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Garnet, Spinel, and Chrome Diopside—Crater Xenocrysts By Margi Jenks Geologists have a term for a mineral crystal in a volcanic rock that did not crystallize from the magma, but was picked up and carried along with it. The term is “xenocryst”. The “xeno” comes from the Greek word that means “stranger or foreign”, and the “cryst” is short for “crystal”. At the Crater the best example of a xenocryst is a diamond. It is not crystallized from the lamproite lava, but instead was carried along in the magma that came up to the Earth’s surface and was erupted by the Crater volcano. In addition to diamonds, the volcanic rocks at the Crater contain other xenocrysts. Three of them that visitors often find are garnet, spinel, and chrome diopside. All three of these crystals are small to tiny in size, less than 1/8th to 1/16th of an inch. Usually our visitors find them when they are combing through the heavy mineral and rock concentrates that result from doing the wet screening diamond mining process. We allow our visitors to take home a five-gallon bucket of these concentrates or gravels per person per day. After the concentrates dry out completely, the visitors then comb or look through them and often find the smaller diamonds. The garnet, spinel, and chrome diopside also attract the eye because they are bright colored and/or shiny. Black spinel, red garnet, chrome diopside, and peridot
Garnet found at the Crater
Spinel crystals are also made of aluminum and magnesium, but instead of silica, the other element is oxygen. Their crystals are generally octahedral and a distinguishing character is that their fracture, called “chonchoidal”, is like glass and looks like the outside of a clam shell. Gem spinel crystals are generally red or pink. Our Crater spinel crystals look black and very shiny, and could be mistaken for very dark brown diamonds. However, the chonchoidal fracture does not occur in diamonds, so this characteristic makes it easy to identify the spinel crystals with a hand lens. Diopside crystals are made of magnesium, calcium, and silica. However, at the Crater we have a type that also contains high percentages of chrome, so it is called “chrome diopside”. All diopside crystals are lighter or darker shades of green. The chrome diopside crystals are a vivid emerald green, which is fairly easy to spot because it is so different. They make short prism-type crystals, but most of the Crater chrome diopside crystals are tiny and without a particular shape. Companies that are exploring for new diamond deposits use these three minerals as diamond deposit indicators when they are found in stream sediments. All of them are formed in the same upper part of the Earth’s mantle and are therefore often associated with kimberlite or lamproite magmas and volcanic rocks. So, finding them in stream sediments may mean that a diamond volcano may be in the area. Fortunately, here at the Crater we already have found the volcano, and so can enjoy all the lovely xenocrysts that are found in it. Search area last plowed: January 7, 2011; Most recent significant rain: February 4, 2011 Total diamonds found in 2011: 34 Diamonds registered for January 30 - February 5, 2011 (100 points = 1carat): No diamonds were registered during this period. |
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Murfreesboro, AR 71958
Email: craterofdiamonds@arkansas.com
Phone: (870) 285-3113
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