The Garnet Family: Spanning the Spectrum of Mineralogy

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Editor Bob joined dealer Benny Fenn in the mountains of Coahuila to dig fine red grossular garnets in limestone.

By Bob Jones

Garnets are among the most common crystallized minerals you can collect. It is one of the very few minerals that spans the entire spectrum of mineralogy. It has been used as a gem for thousands of years. It has always been popular as a collector mineral. It is useful in the industrial world, and scientists study some garnets to determine the pressures and temperatures that create metamorphic rocks located many kilometers deep in the earth. Several varieties of garnets are delightfully beautiful and exotic, while others are common and easy to identify when you are field collecting.

It may seem odd to associate garnets with the business world, but one little-known fact concerns the world famous Wrangell, Alaska, garnets. They played an important role in cracking what the business world calls the “glass ceiling.”

Near the mouth of the Stikine River, close by Wrangell, Alaska, is Garnet Ledge, one of the better-known almandine garnet deposits. Specimens from here have been collected for over a century, starting with gold prospectors, once owned by the Boy Scouts, and Juneau’s Presbyterian Church, and now it is a registered Wilderness area held in trust for the children of Alaska.

Varying Examples

Today, you can see the simple 12-sided almandine crystals in mica schist from the Ledge world-wide in museums, in private collections, and to a lesser degree in jewelry. These crystals are choice examples of an iron aluminum silicate garnet. They are not particularly well known for their gem quality, but Alaskan almandine garnets should be considered special in the business world. It happened that the deposit was once owned and operated by the Alaska Garnet Mining and Manufacturing Company in the early 20th century. This company was very special when it operated, not because of the garnets but because it was the first-ever business corporation in America fully owned and operated by women! Check it out on the internet under Wrangell, Alaska, garnets. What a great story it is!

As for almandine garnets, the crystals are easy to recognize because the mineral usually forms in 12-sided crystals that are very hard, 6.5-7.5 on the Mohs scale. They survive after the host rock has been weathered away. That hardness makes this very common mineral suitable for industrial grinding use, so large deposits of garnets, usually metamorphic as in Alaska and Gore Mountain, New York, are mined. Some of these old mines, once abandoned, become rockhound sites of note, such as in Roxbury, Connecticut.

Amesite var. chromian amesite, garnet var. uvarovite: Saranovskii Mine (Saranovskoe), Saranovskaya Village (Sarany), Gornoazavodskii area, Permskaya Oblast’, Middle Urals, Urals Region, Russia.
(Géry PARENT, CC BY-SA 4.0, via Wikimedia Commons)

Garnets occur in a wide variety of localities and types but are most common in metamorphic rock types. Mineralogists, including Michael O’Donoghue in his sixth edition text Gems, lists no fewer than 16 different members of the garnet family, all silicates. Of these, six varieties are prevalent and described at length in the literature. They are almandine, pyrope, spessartine, uvarovite, grossular, and andradite. They are listed as two sub-groups, pyrospite and ugrandite, based on their chemistry and using portions of their mineral names. The pyralspites are pyrope, almandine, and spessartine, and all based chemically on having aluminum as the main metal. Either iron joins the aluminum to form almandine, magnesium to form pyrope, or manganese to form spessartine. The atoms of those three elements can interchange due to similar size and electron structure to form a series that grade one into the other.

The ugrandite group has three garnets: uvarovite, grossular, and andradite, all basing their chemistry on calcium as the metal. Either iron-forming andradite joins the calcium or chromium to form uvarovite, or aluminum joins the calcium to form grossular garnets. Of the three major rock types in the earth’s crust, garnet is most often formed during metamorphism, so it is found in schist, especially mica schist, gneiss, and other metamorphic rock. It is found much less often in igneous rocks like granite and even in some pegmatite deposits under the right conditions. Because of garnet’s hardness, it survives weathering and so is also found in sedimentary environments.

Telling Characteristics

A fascinating thing about garnets formed in metamorphic rocks is they can hold internal secrets scientists use to establish the conditions under which metamorphic rock forms. Hard to believe, but garnets can tell scientists the approximate pressures and temperatures that particular rock were subjected to during metamorphic action. As the garnet is forming under high pressures and temperature, its crystallization, lattice structure, and internal zoning of the crystal are affected, and this, in turn, tells scientists about the forces involved in metamorphic action. As if that were not enough, garnets may also tell scientists how long ago the rock formed through any trace of led-uranium it may contain. These are all examples of the wonder of the science of mineralogy!

