212 Allen, Crenshaw, Johnston, and Larsen — 



iron in an atmosphere of nitrogen, obtained 1197° ±2°. Fried- 

 rich, who took considerable pains with the composition of his 

 material, preparing it by the fusion of synthetic pyrrhotite 

 with the calculated quantity of iron, found an average of 1171°.* 



Crystals of pyrrhotite. — Measurable crystals of pyrrhotite 

 were readily formed by the action of hydrogen sulphide on 

 solutions of ferrous salts in glass tubes which contained some 

 air. The yield was small and it cannot be stated, in the absence 

 of optical tests, that the different crystals were identical in 

 composition ; in fact, Mr. Larsen finds crystallographic evi- 

 dence that there were differences of composition. The manifest 

 impossibility of finding the composition of individual crystals 

 and of proving that the crystals of any one preparation were 

 identical, made it useless to attack the interesting question, 

 how the angles in crystals of the pyrrhotite series vary with 

 composition. Crystals were prepared at 80°, at 225°, and at 

 some other temperatures, therefore presumably above and 

 below the inversion temperatures, but in the absence of direct 

 evidence on either pure ferrous sulphide, or pyrrhotite of any 

 composition (see p. 206) this can not be stated with certainty. 

 Mr. Larsen believes there is good crystallographic evidence for 

 two crystal forms, the upper form orthorhombic, the lower 

 hexagonal. 



Troilite and its relation to pyrrhotite. — The knowledge of 

 troilite is defective on account of the scarcity and poor develop- 

 ment of material, but the analyses on record, and its behavior 

 towards acids, prove that it is essentially ferrous sulphide, con- 

 taminated generally with the sulphides of cobalt and nickel. 

 The mineral is found only in meteorites embedded in a matrix 

 of metallic iron, and it is evidently the contact with free iron 

 and not the peculiar conditions of meteorite formation which 

 accounts for the lack of dissolved sulphur in troilite. It is 

 probable that the stony portions of meteorites contain ordinary 

 pyrrhotite.f 



Lorenz^: has described a method for the preparation of 

 ferrous sulphide (artificial troilite) which depends on the same 

 principle, i. e., formation in the presence of excess of iron. 

 Metallic iron is simply heated to redness in hydrogen sulphide. 

 Crystalline crusts of silvery luster which are easily detached 

 from the unchanged metal, are produced in this way. Analyses 

 of these crusts by Lorenz approached ferrous sulphide, but not 

 closely. The only determination of sulphur gave 37 per cent, 

 which corresponds to 0'85 per cent of dissolved sulphur. Two 

 products were made by us in this way. They agreed with Lor- 



* Loc. cit. 



t Dana, A System of Min., 6th Ed., p. 73. 



JBer., xxiv, 1501, 1891. 



