FMF - Friends of Minerals Forum, discussion and message board

[alfredo]

[Jordi Fabre]

Very interesting topic Alfredo, so I created a new thread with it! ;-)

Jordi

[Jim Prentiss]

Hi everyone,

When I acquired my first Fluorite, it has octahedral habit, and one of the first things I read about it was that it was because it formed at higher temperature.

Maybe, this too could be applied, as Alfredo has indicated with the Galena.

Well that is my two cents worth for the day.

Jimp

[Peter Megaw]

I'm digging deep into memory here, but at one time it was a major fashion in mineralogy to attempt to correlate crystal habits with temperature, pressure, composition etc. In some cases I think it was borne out, but in most the exceptions quickly swamped the rule. In the case of octahedral galena for example, they are abundant in some mines in the Tri-State and Viburnum Trends...low temperature deposits for sure, so if there's a link to habit it would have to be compostitional.

There is published literature on the arsenic-octahedral pyrite link, but not sure if it stands up today. It may be more specific with the link reduced to octahedral pyrite commonly occuring with enargite.

The problem with octahedral fluorite correlating with temperature is that many high temperature systems (skarns especially) have both octahedral and cubic pyrite, and cube-octahedral is very common.

There is also the common association of bladed calcite (flattened rhombohedra) in boiling epithermal vein systems. In a vein it is a good indicator of the supra-boiling zone, but its presence in any other environment is indicative of nothing special.

My guess is that Peter Richards is quite familiar with this literature and its current level of scientific acceptance.

[John S. White]

Fred Pough was a strong advocate of the theory that with fluorite the habit was a function of the temperature at which it crystallized. In fact, he makes this claim in the various editions of his mineralogy handbooks. But, I agree with Pete that the exceptions are far too numerous to make this theory viable.

[Pete Richards]

Peter Megaw and John White said it right.

There are many studies that report differences in habit as a function of some environmental condition, including temperature of formation. The problem is that many different factors can have similar impacts on crystal habit, including, apparently, rate of growth, relative concentration of chemical constituents (e.g. Iron and sulfur concentrations), presence of impurities in the growth solution, growth from liquid vs. vapor or super-critical fluid vs. magma vs. metamorphic environment, as well as temperature of formation. Some of these factors may be inter-related, so that what appears to be a pattern related to impurity concentrations might actually be an effect of temperature (to make up an example). While crystallographers have had some success in predicting what habit a particular mineral should have, based on its crystal structure, the theories often fail miserably for specific minerals, and generally give very little insight into why a particular mineral has one habit in one place and another habit in another place.

For pyrite specifically, the connection between octahedral habit and higher temperatures is often suggested, though some other factor may be responsible for the octahedral habit than temperature. But it is clear that octahedral pyrite is often found in distinctly low-temperature environments as well - sedimentary environments where the temperature almost certainly never exceeded 100° C. Certainly the octahedral habit cannot be used as a qualitative geothermometer!

Similarly, it seems reasonable that pyrite forming at higher temperatures might incorporate more of certain impurity elements than that forming at lower temperatures. However, it would take a great deal of subtle analyses to establish a connection between octahedral habit and impurity concentration, and even if such a connection were established it would not prove that the impurities CAUSED the octahedral habit.

This is an area where we have some interesting general ideas about how different factors can affect crystal growth and morphology, but I suspect we'll only very rarely be able to confidently say why a particular mineral has a particular habit at a particular locality. Nature guards her secrets well!