For decades, the most common garnet available was almandine; this is because it was heavily mined for its abrasive qualities. You can buy garnet paper in any hardware store, and it is most often crushed almandine. This writer-collector and countless New England mineral collectors cut their collecting eye teeth on the old abrasive almandine garnet locality in Roxbury, Connecticut, a metamorphic schist deposit. The garnets in this location were abundant as perfect dodecahedrons of a deep non-gem red. Some vesuvianite crystals also occurred here. Once the mine was closed, the highway department used the dumps from this location to backfill along roads, and people could collect the loose crystals almost anywhere in town. On my first trip in 1948, I filled the pockets of my jeans with perfect crystals before ever even getting to the deposit.

The Roxbury garnets were chosen as the Connecticut State gem mineral. Connecticut is just one of several states that have chosen garnet as the state gem, including New York and Idaho. The State Gem program was established by the American Federation of Mineralogical Societies, which indicates what rockhounds can do when organized!

By comparison, the almandine garnet deposits in upper New York State at Gore Mountain eclipse the size and quality of the Connecticut deposit. The New York garnets are often gemmy and can be used in jewelry. As for size, the Roxbury crystals tended to be an inch or so in size, while the Gore Mountain crystals often measure several inches across and in what might be described as crystalline masses.

Discovery of Tsavorite

In recent decades, several discoveries of garnets have stirred the mineral and gem world. The most exciting find was made by geologist Campbell Bridges, whom I had the pleasure of entertaining when he and his wife were guests of the Tucson Gem and Mineral Society.

Campbell discovered a deposit of green grossular garnets in 1967 in the Meriani Hills, Tanzania, already known for tanzanite. The grossulars were gemmy and rich green that rivals emeralds. Campbell ‘s further searching found similar geologic formations that extended into Kenya, and there he found another deposit rich in grossular garnets as gemmy and bright as the original find. Working with Tiffany & Co., Campbell mined the garnets and, between them, the companies named the grossular gems tsavorite after the Tsavo National Park near the Kenya deposit.

Grossular garnets are calcium aluminum silicate, and the calcium atoms can be replaced by other elements, which means grossular can be a range of colors: green, pink, gray, brown, even black. But tsavorite is unique in that the rich green color is caused by either vanadium or chromium in its chemistry. Mining this valuable gemstone took place in remote areas, and as Campbell told me once, he kept a huge snake in his tree house campsite to discourage intruders.

Unfortunately, Campbell was attacked and killed, and the world lost an incredible miner and man far too soon. (Editor’s Note: To learn more about Mr. Bridges’ discoveries, his life, and the work that continues in his name and deposits in Kenya, check out Rock & Gem’s Glorious Gemstones Issue #2, available for free, at www.rockngem.com.)

Another recent exciting garnet find is that of spessartine garnets, named for Spessart,

In recent years China has been producing fine specimens of red spessartine garnets on matrix.
(Arkenstone Gallery of Fine Minerals)

Bavaria, where they were first found in quantity. Chemically, spessartine garnet is magnesium aluminum silicate and can be found in several different colors: yellow, red, violet-red, and under the right light, some crystals will show a change of color. Most form in metamorphic rock and skarms, but a recent amazing discovery was made of red spessartine crystals on aquamarine crystals in a gem pegmatite in Pakistan.

Pakistan has been known for decades for a superb variety of pegmatite minerals, with aquamarine crystals leading the way. The fine of aquamarine crystals in the Shigar District, Gilgit-Baltistan, Pakistan, has produced the world’s finest specimen aquamarine specimen now known as “The King of Kashmire.”

The story of the discovery of the “King,” an amazing aquamarine specimen, is well written by Danny Trinchillo. Danny is one of the leading gem and mineral dealers in the world, well known for operating the amazing Pederniera tourmaline pegmatite mine in Brazil, which produced gem crystals over a foot long. The matrix of this beauty, which originally weighed several hundred kilos when lowered from the ceiling of its pocket, is typical quartz, feldspar, and mica. You can read about this amazing discovery in the November-December 2020 issue of Mineralogical Record.

The garnet family of crystals is one of the mainstays of mineralogy and our hobby. They are enough so every collector, even the most inexperienced, can field collect them and learn from them. Yet, the most advanced collector can also own and enjoy a suite of fine, rare garnet crystals. Garnets are truly worthy of being in every collection.