a JVeio Merctiry Mineral from Terlingua, Texas. 205 



Crystallography and Optical Properties ( W. T. Schaller ). 



The crystals are apparently octaliedra of the isometric sys- 

 tem. Twinning on the spinel law has been observed several 

 times, in which case tlie crystals of mosesite are often flattened 

 parallel to a pair of octahedral faces. Mr. Canfield has men- 

 tioned a S-ling, a spinel twin repeated five times. 



The faces of the crystals are never smooth. While bright 

 and often highly polished, they are uneven and only a few of 

 the faces give a single reflection. Most of the faces when 

 measured on the goniometer give a number of signals, no 

 particular one of which is brighter than the others. This has 

 rendered the accurate determination of the form of the crystals 

 impossible. An additional feature that has rendered accurate 

 measurements ditficult is the tendency for a number of crystals 

 to grow together, sometimes in nearly parallel position and 

 again in widely different positions. A mass of signals there- 

 fore resulted from examining such groups of mosesite on the 

 goniometer, and on the minute crystals that were available it 

 was not possible to know what signals to measure and what 

 ones to exclude. Where only two or thi'ee faces were meas- 

 ured in a zone, it was well nigh impossible to be sure of 

 having the zone accurately centered. The measurements, the 

 occurrence of (probable) spinel twins and the optical relations, 

 to be described below, make it fairly certain that the crystals 

 of mosesite are in reality octahedra. No form other than the 

 octahedron was observed. 



On part of a spinel twin, the faces of which gave a mass of 

 very poor reflections, there were measured : 



( = 68" 70° 32' calc. 



^ o' .J = 68° (cleavage face) " " 



( — 68° 28' " " 



o ^ o^ = 41° 13' 38° 56' " 



A second crystal, also a spinel twin, gave : 



I 



,}= '71° 38' 70° 32' " 



I = 68° 51' " " 



I = 72° 28' " " 



\= 37° 06' 38° 56' " 



^ '^ _ (- 38° 56' " " 



A third crystal was adjusted as closely as possible in polar 

 position for two-circle measurement and on revolving the 



108° 54' .... 109° 28' calc. 



107° 53' " 



