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Almandine garnets (Late Cretaceous; Garnet Ledge, Alaska, USA) | by James St. John
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Almandine garnets (Late Cretaceous; Garnet Ledge, Alaska, USA)

Almandine garnets from the Cretaceous of Alaska, USA. (each is ~8 millimeters across)

 

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5500 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

 

The silicates are the most abundant and chemically complex group of minerals. All silicates have silica as the basis for their chemistry. "Silica" refers to SiO2 chemistry. The fundamental molecular unit of silica is one small silicon atom surrounded by four large oxygen atoms in the shape of a triangular pyramid - this is the silica tetrahedron - SiO4. Each oxygen atom is shared by two silicon atoms, so only half of the four oxygens "belong" to each silicon. The resulting formula for silica is thus SiO2, not SiO4.

 

Garnet is a group of silicate minerals. Garnets are expected to be red to dark red in color - many of them are, but several garnet varieties can be other colors, including purple, orange, olive green, deep green, and black. Garnets form 12-sided crystals (dodecahedrons) or crystals with even more faces on them. The crystals become more and more rounded as the crystal face number increases. Garnet has a nonmetallic, glassy luster, whitish streak, is quite hard (H = 7), has no cleavage, and has conchoidal fracture.

 

Common examples of garnet include almandine, grossular, spessartine, and andradite.

 

Almandine is an iron-aluminum garnet (ideally Fe3Al2Si3O12 - iron aluminum silicate) - it is the most common type of garnet. This variety is commonly found as well formed crystals in schists. It is also found in some igneous rocks. Almandine is classically used as a mineral indicator of regional metamorphism. Initially, the development of large, undeformed garnets in metamorphic rocks may seem odd. However, some metamorphic minerals ignore external pressures as they grow. Staurolite and pyrite, both common metamorphic minerals, do the same thing.

 

Many thousands of deep red, gemmy almandine garnets have been collected from Garnet Ledge, Alaska for over a century. The land at Garnet Ledge was given to the Boy Scouts in the early 1960s, and ever since, only the children of Wrangell may collect garnets there for free.

 

Geologic context: Almandine garnets occur in grayish-colored schists containing a mixture of sillimanite, muscovite, biotite, staurolite, chlorite, plagioclase, graphite, and ilmenite. Before metamorphism, the precursor rocks were Late Jurassic-Early Cretaceous fine-grained siliciclastic sedimentary rocks. These were cooked by contact metamorphism during intrusion of a quartz diorite pluton of the Admiralty-Revillagigedo Plutonic Belt (Alaskan Coast Plutonic Complex) during the early Late Cretaceous (at 89-92 million years). The garnets appear to have formed over a one million year interval, from 89 to 90 million years ago. The Garnet Ledge locality is a small, 1.3 square kilometer fragment of roof pendant over the quartz diorite pluton. Summarized from information in Stowell et al. (2001).

 

Locality: Garnet Ledge, adjacent to mouth of Garnet Creek, coastal exposure along the southeastern side of the mouth of the Stikine River, just south of Point Rothsay & just northwest of Garnet Mountain, ~12 km north of the town of Wrangell, southeast of Petersburg, southeastern Alaska, USA (~56° 34’ 17” North latitude, ~132° 21’ 54” West longitude)

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Photo gallery of almandine garnet:

www.mindat.org/gallery.php?min=452

 

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Uploaded on February 5, 2017