Bonaccordite
Chemical formula: Ni<sup>2+</sup><sub>2</sub>Fe<sup>3+</sup>O<sub>2</sub>(BO<sub>3</sub>)
Bonaccordite is an extremely rare, black borate mineral, distinguished by its metallic luster and high hardness.
Description
## Characteristics Bonaccordite is a nickel iron borate, typically occurring as massive or granular aggregates. Its black, opaque mass with a strong metallic luster is its most typical form. Well-formed crystals are extremely rare and small in size. This mineral is heavy and hard, which is its characteristic physical feature. ## Physical Properties Bonaccordite is characterized by significant hardness, ranging from 6.5-7.5 on the Mohs scale, which is a high value for borates. It has a high density, ranging from 5.0 to 5.11 g/cm³. It is completely opaque, and its luster is described as metallic. It exhibits good cleavage in two directions. ## Colors and Varieties This mineral is monochromatic – it occurs exclusively in black. No colored or commercial varieties are known. ## History and Name The name bonaccordite comes from its type locality – the Bon Accord nickel-iron deposit in the Barberton region of South Africa. It was first described in 1974 by S. A. de Waal, E. A. Viljoen, and L. C. Callow. ## Applications Due to its extreme rarity, bonaccordite has no industrial applications. Its significance is purely scientific and collectible, being a desirable object for specialized collectors and research institutions.
Diagnostic features
## Identification Key features for identifying bonaccordite are: black color, strong metallic luster, black streak, and high hardness (6.5-7.5). Its high density is also noticeable even in small samples. The most important diagnostic clue, however, is its association with other rare nickel minerals. ## Distinguishing from Similar Minerals Bonaccordite is visually almost identical to trevorite, with which it often co-occurs. Trevorite, however, is significantly softer (hardness approx. 5) and crystallizes in the isometric system. Distinguishing it from other black, metallic minerals, such as magnetite or chromite, may require testing. Magnetite is strongly magnetic, and chromite usually has a brownish-black streak. ## Crystal Forms Bonaccordite most commonly forms anhedral (irregularly shaped) grains intergrown in massive aggregates. It is rarely observed as small, lath-like or platy crystals.
Geological environment
## Genesis Bonaccordite is a metamorphic mineral. It forms under specific conditions within serpentinized ultramafic rocks, rich in nickel. Its occurrence is associated with processes that occurred in archaic greenstone belt rock complexes. ## Mineral Associations This mineral occurs in association with other rare nickel and iron minerals. The most common associations include trevorite, nimite, willemseite, nichromite, as well as nickel sulfides such as violarite and millerite. ## Localities The most important and historical locality for bonaccordite is the Bon Accord deposit near Barberton (Mpumalanga province, South Africa), which is its type locality. Besides this, its presence has been reported in only a few other places worldwide, including Spain (Carratraca, Málaga) and Russia (Polar Urals).
Rarity
Extremely rare
Collector aspects
## Quality Criteria For collectors, specimens where bonaccordite forms rich aggregates or is clearly visible on the rock matrix are of the highest value. Due to the rarity of well-formed crystals, even microscopic but distinct crystalline forms are highly prized. The value of a specimen is also enhanced by the presence of other rare associated minerals, especially trevorite. ## Popular Localities The only source of collectible specimens is the historical type locality – the Bon Accord deposit in South Africa. Specimens from other localities are extremely rare and usually only available in institutional collections.
Care and storage
## Cleaning Bonaccordite specimens should be cleaned very carefully, preferably with a soft brush dry to remove dust. For heavier soiling, a soft brush dampened with distilled water can be used, followed by thorough drying of the specimen. Ultrasonic cleaners should be avoided. ## What to Avoid This mineral is sensitive to strong acids. Despite its high hardness, it should be protected from impacts and falls, which can cause fractures along cleavage planes. There are no reports of its sensitivity to sunlight or temperature changes. ## Storage It is recommended to store specimens in separate, padded boxes or display cases to prevent scratching by harder minerals (e.g., corundum, diamond) and abrasion against other specimens. Exposure should protect it from dust.