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Rock and mineral ID help
  
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fuss




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PostPosted: Nov 28, 2018 18:09    Post subject: Rock and mineral ID help  

I found this in a rocky gorge that is yards away from the beach at Lake Michigan in WI.
The surface has a silty feel and is soft, I can scratch with some effort with my fingernail. I chipped off a piece and see what looks like quartzite but its softer than typical quartzite it scratches with a #5 pick and with some effort a #4. Also there is some crystals on the surface which are also soft 2-3 in hardness. I was thinking a mudstone covered quartzite with some calcite crystals....how far off is my guess? Thx



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mudorsandstone.jpg



quartzite maybe.jpg
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quartzite maybe.jpg



soft crystal2.jpg
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soft crystal2.jpg



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soft crystal.jpg


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Pete Modreski
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PostPosted: Nov 28, 2018 18:29    Post subject: Re: rock and mineral ID help  

Dear Fuss,
Dolomite rock (more properly also called, "dolostone") has an appearance very much like the fresh interior where you chipped it. Dolomite is very common around Lake Michigan, and that's very likely what this is. Yes, the minerals in the "tension gash" veins are probably calcite. The weathered surface is likely to be softer and silty/slippery feeling.

Look up the tests that distinguish limestone & calcite, from dolomite. In a word--both are soft, easily scratched with a nail or knife. Calcite fizzes in acid; dolomite, fizzes only when it has been crushed to a powder, not noticeably in a solid, crystalline piece.
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Bob Harman




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PostPosted: Nov 28, 2018 19:15    Post subject: Re: rock and mineral ID help  

All I see, especially in the first 2 photos, and especially in the fractured area seen closeup in photo #2, is microcrystalline quartz.
Why is this example not a water rounded cobble of quartzite ??

I pictured dolostone in my Indiana minerals thread as the matrix of the Indiana route 56 roadcuts of Washington County. That is my idea of dolostone and the 2 pictured examples seem quite different to me.
The dolostone in Indiana is not very resistant when compared to quartzite. Shouldn't it have disappeared, while a quartzite nodule be around much much longer in a geologic time frame? BOB
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Pete Modreski
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PostPosted: Nov 29, 2018 12:12    Post subject: Re: Rock and mineral ID help  

Well, the hardness of the rock ought to confirm, dolomite or quartzite!

Actually, what the picture of the interior of the rock reminds me of, is the more crystalline (Precambrian age) Kona Dolomite, such as is found in the UP near Marquette; as opposed to the dolomite of the Niagara Escarpment. Don't know if there's any likelihood of some of that, being glacially transported to the Lake Michigan shore area. The tests should prove what it is!
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fuss




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PostPosted: Nov 29, 2018 20:52    Post subject: Re: Rock and mineral ID help  

I did the acid test and there is significant fizzing action but only on certain parts of the quartzite/dolostone looking surface, some parts hardly bubbled and some seemed not to at all. The same for the crystals a drop on one and it reacted intensely and a drop on a different one had no reaction. I chipped off more of the rock here are a few more images.
Also tested hardness again ...scratches with a #4 pick.



possible dolostone.jpg
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possible dolostone.jpg



possible dolostone3.jpg
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possible dolostone3.jpg



possible dolostone2.jpg
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possible dolostone2.jpg


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fuss




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PostPosted: Nov 29, 2018 20:57    Post subject: Re: Rock and mineral ID help  

By the way, thanks Pete and Bob for the input, I do appreciate it.
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