Deerite - Fe²⁺₁₂Fe³⁺₆Si₁₂O₄₀(OH)₁₀

Deerite was first described in the riebeckite-garnet-stilpnomelane metachert from the Franciscan Formation of California. It was named in honour of Professor William Alexander Deer (1910-2009). Deerite is restricted in occurrence to blueschist facies meta-ironstones. It has been found in suitable metamorphics from: the Franciscan terrain, Oregon, Washington State, the Alps, Corsica, Turkey, Greece, and New Caledonia, such that it can now be considered a normal blueschist facies indicator. The deerite-formation reactions are, in order of increasing temperature, from: magnetite + greenalite + silica; from magnetite + minnesotaite + water; and from magnetite + grunerite + silica + water. In general, however, deerite formation is inter-related with the breakdown of riebeckite and magnetite; reactions dependent on the local aNa⁺:

Riebeckite + water = magnetite + deerite + silica + Na⁺
Riebeckite + water = aegirine + deerite+ silica + Na⁺
Riebeckite + water = deerite + crossite + silica + Na⁺

The relative rarity of deerite even in blueschist terrain devolped under appropriate P-T conditions may be explained by its reluctance in natural examples to incorporate appreciable Al, thus restricting deerite to very iron-rich compositions not often found in subducted sediments and volcanic rocks.

Optical Properties:

• Form: Needle-shaped, acicular crystals with amphibole-like lozenge-shaped cross sections.
• Relief: Moderate.
• Colours: Black (like opaque minerals). Only on thin edge deerite show brown colors.
• Pleochroism: None.
• Interference colors: No interference colours can be seen because of the intense absorption.

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Dark acicular crystals (needle-shaped) of deerite. Laytonville quarry, USA. From RRUFF.

Dark acicular crystals (needle-shaped) of deerite. Laytonville quarry, USA. From RRUFF.

Geology of Laytonville Quarry

Laytonville Quarry (Red Box).

Laytonville Quarry

The Laytonville Quarry is the type locality for the minerals deerite, howieite, and zussmanite (Agrell et al., 1965). Situated 8 km south of Laytonville (Mendocino County, California), is a locality where an unusual variety of minerals has been found in Franciscan glaucophane schists. The rocks exposed in the quarry include both metamorphosed sedimentary and volcanic rocks of the Franciscan Formation (Jurassic and Cretaceous age). The metasedimentary rocks include metachert, metashale, metaironstone, and metalimestone.

The Laytonville quarry contains a series of deep-ocean, subduction related, sediments of varying Fe:Mn, Fe:Al and Fe²⁺:Fe³⁺ contents, but all enriched in Fe Mn carbonates. These sediments have a complex metamorphic history that has involved the early development of aegirine and garnet, followed by various phases of secondary mineral growth, all of which required the addition of water to the metamorphic assemblages under high-P, low-T conditions. During the course of an early hydration episode deerite formed within most of the ironstones, and zussmanite in certain Fe-rich, Mn-poor pelites.
At a later, and apparently lower pressure period in the metamorphic history, howieite, riebeckite, stilpnomelane, and a more manganous variety of zussmanite have overtaken much of the earlier mineralogy.

Sample kindly given to me by Keith A. Peregrine.

Bibliography

• W. A. Deer, R. A. Howie, J. Zussman (1994): Introduzione ai Minerali che costituiscono le rocce. Zanichelli editore.
• R. M. Wood: The iron-rich blueschist facies minerals: Deerite. MINERALOGICAL MAGAZINE, JUNE 1979, VOL. 43, PP- 251-9
• R. M. Wood: The Laytonville Quarry (Mendocino County California) exotic block: iron-rich blueschist-facies subduction-zone metamorphism. MINERALOGICAL MAGAZINE, VOL. 45, 1982, PP. 87 99

Photo
Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and Stilpnomelane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals and quartz. Meta-ironstone. Laytonville quarry, California. XPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and fibrous Howieite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and quartz. Meta-ironstone. Laytonville quarry, California. XPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and Stilpnomelane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and fibrous Howieite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals, Stilponomelane and fibrous Howieite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and stilpnomalane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and stilpnomalane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and stilpnomalane crystals. Meta-ironstone. Laytonville quarry, California. XPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and stilpnomalane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals, fibrous Howieite and stilpnomalane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals, fibrous Howieite and stilpnomalane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and fibrous Howieite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and stilpnomalane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)
Black needle-shaped deerite crystals and stilpnomalane crystals. Meta-ironstone. Laytonville quarry, California. XPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals and fibrous Howieite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 10x (Field of view = 2mm)	Black needle-shaped deerite crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 20x (Field of view = 1mm)
Black needle-shaped deerite crystals and Stilpnomelane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 20x (Field of view = 1mm)	Black needle-shaped deerite crystals and Stilpnomelane crystals. Meta-ironstone. Laytonville quarry, California. PPL image, 20x (Field of view = 1mm)

Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Deerite - Fe2+12Fe3+6Si12O40(OH)10

Geology of Laytonville Quarry

Deerite - Fe²⁺₁₂Fe³⁺₆Si₁₂O₄₀(OH)₁₀