todays rocks

[fuss]

Found these today in a gorge that has a small stream running though it (partially frozen) in SE WI. I think I have a handle on what they are except the last one.

Sandstone with fair sized pyrite (black streak) growing on it. Please correct me if i'm wrong.



This one is really quite interesting and strange..Its almost completely covered in druzy quartz. Not quite sure what under the quartz crystals but could be lime stone or some other rock...The dark section in photo3 is unusual and in photo 4 the LW uv shot shows some interesting fluorescence.









I need some help on this one.. any ideas? it does attract a magnet.









Thx

 

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[DDancer]

Try an acid test on your first two stones to test for sandstone and limestone. I suspect the first picture is not sandstone but sugar quartz, an aggregate of small fine quartz crystals in a massive form, as I see a lot of crystals. Your correct for the pyrite thanks for the streak test.
Your second picture almost reminds me of domolite under the druzy quartz for structure. The black cubic features are nice and have me scratching my head. The UV is quite nice, interesting blue there at the bottom. Overall I'm thinking calicite related material. Acid test and hardness may help.
The last picture reminds me of granite. The black inclusions maybe magnetite however they also remind me of deformed garnet or tourmaline as well.

 

[stdenis_jd]

I'm 100% with DDancer on all counts, I'll go ahead and say in specimen #2 the black cubic/possibly tetrahedral crystals are sphalerite/marmatite on dolomite with partial silicification (quartz replacement)

 

[stdenis_jd]

oh and the fluorescence could be from either the dolomite with associated calcite or the sphalerite (all three can fluoresce)

 

[fuss]

First rock fizzed violently for a sec then absorbed the acid into the rock, hardness around a 3.

Second rock also 3 in hardness also reacts to hcl dilute. The black portion is soft as well, around a 2 in hardness #3 and 4 pick gouged into it when scratched kinda like gypsum does, did not apply acid to the black (dark brown) parts probably should have though.

Third rock....I misplaced...somewhere..when I find it ill test, but I think don't think its a granite.

both rocks streaked white.

also DDancer when you mentioned dolomite were you referring to the rock dolomite (dolostone) or the mineral as if maybe there are some dolomite crystals you see?

Thank you much.

 

[DDancer]

I stand corrected on the first rock fuss, neat formation, I dont think I've ever seen pyrite in sandstone. I'm referring to the mineral dolomite. In the second picture of your second specimen the structure of the mineral under the druzzy coating looks a lot like that of dolomite crystals.

 

[stdenis_jd]

I think the reason the black mineral shows a low hardness is because it is poorly formed and/or in decomposition stages so it's more likely coming apart leaving it susceptible to scratches whereas it normally should not be. Does the mineral seem to be unstable or coming apart when you scratch it? Sphalerite is around a 4 in hardness, so a little imbalance in the Fe, Zn or S could make it softer.

As far as the 3rd rock, calling it a "granite" is kind of a catch all term which I'm guilty of using in regards to intrusive igneous rocks. It looks like metamorphic after igneous - like a "granitoid" that was way down in the lower crust/upper mantle and metamorphosed.

And as DD said, those are textbook dolomite crystal structures :-)

Good job with the tests :-) very helpful

 

[fuss]

I located rock #3 and ran through the basic tests. rock surface is a 2.5-3 in hardness and the dark colored nodule things are a 2 or less.
Streak is brown for the main rock and greenish white on the nodule things..
no reaction on acid test.

I will try and get some detailed surface photos shortly.

 

[fuss]

It did not feel unstable nothing fractured or crumbled while scratching, it felt like scratching a soft chalk.

I think the reason the black mineral shows a low hardness is because it is poorly formed and/or in decomposition stages so it's more likely coming apart leaving it susceptible to scratches whereas it normally should not be. Does the mineral seem to be unstable or coming apart when you scratch it? Sphalerite is around a 4 in hardness, so a little imbalance in the Fe, Zn or S could make it softer.

As far as the 3rd rock, calling it a "granite" is kind of a catch all term which I'm guilty of using in regards to intrusive igneous rocks. It looks like metamorphic after igneous - like a "granitoid" that was way down in the lower crust/upper mantle and metamorphosed.

And as DD said, those are textbook dolomite crystal structures :-)

Good job with the tests :-) very helpful

 

[fuss]

another image added

Not sure if this helps, as close as I can get with my current camera set up. Need to click it a few times to see the details.



thx.

 

[stdenis_jd]

yeah that's a good pic, good enough looks like a non-foliated schist for the most part or like before, an intrusive igneous that has significantly metamorphosed. The blacks are garnets, I suppose it's possible they're amphiboles of some sort but I doubt it. That's my opinion and I'm sticking to it haha

 

[stdenis_jd]

It did not feel unstable nothing fractured or crumbled while scratching, it felt like scratching a soft chalk.

Now it has me a little confused honestly, I really don't know what it is. Not sure if it's a mass of goethite? Other than that I got nada

 

[fuss]

Your opinion is greatly valued! Thank you