldom that, with the 

 highest powers and convergent light, ev^en the beginning of 

 the first ring of the uniaxial interference figure is visible. 

 Individual crystals show under the microscope in transmitted 

 light a golden-yellow color. A mass of crystals has the appear- 

 ance of a glistening dark-brown powder, the color being 

 decidedly darker than that of natrojarosite. 



