82 



MINERALOGY. 



persulphide are frequent and familiar minerals, Iron-pyrites being 

 conspicuous for its sharply defined forms, and Marcasite, or ""White 

 Iron-pyrites," for the fantastic groupings in its crystallisation that 

 have obtained for it the various names of Spear pyrites, Cocks- 

 comb pyrites, &c. 



Molybdenite (MoS 2 ) and Realgar (As 2 S 2 ) are severally molybdenum 

 and arsenic disulphides; the former a rhombohedral, the latter an 

 oblique mineral. 



Here also is included Laurite, the rare ruthenium sulphide. 



Among the trisulphides we find some important compounds of the ( c .) Trisu; 

 triad elements crystallising in the orthorhombic system. They are phides, 8 

 Orpiment, or arsenic trisulphide (As 2 S.J, and the two isomor- 

 phous trisulphides of Antimony and Bismuth, Bismuthite (Bi 2 SJ 

 and Antimonite (Sb.^SJ. Of both the last minerals, and in par- 

 ticular of Antimonite, very fine specimens are in this table-case. 

 Antimonite is an important source of the metal Antimony. 



Tho third section of the division is composed of minerals wherein cer- Section 1 

 tain arsenides, &c, of Section i. are combined with sulphides of Section Arseno-si 

 ii., or which may be looked on as the result of a replacement of half P Lldes ' 1 

 the Arsenic of the minerals in the former section by its equivalent of 

 Sulphur. Of these there is a cubic series, including Cobaltine, or 

 Cobalt-glance, the " Silver White Cobalt" of early mineralogists, a 

 Cobalt Sulphide with part of the Sulphur replaced by Arsenic and 



part of the Cobalt by Iron j" (Co,Fe)(S,As) 2 j . In Gersdorfite or 



Arsenical Nickel-glance, half the Sulphur is replaced by Arsenic, and 

 in Ullmannite or Antimonial Nickel-glance by Antimony and Arsenic. 



In this section, also, the minerals of this chemical type exhibit a 

 dimorphism similar to that of Pyrites and Marcasite among the disul- 

 phides of Section ii., and of Bammelsbergite and Chloanthite among 

 the diarsenides of Section i. ; for in Mispickel and Glaucodote we 

 find arseno-sulphides of Iron and of Cobalt with Iron of the same 

 chemical type as Cobalt-glance, but crystallised in the orthorhombic 

 system. Thus the three homotypic series of cubic diarsenides, di- 

 sulphides, and diarsenosulphides belonging to the three sections 

 of this division might be treated as a single group, while the three 

 corresponding orthosymmetric series maybe looked on as another such 

 group. 



To the fourth section are assigned minerals wherein metallic sul- Section hi 

 phides are so combined with sulphides of Arsenic, Tin, Iron, &c, as Snlphur- 

 to produce a series of sulphur-salts, in the constitution of which Sul- 

 phur plays the part which Oxygen plays in the ordinary oxygen-salts. 

 This section is a numerous one in point of species, and the following 

 are a few minerals included in it that are especially worthy of note. 



In one (and that a somewhat ambiguous) class of these Salts, Iron, 

 either as an iron sesquisulphide (Fe 2 SJ or an iron persulphide 

 (Fe S,), would seem to enter as a constituent of the " acid" ingredient. 

 In this class we meet with two important copper ores, the largely worked 

 Chalco-pyrites or Copper-pyrites, and Erubesciteor Purple Copper-ore. 



