Deerite

Chemical formula: Fe<sup>2+</sup><sub>6</sub>Fe<sup>3+</sup><sub>3</sub>(Si<sub>6</sub>O<sub>17</sub>)O<sub>3</sub>(OH)<sub>5</sub>

A rare, black iron silicate, forming acicular or fibrous crystals in metamorphic rocks.

## Characteristics Deerite is a complex iron silicate belonging to the inosilicate group. It forms characteristic, slender, prismatic or acicular crystals, which often occur as tangled, fibrous, or felted aggregates. The mineral is opaque and has a black color with a vitreous to almost metallic luster on fresh surfaces. ## Physical Properties Deerite is a relatively hard mineral, reaching about 6 on the Mohs scale. Its density is significant, ranging from 3.8 to 3.9 g/cm³, which is typical for iron-rich minerals. The streak is dark green to black. ## Colors and Varieties This mineral is uniform in color and occurs exclusively in black. No color varieties or commercial forms are distinguished. ## History and Name Deerite was first described in 1964. Its name honors William Alexander Deer (1910-2009), a prominent British petrologist and author of the fundamental work "Rock-Forming Minerals". The discovery was made on material from the Laytonville Quarry in Mendocino County, California, USA, which is its type locality. ## Applications Due to its rarity and occurrence as small crystals, deerite has no industrial applications. However, it is an object of scientific interest and is valued by advanced mineral collectors and specialists in metamorphic petrology.

Properties

Mohs hardness
6
Luster
Vitreous to sub-metallic
Streak
Dark green to black
Density
3.8-3.9
Cleavage
Good on {110}
Fracture
Uneven
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Key features for identifying deerite are its black color, acicular or fibrous crystal habit, and high hardness (approx. 6). However, the most important indicator is its environment of occurrence – metamorphic rocks of the glaucophane schist (blueschist) facies. ## Distinguishing from Similar Minerals Deerite can be confused with other black, acicular minerals found in similar rocks, such as hornblende, tourmaline (schorl variety), or aegirine. It differs from hornblende by its cleavage angles. Tourmaline crystallizes in the trigonal system and often shows striations on prism faces. Aegirine may have a similar appearance but often differs in its mineral paragenesis. Certain identification most often requires advanced analytical methods, such as X-ray diffraction (XRD). ## Crystal Forms Deerite forms elongated, slender crystals with a prismatic to acicular habit. They usually occur as radial, tangled, or felted aggregates, less commonly as single, well-formed crystals within the rock mass.

Geological environment

## Genesis Deerite is a characteristic mineral of rocks that have undergone metamorphism under conditions of high pressure and relatively low temperature. It forms in iron-rich siliceous sediments (metacherts) and iron shales within the glaucophane schist (blueschist) facies. ## Mineral Associations This mineral often co-occurs with other high-pressure minerals, such as howieite, zussmanite, glaucophane, riebeckite, crossite, aegirine, grunerite, quartz, and magnetite. ## Localities The most important and well-known occurrences of deerite are within the Franciscan Formation in California, USA, especially at the type locality – the Laytonville quarry. It also occurs in the Western Alps (Italy), on Sifnos Island (Greece), in Japan, and in New Caledonia.

Rarity

Rare

For collectors

## Quality Criteria Most valued by collectors are specimens with well-formed, sharp, and lustrous deerite crystals, rather than just tangled fibers. Samples in which black deerite needles contrast with other associated minerals, such as blue glaucophane, are highly prized. The size of the crystals and the abundance of their occurrence on the rock matrix also significantly increase the value of the specimen. ## Popular Localities The localities in California, USA, particularly in Mendocino County, are considered classic and provide the best deerite specimens in the world. Specimens from this locality serve as the standard for this mineral.

Care and storage

## Cleaning Deerite specimens should be cleaned very carefully, using a soft brush and distilled water to remove dust and loose impurities. Due to the potential fragility and fibrous nature of the aggregates, ultrasonic cleaners and mechanical cleaning are not recommended. ## What to Avoid Avoid contact with strong acids, which can react with iron in the mineral's structure. Specimens, especially those with a fibrous structure, are sensitive to impacts and shocks, and therefore require delicate handling. ## Storage Collector's specimens of deerite are best stored in closed boxes or display cases to protect them from dust and mechanical damage. Avoid placing heavier objects on them.

Sources

Read more