Ohmilite

Chemical formula: Sr₃(Ti₄+,Fe₃+)(Si₂O₆)₂(O,OH)·₂H₂O

A rare strontium, titanium, and iron silicate, forming brown, prismatic crystals, primarily known from Japan.

## Characteristics Ohmilite is a complex silicate belonging to the enigmatite group. It forms elongated, prismatic crystals that often occur as radial or divergent aggregates. It is usually opaque, although thin fragments may be translucent. Its surface is characterized by a vitreous luster, sometimes approaching subadamantine. ## Physical Properties This mineral has a hardness of 5-6 on the Mohs scale, making it relatively scratch-resistant. Its density is approximately 3.25 g/cm³, making it noticeably heavier than most common silicates like quartz or feldspars. It exhibits good cleavage in two directions. ## Colors and Varieties Ohmilite occurs in shades of brown, from reddish-brown to almost black. Its color is a characteristic and relatively constant feature. There are no named color varieties or commercial varieties. ## History and Name The mineral's name comes from its discovery locality in the Ohmi (Ōmi) region, Niigata Prefecture, Japan. It was first described in 1974 by a Japanese team of mineralogists: Mizue Komatsu, Chuzo Nagashima, Kozo Koyama, and Tokiko Tiba. It is a type mineral for this single, specific locality. ## Applications Due to its extreme rarity, ohmilite has no industrial applications. It is solely an object of scientific interest and a valued acquisition for advanced systematic and regional mineral collections.

Properties

Mohs hardness: 5-6
Luster: Vitreous
Streak: Pale brown
Density: 3.25
Cleavage: Good on {110}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Monoclinic

Diagnostic features

## Identification Ohmilite is identified by its characteristic form – prismatic crystals, often in radial aggregates. Key features also include its brown to black color, vitreous luster, relatively high density, and specific occurrence environment. However, definitive confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS). ## Distinguishing from Similar Minerals It can be confused with other dark prismatic minerals, such as tourmaline (schorl), pyroxenes (e.g., aegirine), or amphiboles (e.g., hornblende). It is distinguished from them by its higher density, specific genesis in jadeitic rocks, and excellent cleavage. Co-occurrence with jadeite and albite in the Japanese locality is a strong diagnostic indicator. ## Crystal Forms Crystals are prismatic, with a cross-section close to square or rectangular. They typically form fan-shaped, radial, or chaotically arranged aggregates. Single, well-formed crystals are rarely found.

Geological environment

## Genesis Ohmilite is a mineral of metamorphic origin. It forms under specific conditions, in veins cutting crystalline schists, especially in albite-jadeite-quartz veins. Its presence indicates a unique metasomatic process in high-pressure metamorphic zones. ## Mineral Associations It most commonly co-occurs with minerals typical of its formation environment. These are primarily jadeite, albite, natrolite, analcime, pectolite, titanite, and the rare benitoite. The association with jadeite is particularly characteristic of the type locality. ## Localities This is a mineral with extremely limited occurrence. The main and historical site, from which almost all known specimens originate, is the Ohmi-Kotaki area near Itoigawa City (Niigata Prefecture, Japan). This is its type locality, associated with the Renge metamorphic belt.

Rarity

Very rare

For collectors

## Quality Criteria The most highly prized ohmilite specimens are those with sharp, well-formed, and undamaged prismatic crystals. The size of the crystals and their arrangement into attractive, radial aggregates are of great importance. Specimens on a light rock matrix, especially in association with color-contrasting jadeite or albite, are considered the most desirable. Clarity and intensity of luster also increase collectible value. ## Popular Localities The only source of collectible specimens is the historical type locality in Itoigawa, Japan. This mineral is a symbol of local mineralogy, and specimens from this location are highly valued by collectors specializing in rare minerals or those from specific regions.

Care and storage

## Cleaning Ohmilite specimens should be cleaned very carefully, using a soft brush and distilled water to remove dust and loose contaminants. Ultrasonic cleaners should be avoided, as they could damage the crystals or cause them to detach from the rock matrix. ## What to Avoid This mineral is sensitive to strong acids. Avoid sudden temperature changes and prolonged exposure to direct sunlight, although there is no evidence of fading. Protect from contact with harder minerals to prevent scratching. ## Storage It is recommended to store specimens in separate, padded boxes or display cases to minimize the risk of mechanical damage. Due to its rarity, each specimen should be handled with the utmost care.

Sources

IMA Mineral List (CNMNC)