212 iS. L. Penfielcl — Pyrite crystals from French Creek. 



tended they would give a tetragonal pyramid like fig. 2, except 

 that fig. 2 has been somewhat shortened by the curved nature 

 of the faces. The curious forms which we have been consid- 

 ering I prefer to regard as abnormally developed trigonal-tris- 

 octahedrons. That they are really isometric is proved by the 

 occurrence of the ordinary pyrite form tt(210), i(*-2). The 

 behavior of one of the curved crystals on the reflection goni- 

 ometer is also quite striking. Measuring from pyramid to 

 pyramid over the vertical axis the very points gave sharp reflec- 

 tions of the signal and then on turning the crystal there followed 

 an unbroken band of light, with no sharp reflection of the 

 signal, as long as different parts of the curved surfaces were in 

 a position to reflect the light. The angle between the sharp 

 reflections of the signal, obtained from the very minute flat sur- 

 faces at the_ points, was found to be 109° 36', calculated for 

 o^o (111 /sill) 109° 28'. We see from this that our steep | pyra- 

 mid at the base, becomes by the curving gradually flatter till it 

 corresponds to a unit pyramid or octahedron at the vertex. 



The specific gravity of two of the crystals represented in 

 figs. 2 and 4 was found to be 5,016 and 5,022 respectively. 



I would also mention here a specimen from French Creek 

 which has been in Professor Brush's collection since 1850. 

 The pyrite crystals which are implanted on magnetite are very 

 flat trigonal- trisoctahedrons, appearing at first sight like octahe- 

 drons. These are the only crystals which I have seen from 

 this locality where the vicinal faces form a distinct and clean 

 cat pyramid on the octahedron faces. The faces, however, are 

 not perfect enough to give a distinct and single reflection with 

 the goniometer. The angle between two of the faces is not over 

 3° 50' and consequently the value of m in the symbol ma, a, a 

 cannot exceed ^f although it is useless to assign a definite sym- 

 bol to faces where the angle cannot be measured more exactly, 

 and where a slight variation in the angle causes a very decided 

 change in the symbol. 



Mineralogical Laboratory, Sheffield Scientific School, Dec. 18th, 1888. 



Additional note. — Very recently Mr. Geo. L. English of Philadelphia sent me a 

 suite of French Creek pyrites from his private collection containing six of the 

 elongated pyramids, mostly of the fig. 4 type, also a number of cube octahedron 

 and pyritohedron combinations which are modified and rounded by the occurrence 

 of vicinal faces and one crystal, forming a sort of connecting link between an 

 octahedron and the fig. 2 type, where the octahedron and some trigonal trisocta- 

 hedral faces round off and blunt the apex of the pyramid. He also informs me 

 that the isolated crystals occur imbedded in calcite. s. l. p. 



