»dpSfield.] ADDITIONS TO THE ALUHTTE-JAKOSITE GROUP. 35 
PLTJMBOJAROSITE. 
This material is from Cooks Peak, New Mexico. Jt occurs as a 
listening, crystalline powder and as loosely cohering masses which 
nay easily be crushed by pressure between the lingers. The crystals 
ire very symmetrical, and are exactly like those of natrojarosite (tig. 
[}, although generally only one rhombohedron, r, is present. On the 
iverage the crystals are a trifle smaller and noticeably thinner than 
hose of natrojarosite. A number of crystals were measured on the 
meeting goniometer, the chief difficulty arising rather from the 
ffoinal character of the faces than from their small size. One 
musually large crystal, 0.28 mm. broad and 0.015 mm. thick, was 
Inally found, having the development shown in tig. 2, which is 
inusual, for generall} 7 r (10ll) and not « (0221) is the prevailing rhom- 
ohedron. Fortunately the crystal was so taken up on a minute point 
f wax that the measurement of s^s in three 
hombohedral zones was possible. The results /C^-^^^i 
f ti ve measurements of s s over the upper and x) r .s 
pwer pole edges varied between 109° 5' and Fl(i . 2.— piumbojarosite. 
09° 30', the average being 109° 16'; while 
ix measurements over the middle edges varied between 70° 10' and 
L° 00', the average being 70° 36'. The average of the two supple- 
mentary values gives s^s, 2201^0221=109° 20', which has been 
■>sumed as fundamental, and from it the following axial ratio has 
een calculated: 
c= 1.216. 
n the crystal from which the foregoing measurements were obtained 
le basal plane was vicinal and hence no reliable measurements of 
could be had from it. On a number of other crystals, however, 
e angle of c^r was measured with varying results, the variation 
suiting from the uncertainty of the reflections from the basal planes. 
ur measurements of c^r, which were recorded in the notebook as 
rived from the best reflections, varied between 54° 15' and 54 14', 
e average being 54° 30', while c^r, 0001^1011, by calculation from 
e fundamental measurement, is 54° 32'. Hence it may be assumed 
at the axial ratio as established is reasonably exact. The calculated 
lue of /w, 10IUT101, is 89° 42'. 
In polarized light the crystals exhibit normal optical properties and 
gative birefringence. Being on the average thinner than crystals of 
Urojarosite, it is seldom that, with the highest powers and conver- 
\ nt light, even the beginning of the first ring of the uniaxial inter- 
irence figure is visible. Individual crystals show under (he micro- 
ope in transmitted light a golden-yellow color. A mass of crystals 
s the appearance of a glistening dark-brown powder, the color being 
(cidedly darker than that of natrojarosite. 
