Rock ID for a school project

[jr.nation88]

Our ecology class is looking at restoring a natural glade behind our school. In order to do so, the ecology teacher told them that they first needed to identify the type of rock that primarily makes up the hill side. So, because I teach science, they figured I would be able to do it quickly but that has not been the case.

From first glance I thought dolomite but it did not pass the acid reactivity test. Even after scraping the gunk off I got no effervescence of any sort so I figured that ruled out limestone/dolomite. It seems to have a little quartz sprinkled throughout which made me thing maybe chert that maybe had morphed but i'm stumped.

Any thoughts from those of you who know more than me? Located in SE Missouri in the hill country

 

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[jr.nation88]

another picture with different light

 

[stdenis_jd]

That looks like quartzite to me - is it grainy when you look really close? Quartzite is for the most part just quartz grains (sand) that has metamorphosed. Also a hardness test might help, see if a good steel knife can scratch it and/or vice versa

 

[stdenis_jd]

Also if you have a magnifier app on your phone, if you could take a good clear close-up so we can see the crystal structure (in natural light) that would also help

 

[jr.nation88]

That looks like quartzite to me - is it grainy when you look really close? Quartzite is for the most part just quartz grains (sand) that has metamorphosed. Also a hardness test might help, see if a good steel knife can scratch it and/or vice versa

I like the thought of quartzite, but I'm not seeing the definite grain pattern that I'm used to with quartzite. I'll try to get a better pic when I get a moment. It is harder than a steel knife for sure as it just ground down the tip.

 

[DDancer]

I agree with stdenis, quartzite, a silicate of quartz materials. There may also be chalcedony in the mix as well, closer pictures would help pick that out. *edited*
Well hardness agree's with quartz. Very fine grained quartz mixed with chalcedony will give an indistinct patterning. There may be vugs in the rock where druzzy quartz crystals can be seen. Not an uncommon occurrence with that type of host rock.

 

[Eu_citzen]

Will a knife scratch it? If not; quartzite is as good a guess as it gets.

 

[jr.nation88]

Will a knife scratch it? If not; quartzite is as good a guess as it gets.

Nope, it tore the knife up So now after more observation and comparison, Quartzite seems to be our winner. Thank you all!

 

[Bajahunter]

You do realize that there is a geologic map for every square inch of the USA. It will show you the identity of every rock outcrop and all the different layers of rocks underneath. They might be available in a local map store that sells USGS topo maps, otherwise you can order them from the Missouri Dept. of Natural Resources
Here is a link to an interactive online map. Need to set up Google Earth for it to work.
https://www.americangeosciences.org...e-map-missouris-geology-and-natural-resources

 

[Clay Diggins]

It's pretty easy to find all the geologic maps for any given area. Geologic maps can be very accurate or just give you a general idea of the kind of rock in an area. If you want high quality high resolution geologic maps just go to Land Matters.

Go to the Land Matters Geology map for the State you are studying
Draw a box around the area you have an interest in
Switch to the "i" information tool and click on the map area you have selected
When the Result window pops up look for the Layer: National Geologic Map Database Query and click on the "Map List" link on the right.

A new webpage will open up with links to literally every geologic map made for your area. Dozens. You can download those maps directly to your computer. Choose the Scale 1:24,000 or smaller and you will have a 40 foot resolution or better. That's more accurate than any Google map.

Free, quick, easy and accurate right in your browser.