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Rocks In My Head

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geology et al
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Topic: geology et al
Rocks, stones, gems, minerals, fossils, crystals....there are so many terms and they each mean something slightly different. For example, people will ask me, "Are these agates?" Sometime it just depends on who you ask.

Rocks are what the earth's crust is made up of. Formations. Aggregate. This includes soil. Rocks are composed of minerals, specific chemical compounds or sometimes elements in crystalline form (usually).

So, what can be found in Minnesota? Quartz is common, Agates are a form of quartz and so is jasper. Then there are the oxides of iron, magnetite and hematite. Iron has been a very important mineral in the state. Limonite is abundant and used to make paint pigment (red and yellow ocher). Carbonates are formed from metal, carbon and oxygen, and in Minnesota include calcite, which makes up limestone (a rock), dolomite and siderite. Pyrite and gypsum are also common. Complex silicates (made from silica and oxygen, include feldspars, mica and garnet.

Granite, an igneous rock, is very common. So is gabbro. Sedimentary rocks found in Minnesota include conglomerates, some of which are very attractive. Also sandstone and shale.

The igneous and sedimentary rocks have metamorphized into slate, gneiss, schist, marble, quartzite and other common rocks. I just found a sample of garnet crystals in schist matrix in my collection. It's quite pretty.

I remember my grandfather pointing out to me some outcroppings in a field up by Blackduck. He said they were greenstone, and the remnants of the Killarney Mountains. Indeed, greenstone is the oldest rock in Minnesota and is classified as chlorite schist.

Posted by oh5/ojhoff at 5:31 AM CDT
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