Adachiite
Chemical formula: CaFe<sup>2+</sup><sub>3</sub>Al<sub>6</sub>(Si<sub>5</sub>AlO<sub>18</sub>)(BO<sub>3</sub>)<sub>3</sub>(OH)<sub>3</sub>(OH)
Adachite is an extremely rare mineral from the tourmaline supergroup, distinguished by its black color and occurrence only in Japan.
Properties
- Mohs hardness
- 7
- Luster
- Sub-metallic
- Streak
- Grayish black
- Density
- 3.27
- Cleavage
- None
- Fracture
- Uneven to conchoidal
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Practically the only feature allowing for preliminary field identification is its location – the Kiura mine in Japan. The black color, prismatic crystal habit, and submetallic luster, combined with mineral association (skarn minerals), are clues but do not allow for certain identification. ## Distinguishing from Similar Minerals Adachite is visually identical to other black tourmalines, mainly schorl, foitite, and dravite. Certain differentiation is possible only through advanced chemical analyses (e.g., electron microprobe), which can confirm the dominance of calcium (Ca) and divalent iron (Fe²⁺) in the appropriate positions in the crystal structure. ## Crystal Forms Adachite forms prismatic, columnar crystals, typical of tourmalines, often with characteristic vertical striations on the faces. These crystals rarely occur individually, usually forming radial or chaotic aggregates embedded in the host rock.
Geological environment
## Genesis Adachite is a mineral typical of skarn environments. It forms in the contact zone between a granitic porphyry intrusion and carbonate rocks (limestones) under conditions of contact metamorphism. ## Mineral Associations This mineral occurs in association with other minerals typical of skarns. It most commonly co-occurs with akinite-(Fe), epidote, quartz, calcite, and fluorite. ## Localities The only confirmed locality of adachite in the world is its type locality – the Kiura mine (also called Shin-Kiura) in Saiki City, Ōita Prefecture, on Kyushu Island, Japan.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of adachite collector specimens primarily depends on the degree of crystal development and size. The most prized specimens are those with distinct, radial aggregates of well-formed, lustrous crystals, contrastingly set on a light rock matrix (e.g., quartz or calcite). The size of the aggregates is also an important factor. ## Popular Localities All adachite specimens available on the collector's market come from a single location in the world – the Kiura mine in Japan. This makes every specimen from this locality automatically valuable from the perspective of systematic and mineralogical geography.
Care and storage
## Cleaning Adachite specimens should be cleaned with great care. It is recommended to remove dust with compressed air or a very soft, dry brush. For heavier soiling, a damp cotton swab with distilled water can be used, immediately drying the specimen. ## What to Avoid Avoid ultrasonic and steam cleaners, which can cause cracks in brittle crystals. The mineral is sensitive to strong acids and bases. It should not be subjected to sudden temperature changes. ## Storage Adachite, as a hard mineral, can scratch other, softer specimens. It is best stored in a separate, padded box or on a dedicated stand, away from dust and direct sunlight, which, however, does not affect its color.