NdpSfleld. I ADDITIONS TO THP] ALUNITE-J AROSITE GROUP. 33 
omul having a fairly good basal plane, and from this crystal the fol- 
>wing angles were obtained: 
Calcu- 
Measured. Measured. lated. 
c^ r, 0001 ^1011=51 ° 53' * c^s, 0001 ^0221 =68° 42' 08° 35' 
c^r' } 0001^1101=51 53 c^, 0001^2021=68 48 68 35 
c^r", 0001^0111=52 26 r^, 10ll^Tl01=86 5 85 54 
The crystals belonged to the rhombohedral division of the hex- 
gonal system, and the angle Ca? 1 , 51° 53', which is probably very 
early correct, has been assumed as fundamental, and from it the 
allowing axial ratio has been calculated: 
= 1.104:. 
That the axial ratio as given is very near the truth is shown by the 
ict that the measurements of c ^ s and r ^ r do not vary many min- 
tes from the calculated values; while on a number of other crystals, 
Measurement of the angle Ca/ 1 , though varying considerably, was 
Hind to be not far from 52°. The angles of c ^ v and r ^ r' of the 
rdinaiy potassium jarosite are 55° 10' and 90° 45', respectively. 
Under the microscope the crystals exhibit normal optical properties. 
Pith a high-power lens and convergent light the thicker ciwstals show 
ic dark cross and the beginnings of the first ring of the interference 
pare. The birefringence is negative. The color of single crystals, 
hen seen under the microscope in transmitted light, is golden yellow. 
any of the crystals show numerous brown inclusions. The color 
town by a mass of the crystals is yelJowish brown, and the material 
istens, owing to reflections from the basal planes of the minute 
vstals. 
Phe material used for the chemical analysis was the purest that 
nld be obtained, although crystals containing tin 4 brownish inclu- 
>ns just mentioned could not be avoided, and there were occasional 
own ferruginous particles mixed with the crystals. The specific 
avity of the material was found to be 3.18 at 30. 5 C. The results 
the analysis by Hillebrand are as follows: 
Ratio. 
Fe 2 < ) :; 50. 98 0. 319 3. 29 
Xa,<>" 6.03 .004| , 0] 
K,<> 85 .004/ 
s< > : , 30. 90 . 387 i. oo 
H,( ) below 105° 12 
H 2 above 105° 11.08 .01:; 6.33 
As 2 5 20 
Si0 2 28 
CaO (M 
99.94 
)f the soda 0.22 per cenl is not extra del by hoi water a ft it full ignition of the mineral, and hence 
belong to a feldspar or pome other foreign mineral. Only 5.81 percent is assumed to belong to 
jarosite and used in deriving the molecular value. 
Bull. 202—05 8 
