I have also senn the other rocks of that test. But the one I linked to (Porphyritic Felsic Rhyolite: PFR) comes quite close. Could these stones be found in Alsace? Would a wet PFR behave like that? Or could such a stone have inclusions that are combustible and would lead to explosions? But then other stones that look similar would behave the same, wouldn’t they?
I am not sure if it was a river stone. There was a little brook near our campsite. I didn’t collect that particular rock, so I don’t know where it came from. There also was a heap of stones that were probably used before for campfires. It could have been from there but never been heated enough to explode.
(All rhyolite is felsic, btw.)
Nils, pretty much any rock that is fractured and has those fractures saturated with water could behave as you are describing. In short, “explodes when you put it in a campfire” is non-diagnostic.
Ok. Asuming is was water, that made the rock explode. Then keeping the stone inside should dry the stone. So that when I put it on a fire again, it wouldn’t explode, right? How long does it take to dry such a stone? I’m talking of a fragment of about the size of a normal Coke can (0.33 Liters - 12 oz, 20oz I don’t know what the coresponding American size is).
Wow, I must say, upon closer examination of your latest pic…I wonder if it is a Sedimentary stone with some fossilized organic material in it. Those will blow at fairly low temperatures. When I lived in Arizona we liked to Camp up in Payson, on the Mogollon Rim. Which is strewn with fossils in sedimentary rock. They’d blow when we made stone circles for a fire pit. It’s out there, but possible. Whada-yathink?
I bet it could take years.
Try putting it in an oven, 180 degrees for a few hours. Better put it in a container.
@Phlosphr: I’m now quite sure that it is sedimentary. I used a screwdriver to open up on of the cracks. I was able to loosen quite a large plate. So maybe there could be little fossils inside the stone. I didn’t find (or did’t recognize) any fossil. But I found another on of these little holes and its opposite side, a little “ball”, diameter ~2mm.
At least this theory is nicer than the water-explanation.
@RM Mentock: Do you mean 180 degrees Celsius or Fahrenheit? 180 deg C would make the possible water inside boil and thus lead to explosions, wouldn’t it?
180F, but you knew that. 
Fossils in rock are not usually organic.
And that rhyolite is not sedimentary.
If it is possible to seperate different layers, then it is very propable that it is sedimentary. It still look similar to that rhyolite.
I wish I had a digital camera…
Pantellerite Sorry, I wasn’t in yesterday. I see, you guys have moved back to the OP, so if you consider this a hijack, feel free to ignore me.
According to my books (and the link RM Mentock provided) the Streckeisen Diagram/IUGS scheme classifies igneous rocks by the parts that have a color index of less than 90, so by the lighter parts. The darker parts are not considered in this diagram. There might be some or there might be none - apart from biotit that is always there and is a dark mica. If it’s lacking biotit it’s no longer granite but syenit.
To add something to the discussion: Does anyone know whether oilshale, that NurseCarmen mentioned occures in Alsace? I belive there is some in the northern part of Germany that was even used to extract oil (digging it up instead of drilling and pumping).
T.Mehr, (to continue the hijack), the IUGS classification scheme ignores all mafic (“dark”) minerals because they are not essential in classifying the rock! Rocks with M > 90 use a different IUGS scheme specifically for ultramafic rocks.
Granite ONLY has biotite when the amount of (Ca + Na + K) in the rock (as mol%) is > Al. These granites are termed metaluminous and always crystallize biotite as an accessory phase. Most granites are metaluminous and have biotite, but not all.
If the Grantie is oversaturated with respect to Aluminum (Ca + Na + K) < Al, it is termed peraluminous and–if it crystallizes a mica phase–will crystallize muscovite instead of biotite. These peraluminous granites are granites that lack biotite. Many aplites (sugary-grained granites) are peraluminous and lack biotite.
If the Granite is oversaturated with respect to the Alkali Metals, (Na + K) > Al, then NO mica will crystallize–neither muscovite NOR biotite; we term these rocks peralkalic. Instead of micas, you will see alkali pyroxene (e.g., aegirine-augite), alkali amphibole (e.g., arfvedsonite and riebeckite), and possibly aenigmatite or other oddball minerals. But no micas.
No matter which of these three categories the granite falls in, it is still granite if it has fewer than 90% mafic minerals AND has the appropriate relative amounts of alkali feldspar + quartz + plagioclase feldspar per the IUGS scheme. And that’s it. That’s all. The presence of biotite is not diagnostic of granite–but if found in granite, it is diagnostic of a metaluminous chemistry.
Syentite is a similar, but different rock: if you re-examine the IUGS you will see that it is a rock composed almost entirely of alkali feldspar with minor to absent quartz and plagioclase. Following the scheme I’ve described above, it is possible to have both metaluminous syenties (if biotite is present) and peralkalic syenite (if biotite is absent and alkali minerals are present). Here, once again, the presence of biotite does not justify you classifying the rock as granite–it is a syenite on the basis of the relative proportions of the felspars and quartz.
