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toferlewis  ·  3156 days ago  ·  link  ·    ·  parent  ·  post: A rock I found in the UP of Michigan.

1) Sandstone and shale or mudstone often times have the similar composition - being dominantly quartz grains, but sandstone grains you can usually see, plus they are usually more beige to tan, orange or pink, not dark red/brown like this - think of Florida beach sand, that's clean sand waiting to be sandstone in a few million years. Shale/Mudstone is very very fine-grained quartz and other grains, mostly clay, that you cannot always see with the naked eye. The fact that you cannot see any little grains, leans me towards shale/mudstone or slate - which it also could be and is common in these age of rocks in the area. Slate is nothing more than a mudstone/shale that has gone through low-grade metamorphism. All this means is temperature and or pressure, beyond the normal burial and geothermal gradient has altered it by removing even more water and aligning the mineral grains. It becomes very hard and any grains are hard to see - pool tables have a slate underneath the felt. 2) Calite (Ca) is composed of calcium and calcium does not like to be alone, it always wants to be paired with something and its usually carbonate (CO3) in marine sedimentary environments. Together they form calcium carbonate CaCO3 and this is very basic - it's chalk, or limestone - the main ingredient in Tums or any antacid. Hydrocloric acid, is an acid obviously and when you combine an acid and a base, you get a reaction. If you were to drop 10% HCl on this rock and it began to fizz and stink a little, that is 100% diagnostic that it is calcium carbonate, the gas being released in the fizzing is carbon dioxide. Quartz is silica, made of silicon and silicon is very, very stable, it is extremely happy with oxygen as SiO2, it has no reason to do anything. This is why it is so common in rocks. It's extremely abundant and very stable. The chemical reaction is like the volcano that kids make in science classes with vinegar + baking soda, vinegar is acetic acid and baking soda is sodium bicarbonate, identical reaction from similar acid/base reaction. Sadly vinegar and calcium carbonate do not create the same reaction. It is a much slower reaction that takes days. You could take the rock and drop it a bowl of vinegar and in a few days it will dissolve if it's calcium carbonate, but then you'd be destroying it and that's not the point, a drop of HCl is harmless.