FMF - Friends of Minerals Forum, discussion and message board

[Tom Mazanec]

Do wildfires ever "heat treat" gems?

[alfredo]

Heat treatment requires very slow heating and cooling to avoid cracking crystals by thermal shock. I think wildfires probably come too fast.

[Riccardo Modanesi]

Hi to everybody!
A Bunsen flame is normally used to treat gemstones by heating. It works with pale brown topazes (turning to pink), amethysts (turned to citrine) and weak grey tanzanites (turning to vivid blue and/or violet). I had an experience in Idar-Oberstein some decades ago.
Greetings from Italy by Riccardo.

[mmauthner]

Well, Alfredo, I am not sure about that. In fact, I am convinced that wildfires can produce heat-treated crystals. That said, I am not convinced the results in the case I have in mind were desirable.

Many moons ago, I was digging for quartz crystals at Crystal Park in Montana, I found an area that had a number of crystals in the soil/decomposed granite, but they were devoid of color---completely colorless. As I dug deeper, the crystals I found had color, which improved with depth. I did not apply strict scientific methods at the time; that is, I did not get out a tape measure and exactly record the depth of each find, but my memory has it that the colorless crystals occurred in the top 40-50 cm and the best color was at about a meter depth (one was only allowed to dig to 4 feet). When I stepped back to look at the "section", I noticed that the "zonation" was also defined by charred and non-charred roots. Remembering what I had been told about baking amethyst in bread to avoid thermal shock in heat treatment, I came on the idea that a wildfire had raged here (also evidenced by older charred stumps, etc) and crystals within the top 40-50 cm got hot enough to lose their amethyst-smoky color.

I also remembered that in the case of smoky quartz and amethyst, heat treatment reverses the effect of natural irradiation that causes this coloration in quartz, GIVEN pre-existing chemical conditions in the quartz. I then took things further, in that I had a contact back at the University that operated a Co-60 autoclave, and had him re-irradiate a couple of the specimens, and LO! the colour came back! The interesting thing is, and this perhaps speaks to the pre-existing conditions I mentioned---the resulting color was somewhat pale, so we tossed the pieces back into the autoclave for improvement (an entire weekend) but got no more color out of it, so I believe that the color we achieved is actually the 'original' color.

By the way, we performed all kinds of cool experiments with the autoclave...for my thesis, I tested the tenebrescence of the spodumene from my thesis area: pink to green, back and forth several times, I think we were also the first to produce the raspberry red apatite from Pakistan (very early 1990's). A friend had left his apatite on window sill and the pink color completely disappeared after a few months, and after talking to him about my experiments with my thesis apatite, he asked if I would bring back the color on his apatite. Well, I confidently told him 'sure,' of course thinking it would also bring it back to 'original' color. Well,...surprise!!

Unfortunately, again, I did not apply rigorous scientific method in that I did not record exact exposure durations, radiation strengths, Brand name of the autoclave, take pictures of the hole in Montana, complete with survey rod, etc, etc. so I never formally wrote up my findings, as was later suggested to me by another scientist who lived in a publish or perish world.

Anyway, a long answer to say that, yes, forest fires can affect the color of minerals.
Best,
Mark

[Amir Akhavan]

Interesting observation, Mark. 300-400°C would be enough and pale the crystals within a few hours, higher temperatures would work much quicker.

[alfredo]

Mark, I think that would be a rare and very localized occurrence in Nature - perhaps in some spot where a large tree fell and burns for a few days, or where campers have built an all-night fire in a fire ring. But in general the heat of a forest fire only penetrates a couple cm into the ground. Google soil temperature profile in wildfires.

[James Catmur]

I tend to agree with Alfredo. I find it hard to believe a wildfire could affect a mineral's color

[Bob Morgan]

As a fire fighter I would not be surprised at the temperatures that might occur. I have no particular knowledge of wildfires, but the temperatures in house fires commonly rise above 2000 F. Too much water applied produces unbelievably high temperature steam that pushes downward and penetrates even fire gear.
There. I've shared my ignorance.

[Matt_Zukowski]

In soils with peat or other undecomposed organic matter (a large volume percent of dead tree roots perhaps), forest fires move underground, and so would the thermal effects. One sees this in Alaska and Siberia where forest fires move underground for the winter and then reappear above ground when the surface soil dries out.

It seems to me an open question as to how rare it is to find tree roots penetrating a xl-bearing regolith in sufficient quantity and in a way that can burn and cause the phenomena observed by Mark.