Unknown rocks
I have two pieces of rock which I cannot identify for sure since Im quite new at geology. I would like your opinions of them and what you know or think. I have no instruments at home so the photos is basically what you get. If you aren’t sure, an educated guess is better than nothing.
Rock A
Found on the island of Gotland (as you now know have a primarily Silurian lime stone bedrock). Def. sedimentary. Perhaps a conglomerate of some sort? White circular or oval granules surrounded by grey filling. The white filling however looks a bit precipitated almost. And the white granules seems to erode more quicker than the surrounding area. Also a couple of charcoal-looking black crystals (but a lot harder than ordinary coal. Perhaps a hornblende?) embed inside some of the white granules. No certain trace of any distinct macro fossils. Low density/weight.
Rock B
A rock with heavy density and large milkywhite plagioclase-looking crystals (they look more transparent on the closeup picture than in reality so ignore the transparent effect on the close up) embedded in a black igneous rock. No traces of quartz what I can see. There are also a few random red granules or "holes". Found on the Swedish mainland so it could very well have come from anywhere due to the ice age.
April 27, 2008 + Posted in Mineralogy, Petrology, Paleontology +
Guess for Rock B: Basalt Porphyry
http://csmres.jmu.edu/geollab/fichter/IgnRx/BasPor-1A1.html
BASALT PORPHYRY
Comment by Mikah — April 27, 2008 @ 4:56 pm
Mikah: A good guess indeed.
Comment by Antimonite — April 27, 2008 @ 5:32 pm
I agree with the assessment of B as a porphyritic basalt with plagioclase phenocrysts. The texture is almost (but not quite) glomeroporphyritic.
On sample A I’d suggest that those round features may be pisoliths or oncoliths. No idea what the dark mineral would be, but I doubt hornblende if this rock is indeed a limestone.
Comment by Ron Schott — April 27, 2008 @ 6:36 pm
Ron: Thanks for your input.
Comment by Antimonite — April 27, 2008 @ 7:12 pm
I can’t say about rock B but rock A is definitly a limestone for me dispite the dark material on it that I guess might be manganese or other weathering related “oxides”. I also think its pisoliths or oncoliths.
Comment by Lost Geologist — April 27, 2008 @ 8:40 pm
Lost Geologist: Thanks for your comment. Much appreciated.
Comment by Antimonite — April 28, 2008 @ 6:30 am
However, trying to read up on the matter I only find information about Cretaceous pisoliths or oncoliths. And that time period would be out right impossible for Gotland. Did these thing form already in the Silurian? If not, Ive got to explain how a Cretaceous rock ended up on Gotland. Possible of course, but a little bit difficult.
Comment by Antimonite — April 28, 2008 @ 6:44 am
First of all one of these telediagnosises is always a bit uncertain. It just won’t meet the real thing. Then assuming we are right that leaves 3 options. One, there exist small patches of left overs that survived erosion and weathering. Two, they got transported there. The Ice Age would come to mind, or humans dropping them off. Three, they already formed in Silurian time.
Now I don’t know the location where they are from nor the geology of Gotland except that it has lots of limestone. So…no Sweden experts around?
I’d make a thin section (this would be cheapest and quickest option IMHO) and look for distinct microfossils that would allow dating it if you cannot find any gotland expert.
Comment by Lost Geologist — April 28, 2008 @ 9:48 am
p.s. You aren’t making us do your lab homework by identifying these, are ya?
Just checking…
;)
Comment by Lab Lemming — April 28, 2008 @ 10:49 am
Lost geologist: I dug trough my collection and found one more identical rock that ive completly forgotten all about. But in this other rock there is a clear and distinct rugosa coral which match the Silurian period nicely. And since all of Gotland is a an island of Silurian Limestone its quite likely that so are my loose rocks. But as you said, a microfossil would do the trick. Will look in to that during my more extended courses in paleontology in the fall.
.
Lab: No. Scouts honour. They are just rocks in my own collection.
Comment by Antimonite — April 28, 2008 @ 12:42 pm
Possibly this website could be of help: sequence stratigraphy. Maybe I’m misunderstanding the question, but it seems that pisolites/liths can form in Cambrian time, so why not Silurian? Maybe you have made a new find?
Comment by Silver Fox — April 28, 2008 @ 3:05 pm
Silver Fox: Cool link. I doubt that my find is that sensational, but one can always hope. If I get a certain answer one day I will post a blogpost about it. But that will probably have to wait until the fall.
Comment by Antimonite — April 28, 2008 @ 3:57 pm
I would say it is a Silurian crinoid limestone (i.e. krinoidékalk as we usually call it), a weathered one. The limestone is quite common on Gotland. Have a look on the beaches of Gotland the next time you get there. Look closely at the “raukar” and you will see hundreds of various fossils that constitute the rock (reef living and reef building fauna).
Comment by Monica Bjerkeus — April 29, 2008 @ 11:03 pm
Hmmm, my comment above was of course directed to Rock A and nothing else.
Comment by Monica Bjerkeus — April 29, 2008 @ 11:05 pm
Monica: Aha, thanks!
Comment by Antimonite — April 30, 2008 @ 5:07 am