Suhailite

Chemical formula: (N<sup>3-</sup>H<sub>4</sub>)Fe<sup>2+</sup><sub>3</sub>(Si<sub>3</sub>Al)O<sub>10</sub>(OH)<sub>2</sub>

A black, very rare mica group mineral, an ammonium analogue of annite.

## Characteristics A black mica group mineral, forming very small, hexagonal crystals in the form of flakes and scales, usually not exceeding 0.1 mm in diameter. It is an ammonium (NH₄⁺) analogue of annite, in which the ammonium ion replaces potassium. Macroscopically, specimens resemble common biotite, but are much rarer. ## Physical Properties Crystals exhibit a vitreous luster, and on surfaces of perfect cleavage, a pearly luster. The mineral is translucent in thin fragments and opaque in thicker parts. The calculated density is approximately 3.16 g/cm³. ## Colors and Varieties The only known color of this mineral is black. No color or commercial varieties are distinguished. ## History and Name The mineral was officially described in 2006 by a Japanese research team (J. Harada, M. Bunno, T. Minakawa, H. Hori) under the name **ammonioannite**. The name directly refers to its chemical composition – it is annite with a dominant ammonium ion. The type specimen comes from the Tanno-Hata mine in Japan. ## Applications Due to its extreme rarity and occurrence in the form of microscopic crystals, this mineral has no practical application. It is solely an object of scientific and collecting interest for specialized rare mineral collectors.

Properties

Luster
Vitreous to pearly
Streak
Light brown
Density
3.16
Cleavage
Perfect on {001}
Fracture
Splintery
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Identification in field conditions or in a typical collection is impossible. Macroscopically, the mineral looks like black mica (biotite). Certain identification requires advanced analytical methods, such as electron microprobe (EDS) to confirm the presence of nitrogen and absence of potassium, and X-ray diffraction (XRD) to determine the structure. ## Distinguishing from Similar Minerals It can be confused with other black micas, mainly annite and biotite. Annite has practically identical physical properties, but its structure is dominated by potassium, not the ammonium group. Differentiation without specialized equipment is impossible. ## Crystal Forms It forms small, hexagonal or pseudohexagonal tabular (flaky) crystals. It occurs as scattered, individual scales in the host rock.

Geological environment

## Genesis The mineral forms under low-grade metamorphic conditions affecting sedimentary rocks rich in organic matter. The presence of the ammonium group (NH₄⁺) is directly related to the thermal decomposition of organic compounds (kerogen) contained in the primary sediment. ## Mineral Associations It co-occurs with minerals such as quartz, calcite, and pyrite. ## Localities The only confirmed occurrence worldwide (type locality) is the Tanno-Hata mine, in the Kitakami Mountains, Iwate Prefecture, Japan. The mineral was found there in rocks of the Tanno-Hata Formation, consisting of metamorphosed tuffaceous sandstones.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a specimen primarily depends on its rarity and scientific significance. Since the mineral forms only microscopic crystals, the main criterion is the abundance of small scales on the smallest possible surface of the host rock (matrix). Specimens with well-formed, though still microscopic, hexagonal crystal outlines are most desirable. ## Popular Localities All known collector specimens come from a single location in the world – the Tanno-Hata mine in Japan. This is an extremely difficult mineral to acquire, available almost exclusively through specialized channels.

Care and storage

## Cleaning Specimens should only be cleaned with compressed air or very gently with a soft brush. Due to the microscopic size of the crystals, avoid any contact with water and chemicals that could damage the host rock. ## What to Avoid Avoid ultrasonic cleaners, which could destroy the delicate flakes of the mineral. Crystals are sensitive to mechanical damage (abrasion, scratching). Do not heat the specimen due to the risk of decomposition of the ammonium group. ## Storage It is recommended to store specimens in closed "micromount" boxes, protecting them from dust and mechanical damage. Exposure to strong light is not advisable, although there is no data on its effect on this particular mineral.

External references

Sources

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