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Don't Take Garnet for Granite

A cache by Dare2Geocache Send Message to Owner Message this owner
Hidden : 06/04/2010
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Geocache Description:

You will need a strong magnet at this earthcache site. If you don't have one, check out GC1P7ZW, Black Balsam Berry Bags.

Garnet is commonly found in medium to high-grade metamorphic rock, occurring alongside quartz, feldspar, mica, and associated minerals. It is a stone of great beauty and moderate economic value. Often a deep translucent or transparent red, garnet is a popular gem. One type of garnet is also used to produce commercial abrasives such as sandpaper and polishing grit.

While most minerals have a very specific chemical composition, the term “garnet” refers to a group of minerals which are very similar in both chemical and physical properties. All varieties of garnet are silicates with the general formula of X2+3Y3+2Si3O12, where X may be Ca, Fe2+, Mn, and Mg and Y may be Al, Cr, and Fe3+. In plain English, any of several metals may be substituted into the molecule (thus changing the variety), but each molecule must contain silicon and oxygen linked together in the ratio of 3:12.

Six varieties (and there are many gradations between them) occur fairly frequently in deposits scattered about the world. Almandine (Fe3Al2Si3O12), the most common of all garnets, is usually deep red to reddish brown and sometimes purplish.
deep red almandinereddish brown almandinepurplish almandine

Due to its large iron content, it is paramagnetic (attracted to a magnet, although not magnetic on its own). It is frequently found in western North Carolina, including in several nearby locations in commercial quantities and qualities.

Pyrope (Mg3Al2Si3O12), less widely distributed but more valuable as a gemstone, is rose-red to reddish purple and is also fairly common in the mountains of NC.

rose-red pyrope reddish-purple pyrope

A rose-pink to red solid solution (gradation) between pyrope and almandine, known as rhodolite, is a prized gemstone found only in the Cowee Valley of Macon County, near Franklin, NC. rhodolite

All varieties of garnet form dodecahedral and trapezohedral many-sided isometric (equal-dimensioned) crystals. The ancient Romans thought the red, roundish individual crystals and grape-like groups of crystals had the appearance of pomegranate seeds and named them granatium which means a pomegranate. The original scientific name given this mineral group was granat which was later corrupted into garnet.

rough garnet crystalspomegranate with seeds

garnet sandpaper Garnets are relatively hard (6.5-7.5 on the Mohs hardness scale) and show conchoidal fracture (break into sharp, irregular pieces). These characteristics make them ideal for use as abrasives when ground into fine grit. Most garnet that is mined commercially is used for this purpose.

Only a very small number of garnet crystals are of sufficient quality to cut as gemstones. Most of these are pyrope.

cut pyrope gemstone

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Now that you know all about garnet, let's go find some!

Logging Requirement

In order to log this earthcache, email the answers to the following questions to Do not post your answers.

(1) The coordinates take you to two largish rocks imbedded in the trail. Describe the garnets that you find in these two rocks. (Size, color, luster, degree of transparency, crystal shape, quantity, pattern of distribution, etc.)

(2) Describe the four main colors occurring in the two rocks.

(3) Use your strong magnet to test the dark reddish sandy erosional fragments in the trail nearby. Make sure the sand is dry. (Make a thin layer of garnet sand in your hand and "blow dry" if necessary.) Which variety is the majority of this garnet deposit?

Warning! Do not attempt to remove these garnets from their matrix. It won’t work, and you will destroy the crystals. “Take only photographs, leave only footprints.”


Additional Hints (No hints available.)

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