210 S. L. Penfield — Pyrite crystals from French Creek. 



and admit of only approximate measurement with the contact 

 goniometer. The crystals have a remarkably perfect geometri- 

 cal development, that is, similar faces are developed to almost 

 exactly the same size and extent. 



The first three crystals to be described, which are in the 

 Bement collection, appear as tetragonal pyramids. By meas- 

 urement of the interfacial angles over and near to the middle 

 edges the faces were found to be steep enough to cut the ver- 

 tical axes at l - 25, l - 50 and 1-80 respectively, but owing to the 

 curving the distances at which the faces actually intercept the 

 vertical axes are less. Figures 1, 2 and 3 represent the three 

 crystals, drawn with the same length of the lateral axes and 

 with the pole edges straight for a short distance from the lat- 

 eral axes and steep enough to cut the vertical axes at 1*25, 1*50 

 and 1*80 respectively, but curved toward the top so that the 

 vertical axes are really cut at 1'16, 1*25 and 1*50 respectively, 

 according to actual measurement of the diameters of the crys- 

 tals. The crystals are of good size and measure in the direc- 

 tion of the vertical axes respectively, 22, 22 and 33 mm . 



The remaining crystals are perhaps more interesting owing 

 to the occurrence of pyritohedral or pentagonal dodecahedral 

 faces, which in all of the crystals occur only at the extremities 

 of the lateral axes. The faces are rough, but approximate meas- 

 urements with the contact goniometer determine the crystals to 

 be the ordinary pyrite form e, 7r(210), ${i-2). The pyramid is in 

 all cases the curved § form, r, like fig. 2. The pyramid faces are 

 always striated near to and about the front pyritohedral faces, 

 the strise being a little steeper than the combination edge 

 between e and r and having about the direction of the combi- 

 nation edge tt(421), -|(4-2) and r. The pyritohedral faces have 

 very different shapes at the extremities of the two lateral axes 

 and the crystals, having only three symmetry planes, resemble 

 orthorhombic forms. The two crystals in the Brush collection. 



