OF PYRITES AND GYPSUM. 3X1 



well illustrated by Dr. Plot, the historian of Oxfordshire, who in 

 reference to the teim remarks that under this ^^ Genus may be 

 reckon'd not only F-yrites strictly taken, but Flints, Pebbles. Sand, 

 and whatever else by any quick and sudden attrition may have its 

 parts kindled into sparks."^ 



In consequence of readily giving forth sparks by concussion,, 

 pyrites was known to old writers by such names as lapis igniavius 

 and lapis luminis. In Cornwall, where pyrites is common in the 

 veinstones of the copper and tin-mines, it is generally called 

 mundicy a word which Borlase, the historian of Cornwall, derives, 

 though on what authority I know not," from the cleanly shining 

 appearance, both of its surface and structure."" The " shining 

 appearance," or metal-like brilliancy, is a character common to 

 all the minerals which at present pass under the name of 

 Pyrites. 



In modern times the term Pyrites has come to be used by 

 mineralogists as a family-name, embracing a group of native 

 sulphides, all possessing a metallic lustre and a considerable 

 degree of hardness, but not being strictly isomorphous — that is 

 to say, they do not always crystallize in forms which are 

 geometrically compatible with each other. The different kinds 

 of pyrites are distinguished by prefixes, so that we speak of 

 iron-pyrites, copper-pyrites, nickel-pyrites, cobalt-pyrites, and so 

 forth. But since iron-pyrites is far and away the most common 

 member of the family, it is this mineral which is always meant 

 when the word is used without qualification. The name 

 sideropyrites has sometimes been applied to iron-pyrites in order 

 to distinguish it from chalcopyvites, or copper-pyrites. 



Iron-pyrites is chemically a compound of iron and sulphur^ 

 termed iron disnlphide, and we may regard each molecule as 

 containing one atom of iron combined with two atoms of sulphur. 

 Its formula is consequently FeSg and its percentage composition 

 46*6 parts by weight of iron to 53*4 parts of sulphur. It is the 

 sulphur rather than the iron that in most cases gives economic 

 value to the mineral. Immense quantities are used for the 

 manufacture of sulphuric acid — an application of the mineral 

 which resulted from the action of the King of Naples in granting 

 a monopoly of Sicilian sulphur to a French company, in the 



^The Natural History of Oxfordshire. By R. P., LL.D. Oxford, 1677, p. 71. 

 6 The Natural History of Cornivall. By William Borlase. Oxford, 1758. P. 131. 