A correction to my post above:
Metaluminous rocks are defined by:
(Ca + Na + K) > Al; AND
(Na + K) < Al
On further review, I also suspect that T. Mehr might not understand how a ternary diagram is read. My apologies if you do, but it works like this:
The three end-components are Q (Quartz), A (Alkali Feldspar), and P (Plagioclase).
You have in your rock in addition to Q, A, and P, Bt (biotite), Hb (Hornblende), and Mt (Magnetite). To classify your rock, you determine the vol% of each of the components present, which should total to 100%.
Bt, Hb, and Mt are mafic minerals. We sum their abundances together and call this quantity M (for mafic), such that now:
Q + A + P + M = 100%
Since only Q, A, and P are essential and necessary for plotting on the diagram, we drop M. Completely. We don’t even care what M really is. M could be biotite, arfvedsonite, or anything–we just don’t care. (Note, we also drop any other accessory minerals that are not explicitly Q, A, or P).
So, say we start out with Q = 20%, A = 40%, P = 25%, M = 15%. We drop the M and see now that:
Q + A + P = 85%.
To use the ternary diagram, we renormallize each of the three components by dividing them by the new sum:
new Q = Q/85%,
new A = A/85%, and
new P = P/85%, such that
new Q + new A + new P = 100%. From our example,
new Q = 23.5%
new A = 47.1%
new P = 29.4%
THESE are the numbers we plot on our QAP ternary diagram to classify the plutonic rocks. Please note that we dropped M, didn’t care what it really was, and act as though Q, A, and P are the only phases present. If our M = all biotite, this would plot as granite; if our M = all riebeckite, this would plot as granite; if our M = all magntite (unlikely, but), this would plot as granite; if our M = etc…
Pantellerite, (to end the hijack).
Now, either there are slightly different concepts in the nomenclature (US/Europe or general) or I have to throw my book away. It will not even classify aplites as granites but rather as stemming from the pluton and having a very similar chemistry as it.
To excuse my nit-picking, we had to learn a sort of mnemonic rhyme that basically said: Don’t forget the damn mica when classifying granite.
…and thanks for the effort, I knew how to read a Streckeisen.
What book is that? What is its publication date? Looking back, the only cite from you I see is Websters.
I think there has been a “refinement” of the rock classification schemes, to make them more qualitative. And that is what some of the argument is about, I’m sure.
Well, you raise a good point. The classification of igneous rocks was a big problem for a long time. Up until the 70’s, there were almost as many classification schemes as there were igneous petrologists (Irvine and Barragar, Cross et al., etc.) The IUGS subcommision on systematic petrology tried to put an end to that and come up with a single scheme that would be universally accepted–and the Streckeisen QAPF (for plutionic rocks) and the TAS (for volcanic rocks) became that. Obviously, that doesn’t mean that you HAVE to use it, but if you don’t, good luck getting published (unless you can justify using a non-IUGS approved scheme)!
Regarding biotite and identifying granite, it’s like this:
Biotite crystallizes late. If you are looking at an igneous rock and see biotite then you can, with great certainty, say: “biotite is a late-crystallizing mineral and is therefore most common in felsic rocks! Therefore, I am probably looking at a granite (if coarse-grained) or dacite or rhyolite (if fine-grained)!” And more specifically, you are looking at a metaluminous granite, dacite, or rhyolite.
However, the lack of biotite does not preclude the rock from being granite or rhyolite. You might instead notice if the rock has muscovite mica (in which case you’d say “A-ha! It’s a peraluminous granite!”) or some funky mineral like riebeckite (in which case you’d say “A-ha! It’s a peralkalic granite!”)
IUGS aside, here’s a relevant quote from an old source that I have in my collection of igneous petrology texts (yes, you read that right: sad but true).
From:
Wahlstrom, E.E. (1947) Igneous Minerals and Rocks. New York: John Wiley & Sons, Inc., 367 p.
From page 251:
My book is called ‘Rocks’ by Maresch and Medenbach from 1987. I just couldn’t find it on amazon, so it’s probably a little outdated. I guess I’ll accept your wisdom. Didn’t get us any closer to identify Nils’ stone either.
Is this the book (in updated form)? (From WorldCat):
Title: Rocas /
Author(s): Maresch, Walter. ; Medenbach, Olaf. ; (Author - aut.); Trochim, Hans Dieter. ; (Collector - col.); Sala, Rosa, ; tr.
Publication: Barcelona : Blume,
Year: 1995
Description: 287 p. : col. il., fot. ; 20 cm.
Language: Spanish
Series: Guías de Naturaleza Blume;
Standard No: ISBN: 8487535216
SUBJECT(S)
Identifier: Rocas; Guías.
Note(s): Título original: Gesteine.- Traducido del alemán.
Responsibility: Walter Maresch, Olaf Medenbach ; colaboración: Hans Dieter Trochim ; traducción: Rosa Sala.
Vendor Info: Puvill Libros (PUVL) $17.93
Document Type: Book
WorldCat states that only TWO libraries worldwide (one in Madrid, one in Mexico City) have this book!
This is the book! But I have the original german one 