What kind of rock/stone was that?

About a week ago I went camping with some friends. We’ve been to Alsace in France if that is to any importance to my following question. We build a stone oven to make some delicious “Tartes flambées” (a bit like Pizza). In order for the oven to be heated we made a campfire around and under it (the groundstone was a little elevated). Until now everything was as usual ,we’ve built these kind of ovens many times.

Soon after the fire was lit we started to hear loud bangs from the oven. It is usual to hear light cracks or even bangs from stones when you heat them like that, but these sound were really loud, like gunshots. We continued to fire the oven because we thought there were some big tensions in one of the stones and everything would be okay when these tensions were gone. But thing got worse, the groundstone would start shooting little stones all over the place. Sometimes bigger pieces of the stone fell into the embers of the fire and these pieces would start exploding at about one bang a second! We saw marble-sized pieces hit or tent which was situated about 8 metres/yards away from the fireplace. So we let the fire go out when the ground stone completely broke apart and had and evening without a warm meal or a campfire to sit around. Fortunately none of us got injured.

What kind of stone was that that? I have seen a lot of fireplaces (with stones around them) and never seen such a spectacle. Neither has one of my friends, most of them rather experienced “outdoor guys”.
The stone has a light redish colour and the surface structure is rather uneven (not like e.g. flintstone which has a very smooth surface).

Nils

sounds like it might be some kind of granite.

No granite for sure.

  1. It’s not red.
  2. It’s very compact.

My guess would be a rock that is very brittle and had a lot of water in its cracks. This would make it explode, when the water is heated fast. I have seen similar things happen (although not that violent) when you throw small stones in the fire. Usually sedimentary rocks, not granite though. That would also fit the geology of Alsace.

** Is there not such a thing as red or pink granite?

How on earth could it have been granite. I’m from the granite capitol of the US and I have fired oven like this on several occasions using granite and nothing of the sort has ever happened. I’d assume you were firing some form of ignacious or sedamentary stone that had sulfer or graphite that was firing in the stone.
Granite it known for it’s strength, and stability…not for having holes that water can get trapped in.

Yes Mangetout and it’s found in France to boot!. Though I can not see it firing either.

I’ve been an avid rock hound my whole life and i’d still vote against granite.

I don’t know what type of rock it is, but would second T. Mehr’s response.

From http://www.campnecessities.com/building_a_fire.htm
Flat rocks might be a shale like material that can have air pockets that can heat up and explode the rock. Similarly, don’t get rocks out of a stream bed or lake, these rocks could have water pockets in them that could explode.

Actually there are several varieties of Pink granite from all over the world

Usually the main three ingredients are
feldspar, quartz and mica. But I take ** Phlosphrs** word for it, that there could be some other stuff that colours it red.

I was under the impression that granite could explode when heated, based on a memory of reading that it was worked in ancient times by heating and cooling. I think I might just have been wrong about all of this though.

Phlosphr some nice link. Learnt something today!

Amberlei what you say about rocks out of a stream bed is exactly what I
have experienced, as posted above. We allways used to have a fire on the banks
of the river, and those stones would explode more often than not when you threw
them to the fire. And they were sedimentary rocks, too.

I took two pieces of that particular rock with me. I will keep them inside to let them dry. If they still explode when dry I at least ruled out water as the reason. But I guess it takes quite a long time for stones to dry, so I hoped any of you dopers had some good ideas what it could have been. So maybe sulfer or graphite could be other reasons?
Unfortuantly I have no digital camera to make a photo of that stone, probably this would help.

Could you describe them a little more fully? - Are they coarse and rough like a brick; Are they crumbly or flaky? Would you describe them as especially heavy for their size? Is there any kind of ‘grain’ or lines of stratification? etc…

There’s no way of knowing without a better description (esp. with respect to the mineralogy).

Regarding granite, there are only three essential minerals: Quartz, Alkali Feldspar, and Plagioclase Feldspar. Usually Alkali Feldspar > Plagioclase. The sodic plagioclase in granite is typically white to light grey; alkali feldspar (microcline, e.g.) is often pink-red, but can also be light grey.

Mica (muscovite or biotite), hornblende, or other minerals (topaz, garnet, etc.) may or may not be present as accessory minerals depending on the chemistry of the granite (oversaturated in Aluminum = Muscovite, Topaz, and/or Garnet; “normal” = Biotite and/or Hornblende).

Here’s the IUGS (read: “OFFICIAL”) classification of coarse-grained igneous rocks. Note that Q = Quartz, A = Alkali Feldspar, and P = Plagioclase. No other minerals are needed in this scheme and that the relative proportion of Q, A, and P are normalized to sum to 100%:

“Identify a rock for a man, he’ll know what that rock is; teach a man how to identify rocks, and he’ll quit asking you to identify rocks for him.” Or something like that.

There’s no way of knowing without a better description (esp. with respect to the mineralogy).

Regarding granite, there are only three essential minerals: Quartz, Alkali Feldspar, and Plagioclase Feldspar. Usually Alkali Feldspar > Plagioclase. The sodic plagioclase in granite is typically white to light grey; alkali feldspar (microcline, e.g.) is often pink-red, but can also be light grey.

Mica (muscovite or biotite), hornblende, or other minerals (topaz, garnet, etc.) may or may not be present as accessory minerals depending on the chemistry of the granite (oversaturated in Aluminum = Muscovite, Topaz, and/or Garnet; “normal” = Biotite and/or Hornblende).

Here’s the IUGS (read: “OFFICIAL”) classification of coarse-grained igneous rocks. Note that Q = Quartz, A = Alkali Feldspar, and P = Plagioclase. No other minerals are needed in this scheme and that the relative proportion of Q, A, and P are normalized to sum to 100%:

“Identify a rock for a man, he’ll know what that rock is; teach a man how to identify rocks, and he’ll quit asking you to identify rocks for him.” Or something like that.

Well they are coarse and rough, but not like a brick. Definately not crumbly or flaky The weight seems just right for their size. There are very little inclusion in the stone that blink a bit when I move the stone in the light of my desk-lamp. I’m not sure if I see stratifikation, but there is some kind of a longish texture on the side of the stone. There are also little cracks along these textures, but I don’t know if they have been there before or if they were caused by the explosions. There are also cracks not aligned with the stratifikation.
Before the explosions the rock was about 40cm x 30 cm x 20cm (16 x 12 x 8 inches).
I guess I’m not that good in describing stones…

Maybe I should add, that most of the pieces that were catapulted out of the fireplace had very sharp sides, I later used on of them to cut some strings (I didn’t have my knife with me and they were lying around the place).

Sounds like late cooling metamorphic stone similar to this .

How many colors were there? just two? or more?

Metamorphic stone is a stone that has been changed during it’s forming from one thing into a conglomerate of different things. Graphite can be found in it. And it’s color spectrum is quite vast. I’m guessing you found a slab of this and tried to cook on it. It could have had some combustable minerals in the mix and it did what it’s properties told it to.

See here for more info. Scroll down and read the properties…

Now ** Phlosphr**, conglomerate is defenitely not metamorphic.
And I still belive you need mica in a granite.

Damn it, you DO NOT need mica in a granite!