382 Allen and Crenshaw — Sulphides of Zinc, 



always the Stable form which conies out of the alkaline solu- 

 tions, while the unstable is obtained only from the acid solutions. 

 The alkaline solutions never give rise to any other than the 

 stable forms, while the acid solutions may give rise to either 

 stable or unstable or both, according- to conditions. 



This statement is subject to only one qualification, viz., our 

 synthetic black crystalline mercuric sulphide was not positively 

 identified as metacinnabar though it was probably that. 



AV T e find in chemical literature one well attested instance of 

 the same rule. Schoch * finds two crystalline oxychlorides of 

 mercury of the composition HgCl 2 .2HgO. One of these is red 

 and changes readily into the other, which is black by the action 

 of solutions of alkali carbonate or chloride. The latter form is 

 therefore the stable one, at any rate at low temperatures. The 

 conditions for the formation of these two bodies are essentially 

 identical, except that for the preparation of the red form 

 (unstable) the solution must be slightly acid while for the 

 black form (stable) the solution must be slightly alkaline. 



The geologic evidence so far as it is at hand seems to agree 

 remarkably well with the above facts. Sphalerite, pyrite and 

 cinnabar are primary minerals ; marcasite, metacinnabar and 

 apparently wurtzite are characteristic secondary minerals. As 

 explained above, pyrite, sphalerite and cinnabar maybe formed 

 from acid solutions by hydrogen sulphide under certain condi- 

 tions, consequently we are not surprised to find that they may 

 be secondary as well as primary. There is one fact concerning 

 the occurrence of marcasite and metacinnabar which should 

 be mentioned. Both are sometimes associated with calcite. 

 Whether or not they are paragenetic is a doubtful question. 

 (Lindgren.) If the calcite is not subsequent to the sulphides 

 one would be led to suspect that the original solution- must 

 have contained bicarbonates and sulphates. The synthetic 

 Avork would lead us to expect pyrite from such a solution ; still 

 the quantity of free acid required for pure marcasite is very 

 small at low temperatures and it may be that the reaction 

 FeS0 4 + H 2 S + S=FeS 2 + H 2 SO, 1 would produce marcasite when 

 the initial concentration of acid was no greater than it is in 

 calcium bicarbonate solutions. It would probably be difficult 

 to verify or refute this by experiment. We are usually obliged 

 to work in the laboratory at higher temperatures and in more 

 concentrated solutions than nature does, in order to get any 

 crystals at all in our limited periods of time. 



It will be interesting to see whether other similar cases of 

 polymorphism (or isomerism) exist where the crystal form is 

 determined by the chemical composition of the solution. 

 * Am. Chem. Jour., xxix, 335, 1903. 



