Mini Me Geology Blog » metamorphic rock

Do You Have a Sandstone or a Quartzite?

tracyb — Fri, 21 Oct 2011 14:31:20 +0000

Quartzites are metamorphic rocks that were once quartz sandstones. The quartz sandstones change to quartzite from high heat and pressure deep inside the Earth. Sometimes quartzite rocks have the same, bedded look as the original sandstone, making the two hard to tell apart. Learn more about a fun test for these rocks here.

Today’s Ask-a-Geologist Question: Marble vs. Quartzite

tracyb — Fri, 17 Jul 2009 11:44:47 +0000

Question: How could mineral hardness be used to tell quartzite (shown right) from marble (shown left)?

Answer: I don’t think that hardness would be the best test. In theory, quartzite should be harder because it is made of Quartz which has a hardness of 7 on the Mohs scale and Marble is made of calcite which is has a hardness of 3 on Mohs scale. The best test is the “fizz test.” If you drop a dilute acid on the rocks, the marble will bubble or fizz. You can use vinegar or lemon juice since they are very weak acids and see if you can observe the fizz. Anything made of calcite should fizz but quartz will not.

Today’s Ask-a-Geologist Question: Banded Textures

tracyb — Wed, 08 Jul 2009 15:17:10 +0000

Question: What term for metamorphic rock with banded texture?

Answer: You may be referring to “foliation” which is the layering within a metamorphic rock. Foliation occurs when there is a strong pressure applied to the rock in one direction, just like when you put your hands flat together and push. When this pressure is applied, certain minerals, like mica and chlorite, grow with their long direction perpendicular to the pressure. This make the rock appear with layering and/or banding.

Why We love Garnet Schist

tracyb — Mon, 08 Sep 2008 03:03:47 +0000

Garnet Schist is a beautiful gray, silvery or brown colored metamorphic rock which shiny mica layers and an abundance of well-formed garnet crystals. Garnet schist is the same rock as a mica schist with the exception of the inclusion of the garnets (too cool!). The original rock type is typically a clayey rock with some calcite content such as a siltstone or a mudstone. The major minerals in a schist are quartz and mica. During the compression of the rock (metamorphism) the mica grains, which are thin and flat, are squeezed into sheet-like layers and give a silky, shiny appearance to the rock. The texture of a schist is usually fine-grained; however, in a garnet schist the garnet crystals are large enough to be seen by the naked eye and appear to be “floating” in the schist rock. If schist rocks are broken, they will bread into wavy surfaces. Get your sample of Garnet Schist today!

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