358 Palache and Wood — Crystallographic Study of Millerite. 



q, (6174). This form rests on two readings made on a single 

 crystal. This crystal was the richest in forms of any measured. 

 The agreement is not very bad. The form is at least indicated. 



Crystal. Signal. , Headings. > 



<t> 01 P 



83° 05' 23° 05' 32° 00' 



97° 33' 22° 27' 32° 00' 



22° 46' 32° 00' 



22° 24' 31° 47' 



2 



poor 



2 



very poor 



Mean 



poor 



Calculated 





w, (4265). Only one reading supports this form. It in con- 

 sequence is very doubtful. 



Crystal. Signal. , Headings. , 



5 fair 70° 26' lO ^' 21° 47' 

 Calculated 10° 53' 21° 48' 



Resume. 



The Orford millerite has yielded a number of terminated, 

 measureable crystals from which the new axial ratio a : e — 

 1 : 0*2183 is calculated. In addition to the forms previously 

 known, one new prism, two rhombohedrons and two scaleno- 

 hedrons are definitely established and a number of uncertain 

 forms observed but not established. 



The presence of a gliding plane parallel to the negative 

 rhombohedron, along which pressure twins of great perfection 

 can be easily obtained, is also determined. 



Millerite has been thought to be hemimorphic like the nearly 

 related cadmium sulphide, greenockite, and has been so classi- 

 fied by Groth. The sharply defined trigonal character of many 

 of the prisms of Orford millerite seem to give some basis for 

 this assumption ; and doubly terminated crystals were therefore 

 eagerly sought for in the hope that the question might be settled 

 by definite observations. A few crystals doubly terminated were 

 found but they were poor crystals and so far as measurements 

 could be made upon them showed no unlikeness between the 

 two ends. Our evidence on this matter is therefore not con- 

 clusive. 



On the surface of a single hand specimen of very rich green 

 chrome garnet which was originally covered with a thin layer 

 of calcite, there appeared on the removal of the latter with acid 

 a number of tiny clusters of crystals of a gray to white metallic 

 mineral which seem to be rammelsbergite, an arsenide of nickel. 

 The crystals are minute and invariably so deeply striated that 

 no satisfactory measurements could be obtained. Still inasmuch 

 as this mineral has never been found before in crystals that per- 

 mitted of measurement except in one zone, a number of the 

 crystals were studied carefully and a provisional axial ratio was 

 calculated for the species. It was found to be like the other 



