Kotoite

Chemical formula: Mg<sub>3</sub>(BO<sub>3</sub>)<sub>2</sub>

Kotoite is a rare magnesium borate, most commonly found as compact, granular aggregates in contact-metamorphic rocks.

## Characteristics Kotoite is a mineral from the borate group, chemically a magnesium borate. It usually forms massive, fine-grained, or compact aggregates, rarely developing crystals visible to the naked eye. If crystals are present, they take the form of short prisms or bipyramids, but their size rarely exceeds a few millimeters. The mineral is most often colorless, white, or grayish, with a vitreous luster. ## Physical Properties The hardness of kotoite on the Mohs scale is 6.5, making it a relatively hard mineral. Its density is approximately 3.1 g/cm³. It is brittle, and its fracture can be conchoidal to uneven. It exhibits good cleavage in one direction. It is transparent to translucent, depending on its purity and form of occurrence. ## Colors and Varieties This mineral does not have defined color varieties or commercial varieties. Its coloration is usually uniform and limited to shades of white, gray, and colorless. The coloration can sometimes be caused by impurities or inclusions of other minerals. ## History and Name Kotoite was first described in 1939 by the Japanese mineralogist Takeo Watanabe. The mineral is named in honor of Professor Bundjirō Kotō (1856–1935), a geologist and petrologist from the University of Tokyo, who was the discoverer's mentor. The mineral was discovered in the Hol Kol mine in North Korea. ## Uses Due to its rarity, kotoite has no industrial application. It is solely an object of scientific and collector interest.

Properties

Mohs hardness
6.5
Luster
Vitreous
Streak
White
Density
3.1
Cleavage
Good on {010}, poor on {100}
Fracture
Conchoidal to uneven
Transparency
Transparent to translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification A key diagnostic feature of kotoite is its occurrence environment - magnesian skarns. In the field and in collections, it is recognized by its massive, granular form and white or gray color. It is relatively hard (6.5 on the Mohs scale). Its strong, violet or blue-white fluorescence under shortwave ultraviolet (SW-UV) light is very characteristic. ## Distinguishing from similar minerals Kotoite can be confused with other white, massive minerals from skarns, such as forsterite, dolomite, or calcite. It is distinguished from carbonates (dolomite, calcite) by its lack of reaction with hydrochloric acid and significantly higher hardness. From forsterite and other magnesium silicates, it can be distinguished by its specific, strong fluorescence under UV light, which those minerals usually do not exhibit. ## Crystal forms Kotoite most often occurs in the form of compact, granular, or felted aggregates. Well-formed crystals are very rare and usually microscopic in size. They adopt the habit of hexagonal, short prisms or rhombic bipyramids.

Geological environment

## Genesis Kotoite is a typical metamorphic mineral, formed under conditions of contact metamorphism. It forms in contact zones of granitoid intrusions with magnesium-rich carbonate rocks, such as dolomites and magnesian limestones. This process leads to the formation of specific rocks called magnesian skarns. ## Mineral associations This mineral often co-occurs with other minerals typical of magnesian skarns. Its most common associations include forsterite, spinel, clinohumite, fluoborite, ludwigite, as well as other borates such as suanite and szaibelyite. ## Localities The most important kotoite localities in the world are few. The type locality is the Hol Kol mine (Suian) in North Korea, from which classic specimens originate. This mineral has also been found in the Baita Bihor mine in Romania, in the magnesite deposit in Liaoning, China, in several locations in Japan (e.g., Neichi mine), and in Yakutia, Russia.

Rarity

Rare

For collectors

## Quality criteria The most valued by collectors are specimens with visible, even if small, well-formed kotoite crystals. Since these are extremely rare, rich granular aggregates contrasting with the rock matrix also have high value. The attractiveness of a specimen is enhanced by the presence of well-formed associated minerals, such as clinohumite or spinel. ## Popular localities Historically, the most important and well-known locality is the Hol Kol mine in North Korea, but specimens from there are practically unavailable on the market. Currently, the most sought-after samples come from the Baita Bihor mine in Romania, which has provided the best known crystals of this mineral.

Care and storage

## Cleaning Kotoite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, it can be rinsed with distilled water and then thoroughly dried. Ultrasonic cleaners are not recommended, as they can damage the brittle structure of the mineral. ## What to avoid Kotoite, as a borate, is sensitive to acids, which can damage it. Contact with all household and laboratory chemicals should be avoided. The mineral is brittle, so it must be protected from impacts and falls. It should not be exposed to sudden temperature changes. ## Storage Kotoite specimens are best stored in separate, padded boxes or display cases to prevent scratching by harder minerals. Display should be in stable conditions, away from direct sunlight, dust, and sources of moisture.

Sources

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