200 Genth and Penfield — Contributions to Mineralogy. 







Measured. 



Calculated. 



Ca.HL, 



001 a no 



92° 31' 



92° 48' 



c *p, 



001 A 111 



42° 45' 



42 e 46' 



C a 0, 



001 aIII 



40° 15' 



40° 18' 



b a p, 



010 .111 



72° 53' 



73° 2' 



p a X, 



111 ^ 101 



25° 59' 



26° 0' 



Ca X, 



001 ~ 101 



36° 26' 



36° 25' 



x /\ n, 



101 a 1:21 



38° 24' 



38° 26' 



c a d, 



001 aOU 



28° 32' 



28° 33' 



c~h, 



001 ~012 



16° 25' 



16° 30' 



c^f, 



001 ^021 



52° 27' 



52° 28' 



a a b, 



cleavage 



82° 39' 



82° 42' 



The pinacoids a and b were vertically striated and in combi- 

 nation with vicinal faces so that no satisfactory measurements 

 were made in the vertical zone. The faces at the ends of the 

 crystals, although small, gave very good reflections of the 

 signal, the result of which can be seen in the very satisfactory 

 agreement in the above table between the measured and the 

 calculated angles. The cleavage is very perfect parallel to the 

 pinacoids a, 100 and b, 010. The mineral also occurs in radia- 

 ting, bladed crystalline masses, with cleavage surfaces some- 

 times 35 mm long and 8-10 mm wide at the broadest portion. 

 The angle between the two pinacoids a and b, 82° 39' in the 

 above table was obtained from this material, the reflection 

 from both cleavages being very sharp and distinct. Wulfing 

 gives for two faces in the prismatic zone (he does not state 

 that they are cleavage) 81° 53', 82° 11', 82° 38' and 83° ll'. 



The color is a brownish red, amaranth-red. The optical 

 properties agree closely with the determinations made by 

 Wulfing."* Crystal or cleavage plates parallel to the pinacoid 

 a, 100, show under the polarizing microscope a brownish red 

 color, and very little action on parallel polarized light, but 

 with convergent light an optical axis and a bisectrix can be 

 seen, slightly removed from the center of the field, also part of 

 the ring system of the other axis. The plane of the optic axes 

 makes an angle of about 38° with the vertical axis, its trace on 

 100 being from right above to left below. The pleochroism is 

 not very strong, the color being darker in the direction of the 

 plane of the optical axes than at right angles to it. One of 

 these cleavage plates was used in the axial angle apparatus, and 

 although it was not at right angles to the acute bisectrix, it 

 yielded a measurement of the apparent optic axial angle in air, 

 which is very characteristic, 



2E for yellow, Na flame, 63° 3' 

 2E for red, Li flame, 59° 3' 



The section was practically opaque to the green light of a thal- 

 lium flame. The strong dispersion of the optic axes p < u is 



* Loc. cit. 



