FMF - Friends of Minerals Forum, discussion and message board

[Richard Arseneau]

Hello, I purchased a Fluorite specimen today and am curious about the small white nodules on the surface. The piece is from Huanggang Mine in China. The purple nodules are obviously Fluorite. Are the white nodules also Fluorite? Also, perhaps a more difficult question, what kind of change in the chemistry at work during the formation of this specimen could cause Quartz to stop forming and fluorite to start? I have been collecting Fluorite for a while now, I think this is the most intriguing specimen I have.

Thanks,
Richard

[Richard Arseneau]

After some research, I am inclined to believe the white nodules are very small Suttrop-type quartz crystals. So It seems the piece has had at least three distinct periods of growth. The original quartz formation, after which something changed and the surface was covered with Suttrop-type quartz, and finally, the fluorite formed. Any thoughts on this line of reasoning?

Thanks,
Richard

[Peter Megaw]

I love specimens like this...they tell interesting stories.

There are lots of explanations for why this happened....all generally relating to changing conditions in the void it grew in...but the question is how radically did the changes have to occur to affect the differences noted.

1. The paragenesis of fluorite on quartz is quite common. In simplest terms it reflects a shift from conditions favoring growth of quartz (which could reflect exhaustion of SiO2 in the ore fluids) to those permitting growth of fluorite (lower temperature). We can use phase diagrams built in several different ways (temperature, pressure. Redox, XCO2, XF etc.) to improve understanding, but none will tell us for sure what happened.

2. The shift in style of quartz growth potentially has a different genesis. Crystal growth depends on an interplay between growth rate and nucleation rate. For big crystals you need a high growth rate on few nucleii. If the nucleation rate increases the SiO2 gets distributed across many small crystals and large single crystals can't grow anymore.(this is not what many intuit...that big crystals take a long time to grow...the opposite is true...big crystals have to grow quickly before other nucleii steal the ingredient supply).

3. It is possible to argue that this piece reflects different stages along a simple path. Big quartzes grew until the nucleation rate exploded causing finer grained quartz to take over until most of the SiO2 present was consumed and things cooled enough for Fluorite growth to begin (if the tiny balls are fluorite this could reflect a similar nucleation rate change in fluorite.)

4. However, given what we know about the type of ore deposit this came from, we know these systems often have multiple pulses of ore fluids of differing compositions with resulting crystal growth happening under differing conditions, so the big textural and compositional changes could reflect a new fluid pulse (the banding of the finer quartz stage suggests several pulses within one stage). You can even get fluid mixing with groundwater dominant fluids collapsing onto hydrothermal fluids rising from depth...in which case the fluorite might come from a different source from the quartz.

[Richard Arseneau]

Thanks for the reply! Although I didn't understand every term, I took a lot from that explanation. As always there is no one simple answer, but I have a better understanding of some of the factors involved. Certainly enough to continue investigating in new directions.

I am also intrigued by the story being told by this piece., thanks again.

Richard

[Peter Megaw]

Sorry for the jargon Richard...happy to reduce things to real language.

[Richard Arseneau]

No need to be sorry! I would much rather receive a technical answer than an uninformed one. The biggest thing that was new to me was phase diagrams and some of the variables involved. Overall, I found the post very informative and thought-provoking.

[Robert Seitz]

I really appreciate the specimens in my collection with multiple generations of growth This whether well formed crystals or slabs cut from zones of mineralization (or not) in orebodies, etc.They really do tell stories.