218 Allen, Crenshaw, Johnston, and Larsen- 



III Crystallographic Study. 



As the sulphides of iron are all opaque, the ordinary optical 

 tests are not available and the mineralogic study is confined 

 to the determination of the color, luster, cleavage, magnetic 

 properties, crystal form, chemical properties, etc. Color and 

 magnetism are important diagnostic properties of the sulphides 

 of iron, while the crystal form and the chemical properties 

 give positive evidence of the identity of the artificial prepara- 

 tions with the natural minerals. In general the synthetic iron 

 sulphides prepared in the dry way or by the inversion of one 

 form into another in the solid state are massive and without 

 crystal form, and only the color, magnetic and chemical prop- 

 erties can be determined. The iron sulphides precipitated 

 from solutions usually consist of a network of crystals, or of 

 drusy crusts. Some preparations have crystals which are large 

 enough for measurement on the Goldschmidt goniometer ; 

 most of the crystals measured are from 0"2 to 04 mi11 in length, 

 though one crystal of /5-pyrrhotite is about a millimeter in 

 length, and one crystal of marcasite is even a little larger. In 

 many of the preparations the crystals are all very minute — 

 less than a tenth of a millimeter in length and often much 

 less — and the crystallographic study was confined to an exam- 

 ination of the material under the microscope. The crystal 

 habit, however, of each of the four minerals is characteristic, 

 and even the minute crystals can usually be determined. 



Pyrite. 



In some preparations the larger crystals of pyrite are half a 

 millimeter across and can be easily recognized with a pocket 

 lens by their color and crystal form. Much smaller crystals 

 can be determined by the use of the microscope. The crystals 

 usually show both the cube and the octahedral faces, but both 

 forms also occur alone ; pyritohedrons were not observed ; the 

 faces are always much warped and very imperfect. For the 

 several crystals measured on the goniometer, the angles were 

 seldom over half a degree from the theoretical value, and a 

 closer agreement cannot be expected. 



Marcasite. 

 The color of the synthetic marcasite is identical with that of 

 the natural mineral, but the color test is not altogether satisfac- 

 tory on minute crystals, even when brightened by heating in 

 acid. The crystal habit, however, is characteristic, and the 

 goniometric measurements of some of the larger crystals show 

 a very close agreement with those of the natural mineral. 



