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November 26, 2017 Weekly Geology Guest, Garnet

November 25, 2017
Greetings from the Bluff Park Back Porch, way up yonder on Shades Mountain (1,109′) in Alabama:
For the next several weeks (months) we will delve into Industrial Minerals, While they are not glamorous, they are terribly important to our modern way of life. 

Typical examples of industrial rocks and minerals are limestone, clays, sand, gravel, diatomite, kaolin, bentonite, silica, barite, gypsum, and talc. Some examples of applications for industrial minerals are construction, ceramics, paints, electronics, filtration, plastics, glass, detergents and paper.




Garnet is the name used for a large group of rock-forming minerals. These minerals share a common crystal structure and a generalized chemical composition of X3Y2(SiO4)3. In that composition, “X” can be Ca, Mg, Fe2+ or Mn2+, and “Y” can be Al, Fe3+, Mn3+, V3+ or Cr3+.

These minerals are found throughout the world in metamorphic, igneous, and sedimentary rocks. Most garnet found near Earth’s surface forms when a sedimentary rock with a high aluminum content, such as shale, is subjected to heat and pressure intense enough to produce schist or gneiss. Garnet is also found in the rocks of contact metamorphism, subsurface magma chambers, lava flows, deep-source volcanic eruptions, and the soils and sediments formed when garnet-bearing rocks are weathered and eroded.

Most people associate the word “garnet” with a red gemstone; however, they are often surprised to learn that garnet occurs in many other colors and has many other uses. In the United States, the major industrial uses of garnet in 2012 were waterjet cutting (35%), abrasive blasting media (30%), water filtration granules (20%), and abrasive powders (10%).

Uses of Garnet

Garnet Minerals

Mineral Composition Specific Gravity Hardness Colors Almandine Fe3Al2(SiO4)3 4.20 7 – 7.5 red, brown Pyrope Mg3Al2(SiO4)3 3.56 7 – 7.5 red to purple Spessartine Mn3Al2(SiO4)3 4.18 6.5 – 7.5 orange to red to brown Andradite Ca3Fe2(SiO4)3 3.90 6.5 – 7 green, yellow, black Grossular Ca3Al2(SiO4)3 3.57 6.5 – 7.5 green, yellow, red, pink, clear Uvarovite Ca3Cr2(SiO4)3 3.85 6.5 – 7 green

crushed and graded garnet abrasive granules
Garnet abrasive: This photo shows garnet granules that have been crushed and size-graded for use as abrasive, cutting, and filter media. They are used in waterjet cutting, “sand” blasting, sandpaper, water filtration, and a number of other uses. Almandine is the hardest garnet and also the most abundant. It is the garnet of choice for most abrasive applications. Photo by the United States Geological Survey.

Almandine in metamorphic rock

Eurico Zimbres (FGEL/UERJ) and Tom Epaminondas (mineral collector). 2004.  Paraíba, Brazil.

Grossular on display at the U.S. National Museum of Natural History. The green gem at right is a type of grossular known as tsavorite.
2005. from Wikipedia

Pendant in uvarovite, a rare bright-green garnet.
Photographed by Adrian Pingstone in February 2003 and released to the public domain.

Garnet var. Spessartine, Putian City, Putian Prefecture, Fujian Province, China

2/02/2009 Source Own work, specimen from the Willems Miner Collection Author JJ Harrison ( Permission
(Reusing this file)

Own work, share alike, attribution required (Creative Commons CC-BY-SA-2.

Lookie, A Real Garnet, but too big for mantle.
Jutta standing next to a garnetiferous amphibolite boulder on display at Pit #1. This boulder is a fine example showing the large pyrope / almandine garnet porphyroblasts that the Gore Mountain mine is famous for.

Garnet Miners


Almandine on gray-green schist Locality : Granatenkogel Mt. (north slope), Gaisberg valley, Obergurgl, Ötz valley,North Tyrol, Tyrol, Austria Size 19x11x7cm
Date 9 January 2011, Didier Descouens


List of Industrial Minerals
Editors Note:  It is the intent of this site to keep this discussion as simple as possible, so as to educate the interested general public and not to discuss with the geology crowd the latest geologic theories and nuances.  Thanks, R
“No copyright infringement intended.
The rights belong to their respective owners”
Copyright Disclaimer: Under Section 107 of the Copyright Act 1976, allowance is made for “fair use” for purposes such as criticism, comment, news reporting, teaching, scholarship, and research. Fair use is a use permitted by copyright statute that might otherwise be infringing. Non-profit, educational or personal use tips the balance in favor of fair use.
Author’s Request:  If you see any pictures that you know the source and photographer, let me know immediately.  Thanks!  R

Images have been searched by TinEye Reverse Image Search.

We will now continue down the Industrial Mineral dusty trail and continue with Gem Minerals
Enjoy the adventure!

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