S. L. Penfield — Analcite from Phoenix Mine. 113 



is the case even when the sections are quite thick for micro- 

 scopic purposes, so that in reality there are four separate indi- 

 viduals cut through at that point. Under the microscope the 

 sections show a slight double refraction, but no division into 

 definite fields, as observed by A. Arzruni and S. Koch,* could 

 be observed. The patches of dark and light were very unequally 

 distributed, but the extinction was in all cases at a maximum 

 when the principal isometric axes were parallel to the planes of 

 the polarizer or analyzer, as observed by the above mentioned 

 authors. A section nearer the center of the crystals showed the 

 four lines of separation as above, but no separation parallel to 

 the dodecahedral symmetry planes, as might be expected below 

 the point 6, could be observed. In addition, a rectangular patch 

 surrounded by an opaque rim was observed in the centre of the 

 section, being the section through a central crystal or nucleus. 

 A thick cross section between the points b and c, fig. 1, is given 

 in fig. 3. The outer portion a represents the clear transparent 

 material of the crystal with the four division lines, while in the 

 interior a perfect tetragonal-trisoctahedron can be seen. The 

 material of this inner crystal is transparent, but it has an 

 opaque, white coating, and the whole outline of the crystal can 

 be seen as distinctly as if it were a model mounted in some 

 transparent material. After observing this in the section, it can 

 readily be seen that all of the larger crystals have an opaque 

 center. The transparent exterior portion shows with crossed 

 nicols under the microscope colored patches running in radial 

 directions from the center, the maximum of extinction being for 

 all of the four secants when the principal axes are parallel to 

 the polarizer or analyzer. 



The outer portion would thus seem to be a secondary growth 

 about an older analcite crystal serving as a nucleus. Each face 

 of the original crystal seems to have acted separately in the 

 orientation of the secondary deposit. The three crystals or parts 

 of the crystal in each octant seems to be parallel in molecular 

 arrangement because no lines of demarcation or separation be- 

 tween them can be detected, while the material of the eight 

 octants seems to be not quite parallel in arrangement, as indicated 

 by the division lines and easy separation which were in all cases 

 observed. 



Mineralogical Laboratory, Sheffield Scientific School, May 12th, 1885. 

 *Zeitschr. Kryst., v, 483. 



