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Geology is Everywhere: Pegmatite Store Earthcache

A cache by foxult Send Message to Owner Message this owner
Hidden : 09/19/2018
1.5 out of 5
1.5 out of 5

Size: Size:   other (other)

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Geocache Description:

The coords put you on a public sidewalk outside a building built ca. 1936 with an unusual stone facade. You will need a measuring device (e.g. - a ruler with metric units) to complete this Earthcache. Parking is close by.



Intrusive igneous rocks, in general, form from the solidification of magma (melted rock) inside the Earth. Because the magma never makes to the surface before solidifying, the liquid cools slowly, which allows the minerals to grow bigger (coarse grained). Magma that makes it to the surface is called “lava,” and it cools quickly, so its minerals barely have time to grow and are not usually visible to the naked eye (fine grained).


Granite, diorite, gabbro, and peridotite are types of intrusive igneous rocks, so their minerals are coarse grained, or visible to the naked eye. Pegmatite is an intrusive igneous rock closely related to granite but with one important characteristic that separates it from “normal” granite: the minerals in pegmatite are greater than 2.5 cm (1 inch) long, which means they are very easy to see with your naked eyes--no magnifying glass needed! If you go to a museum that displays giant minerals, it’s quite likely those minerals derived from a pegmatite somewhere.


(Photo of some pegmatite I took at the Keck Museum in Reno, NV, October 2016)


Pegmatite Minerals are Big? Of Coarse.

As the magma hardens, the elements in the liquid combine to form minerals as easily as they can, but at the very end there are some elements that don’t combine easily with others, so there’s a little bit of magma left containing these exotic (unusual) elements and hot water. Because these elements don’t combine easily, the few minerals that do form have less competition for space and can grow very big. Eventually, though, all the magma cools enough to harden completely, and the exotic elements (e.g. - gold, , like that found close to Charlotte in the early 1800s) form into minerals filling in whatever spaces are left in the rocks.


The Mineralogy of Pegmatite

The minerals in pegmatite are similar to those in granite (with the exception of some exotic minerals found in pegmatite).


A brief field guide to the identification of minerals typical in granite and pegmatite.

White, with a glassy luster (shiny like glass) = milky quartz.

White, without a glassy luster = orthoclase.

Black, flaky (peels into sheets) = biotite (a type of mica).

Not black, yet still flaky = muscovite (another type of mica).

Black, not flaky = hornblende.


One final note: According to the Rockd app on my phone, the rocks in this area formed during the Ordovician/Devonian periods, around 485.4-358.9 million years ago.

Want to know even more about pegmatite? Go here: site

Sandatlas site


1) Examine the wall. What mineral dominates (98+%) all the pegmatite blocks that make up the outer walls of the store?

2) What famous exotic mineral associated with pegmatite was found in the Charlotte area (Midland, NC) in the early 1800s?

3) Use your ruler to measure one of the pegmatite blocks. This will give you a length and width. If you can’t measure a corner block, estimate the depth of the block. Calculate the weight of the block by using this formula:

(L x W x D) x 2.65 g/cm^3 = ___ g,  where 2.65g/cm^3 is the density of the dominant mineral.

Show your work.

4) Optional: You’ve established how heavy one block is, and they’re all roughly the same size. If you’re up to the challenge, count the total blocks it took to build the outside of the building and multiply that number by the weight you calculated for a single block to get the rough total weight of the outer walls.

Imagine how much work it would have taken to haul all those rocks.

5) Optional: Post a photo of you and your crew (if there is one) having a good time at this Earthcache.


Additional Hints (No hints available.)

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