220 Allen^ Crenshaw, Johnston, and~Larsen — 



Crystals of niarcasite large enough for measurement on the 

 goniometer were found in several preparations ; most of them 

 are only a few tenths of a millimeter in length, but a few are 

 about a millimeter long. Seven fairly satisfactory crystals 

 from two different preparations were measured. Only one of 

 these is not twinned. The signals were seldom sharp, blurred 

 signals, striated zones, and vicinal faces being the rule. The 

 faces of J 111 } usually gave fairly sharp reflections, but between 

 (111) and (111) there was a more or less complete band of sig- 

 nals, and that for the face (110) seldom stood out sharply from 

 the others. In some of the tabular twinned crystals the faces 

 of • 110 \ which are parallel to the twinning plane yielded bright 

 signals, or else there were several bright signals near these faces. 

 The reflections from the faces of -J 011} were usually bright, but 

 vicinal development often made their exact position uncertain. 

 Between (Oil) and (Oil) there was usually a dim series of sig- 

 nals, but the reflection signals of the faces of {011} did not 

 stand out from the others. The faces of { 101 } gave fairly 

 sharp signals. In one crystal the reflections of the faces of 

 J 772} took the place of those from {111} at the ends of the 

 series of signals of this zone. Most of the twinned, tabular 

 crystals gave a dim but nearly continuous line of signals from 

 the large faces (110) through the face (011) and the corre- 

 sponding face (011 ) to (110) . The faces {101} of both individ- 

 uals lie on this zone, whose symbol is h+l~k. It is poorly 

 developed on the untwinned crystal. 



Crystal angles of synthetic marcasite. — Of the seven crys- 

 tals to be described, the first five were from a preparation of 

 October 16, 1908, and were formed by the action of H 2 S on an 

 acid solution of FeS0 4 for four days at a maximum tempera- 

 ture of 300°. The crystals of this preparation are all of the 

 habit shown in ^. 15. Crystals 6 and 7 were from a prepara- 

 tion of October 28, 1908, and were formed at a maximum tem- 

 perature of 220°. The common habit of the crystals of this 

 preparation is shown in fig. 16, but the pyramids and prisms 

 are often less prominent ; there are some twinned crystals 

 similar to those shown in fig. 15. 



Table IX lists the weighted average of the angles measured 

 for each crystal, the crystal constants of synthetic marcasite, 

 and the corresponding constants of the natural mineral as 

 given by Gemacher and Goldschmidt. 



The agreement shown by Table I between the angles and 

 axial ratios of natural marcasite and those of the synthetic 

 mineral is very good. Both commonly show twinning with 

 (110) as twinning plane, and both have the two unit domes and 

 the pyramids as common faces. The base, however, which is 





