34: 



IOWA ACADEMY OF SCIENCES. 



are filled with small crystals varying in size from almost microscopic dimensions 

 to a centimetre or more in length. They are brown in color, varying on the one 

 hand to greenish brown, and on the other to nearly black, especially those which 

 are weathered. Their crystallographic properties are interesting on account of 

 the different kinds of symmetry they have approached by the excessive development 

 of planes in certain zones. They are all rhombic-dodecahedrons, the edges of 

 which are occasionally truncated by minute icoistetrahedrons never large enough 

 to alter the general appearance of the crystal. In addition to the regularly formed 

 dodecahedrons four different types have been noticed. 



First — The four planes normal to the plane 

 of the lateral axis are elongated, as in Fig. 1, 

 giving the crystal an apparent tetragonal sym- 

 metry, and resembling a combination of the 

 unit, prism, oo P, with the pyramid of the 

 second order P. co- But as all the angles are 

 either 90" or 120" there is no doubt but that 

 t is a modification of the rhombic dodeca- 

 liedon. Fig. 1. 



Second— In this case six planes are elongated, 

 as in Fig. 2, so that the crystal assumes the sym- 

 metry of the hexagonal system, and appears to be 

 a combination of the hexagonal prisim as oo P. 

 and the rhomboliedron R. The angles are here 

 also 120", just what they should be for the hex- 

 agonal prism, making the resemblance all the 

 more striking. These two figures are similar to 

 two of those given for garnets in Dana's System 

 F'f^- '•^- of Mineralogy, p. 266. 



Third— I'lais case differs from the first 

 given in the excessive development of two 

 of the faces resembling the pyramid of the 

 second order, as is shown in Fig. 3. In 

 this way it seems to have a monoclinic 

 symmetry, and to be formed by a com- 

 bination of the two lateral pinacoid faces, 

 00 P55 and cxdPqc w'ith the positive and 

 negative pyramids + P. ■ Pj„_ 3 



Fourth— ]n this case the six faces are elongated as in 

 Fig. 2, and also two of the, faces resembling the rhom- 

 bohedrons, while the third is very small, as in Fig. 4, 

 giving the crystal apparently the monoclinic symmetry, 

 and seeming to be composed of the clinopinacoid, 00 

 PS, the unit prism 00 P, the clino dome, P 00, and 

 the plus orthodome -j- Po), a combination which is 

 not contrary to the monoclinic symmetry. Fig. 4 is 

 drawn in this position in order to illustrate the pseudo 

 symmetry, and should be rotated 45*^ to the left if c is 

 placed vertically. 



I'^is. 4. Small fragments of the mineral examined in polar- 



ized light, with the nicols crossed, transmit some light, showing that the optica 

 anomalies so common in garnets occur here as well. 



