PRECIOUS METALS 53 



gold-bearing belts, reach the sea. In the Appalachian belt, there 

 is gold in the schistose rocks. Therefore along the Atlantic 

 coast it is manifestly present in the sea water. Australia has 

 many gold deposits, and meteoric waters flowing in rivers to the 

 sea naturally carry some gold to the sea. 



The compounds of gold occurring as minerals of economic im- 

 portance are few. The combination is generally with telluruim 

 as petzite, (Ag,Au)2Te, in which the silver and the gold vary some- 

 what but represent the unit structure in the mineral. 



Hessite shows no gold in the formula, Ag 2 Te, but gold may re- 

 place the silver to a considerable extent. There is theoretically 

 at least every gradation between the true telluride of gold on 

 the one hand and the telluride of silver on the other. In the 

 telluride of gold, silver is generally present and in the telluride of 

 silver, gold is usually found. Calaverite, sylvanite, krennerite 

 and nagyagite are also important sources of gold. The last named 

 mineral is best catalogued as a sulpho-telluride of lead and gold. 

 Kalgoorlite and coolgardite are examples of the tellurides of gold, 

 silver and mercury. Gold occurs also as an amalgam with mer- 

 cury, and as an alloy with copper, bismuth, platinum and rhodium. 



A word may be said here with reference to the occurrence of 

 gold as a sulphide. The proofs of its occurrence as such are 

 inadequate. The sulphide of gold is exceedingly unstable when- 

 ever the precipitation occurs in chemical laboratories or the labo- 

 ratory of nature by the action of hydrogen sulphide upon the 

 neutral or slightly acid solution of the metal. Its instability 

 favors its immediate reduction to the elemental state, provided 

 any reducing agent is present. 



The sulphide of iron which is an exceedingly common associate 

 with gold serves as such a reducing agent. Therefore whenever 

 gold is found encased in pyrite, it is always present as free gold 

 and not as a sulphide. Therefore the existence of auric sulphide, 

 Au 2 Ss, in nature may be questioned. 



Gold is found widely diffused in nature although one of the 

 scarcer metals. It appears both in the igneous rocks and the 

 sedimentaries, and manifests itself in the metamorphic rocks both 

 of igneous and sedimentary origin. When occurring in igneous 

 rocks the ore is of primary origin; when in fissure veins it is of 

 pneumatolytic origin; when in sedimentary rocks it is of secondary 

 origin. Its favorite gangue mineral is quartz often associated 

 with fluorite and pyrite. Its occurrence in the granites of Mexico 



