o/" Copper and of Iron. i 3 g 



SECTION II. 



Arfen'iates of Iron. 



Mattrell mine, which is immediately contiguous to Huel 

 Gorland mine, in the county of Cornwall, has produced 

 fome Specimens of arfeniates of copper e.xaclly fimilar to 

 thofe defcribed in the former part of this paper. But this 

 mineisltill more interefting to mineraK)aifts, on account 

 ot a coiiibination found therein, of arl'enic acid with iron, 

 and alfo a double combination of that acid with both iron 

 and copper. 



The firit-mentioned of thefe arfeniates feems analogous lo 

 thofe cryftals, or cubes, of a fine green colour, of which fome 

 Ipccimcns had already been found in Carrarach and Tmcroft 

 mines, and which Klaproth, in his Memoir upon the Mine- 

 ralogy of Cornwall, confidered as belonging to the arfeniates 

 of copper ; but, according to the analyfis made by Mr. Che- 

 nevix, with all the care which his extenfive knowledge and 

 extreme zeal for fcience would naturally lead him to employ, 

 it appears to be a true arfeniate of iron, containing only a 

 fmall quantity of copper ; and even that quantity ieems to 

 be merely an accidental mixture. As, in the fpecimcns from 

 the old mines of Tincroft and Carrarach, the greatefl part 

 of the cryftals adhered to vitreous gray copper ore, it is pof- 

 lible that fome particles of that ore remained attached to the 

 cryftals; or, as I liave frequently found to be the cafe, that 

 fome fuch particles had penetrated into the cryftals, and that 

 Mr. Klaproth had been thereby deceived, by finding in 

 the button left by the blowpipe a much greater proportion 

 of copper than this ore really contains. The natural decom- 

 pofuion of this arfeniate, which produces an oxide of iron of 

 a fine rcddifli yellow colour, ftrongly confirms the refult of 

 Mr. Chenevix's analyfis. 



Gmelin, in his Principles of Mineralogv, printed at Gut- 

 tlngen in the year 1790, had already fuppofed that thefe cryf- 

 tals could not belong to the fubftance which, in minera!o(j;ical 

 publicatioiisi, had been called arfenical copper ore. He had 

 confequently feparated them, leaving them, however, among 

 the ores of cf)ppcr, under the name of ivurfel criz. 



The double combination of the arl'enic acid with iron and 

 copper, although it had appeared to cxift in the arfeniate 

 juft fpoken of, in the mines of Tincroft and Carrarach, had 

 not excited the attention of mineralogifis. Jt is however 

 noffible that the tranfpareiicv, the brilliancv, and the pale 

 blue colour of its crvftafs, might occafion tliem to be miflakeii 

 for cryftali of a llony nature. liciidcs, their Imalluefs miglit 



calily 



