P aiaclie and Wood — Crystallogra/phic Study of Millerite. 359 



members of the marcasite group in habit and the flat dome 

 which was found on all measured crystals was taken by analogy 

 with arsenopyrite as the brachydome (014). The prism zone 

 was so deeply striated that no reliance could be placed on the 

 readings made from it ; but an orthodome was found on nearly 

 all the crystals very faintly developed but which gave fairly 

 uniform readings for p. This was taken as (102), a choice that 

 gave the simplest indices for the prism forms and a ratio most 

 nearly like that of the other members of the group, although 

 differing widely from any of them. The observations follow : 



Rdmmelsbergite f . 



oi 



av. of 7 



, i 



> = 



0° 00', 



P 



= 16 



l° 06' 



\Po 



— 2 



•0176 





|0 



av. of 4 



, 4 



. = 90° 00', 



P 



— 4t 



i° 15' 



W» 



= 1 



•1545 









a : 



b:c 



= o- 



572 



22 



: 1 : 



1-1545 









Form. 



i 



Calculated. 









Measured. 





No. of 



G. 



Miller 







p 









p 



i 



observations, 



GC 



010 



00° 



00' 



90° 



00' 





00° 



00' 



90° 



00' 





6 



00 



110 



60° 



13' 



90° 



00' 





60° 



44' 



90° 



00' 





3 



oo 2 



120 



74° 



02' 



90° 



00' 





74° 



26' 



90° 



00' 





4 



2 oc 



210 



41° 



09' 



90° 



00' 





42° 



30' 



90° 



00' 





2 



oi 



014 



00° 



00' 



16° 



06' 





00° 



00' 



16° 



06' 





I 



o* 



013 



00° 



00' 



21° 



03' 





00° 



00' 



20° 



30' 





2 



ol 



012 



00° 



00' 



29° 



59' 





00° 



00' 



29° 



45' 





1 



02 



021 



00° 



00' 



66° 



35' 





00° 



00' 



66° 



50' 





1 



io 



102 



90° 



00' 



45° 



15' 





90° 



00' 



45° 



15' 





4 



The amount of the mineral present on our specimens was so 

 small that sufficient for analysis could not be secured. Its 

 doubtful determination as rammelsbergite is based on blow- 

 pipe reactions for arsenic and nickel obtained on minute crystals. 

 No test for sulphur could be obtained. It is hoped that more 

 and better material may ultimately be obtained which will 

 enable the character of this mineral to be definitely established. 



EXPLANATION OF THE FIGUEES. 



Fig. 1 shows a photograph of a specimen from which calcite has been partly 

 removed by solution in hydrochloric acid. Several large straight prisms of 

 the millerite may be seen still partly embedded in the calcite. On the surface 

 of the pyroxene matrix may be seen several smaller broken and twisted 

 millerite crystals. 



Fig. 2 shows a twin crystal produced by pressure, twinned parallel to the 

 rhombohedron e, (0112). The upper termination is formed by single planes 

 of the two cleavages, parallel to r and e ; the lower termination is by a single 

 plane of the r cleavage. 



Fig. 3 shows in plan and perspective the commonest type of crystal found. 

 The proportions of the rhombohedrons may vary but some or all of the faces 

 of the three are generally found. 



Fig. 4 and Fig. 5 show the mode in which the scalenohedrons occur in 

 combination. Of these forms u is much the more frequent, s having been 

 observed on but two crystals. 



Harvard Mineralogical Laboratory, 

 June 1904. 



