Prismatine

Chemical formula: (Mg,Al,Fe²⁺,Fe³⁺)₆Al₄(Si,Al)₄(B,Si,Al)(O,OH,F)₂₂

Prismatine is a rare borosilicate with a complex chemical composition, forming prismatic, almost black crystals.

## Characteristics Prismatine is a complex borosilicate belonging to the kornerupine group. It forms elongated, prismatic crystals, often with a nearly rectangular cross-section. It typically occurs as embedded, well-formed crystals within the host rock. Specimens are most often opaque, ranging in color from dark green and dark brown to almost black. ## Physical Properties This mineral is characterized by significant hardness, approximately 7 on the Mohs scale. It has a vitreous luster and is brittle. The density of prismatine ranges from 3.29 to 3.41 g/cm³. ## Colors and Varieties The dominant colors of prismatine are dark green, brown, and black. Rarer specimens exhibit lighter, greenish or yellowish-brown hues. No named varieties of this mineral are distinguished. ## History and Name The name "prismatine," given in 1886 by Augustin Alexis Damour, refers to the typical prismatic crystal habit of this mineral. It was first described based on specimens found in Waldheim, Saxony (Germany). ## Applications Due to its rarity and generally small crystal sizes, prismatine has no industrial applications. It is solely an object of interest for collectors and scientists.

Properties

Mohs hardness: 6.5-7
Color: Gray-green, gold brown, purplish gray, gray
Luster: Vitreous
Streak: Gray
Density: 3.341
Cleavage: {110}
Fracture: Uneven, Sub-Conchoidal
Transparency: Translucent,Opaque
Crystal system: Orthorhombic

Diagnostic features

## Identification Key diagnostic features of prismatine include its prismatic crystal habit, high hardness (approx. 7), dark color (green, brown to black), and vitreous luster. Its occurrence in a specific metamorphic environment is also an important clue. ## Distinguishing from Similar Minerals It can be confused with tourmaline (schorl), hornblende, or pyroxenes. It is distinguished from schorl by its crystal cross-section (usually tetragonal, not trigonal) and slightly different hardness. It differs from hornblende and pyroxenes by its higher hardness and lack of distinct cleavage at specific angles. ## Crystal Forms Prismatine forms columnar, elongated crystals with a prismatic habit. Crystals often exhibit striations on the side faces, parallel to the main axis. It occurs as single, embedded crystals or in radial aggregates.

Geological environment

## Genesis Prismatine is a mineral of metamorphic rocks that have undergone transformations under conditions of high temperatures and pressures (amphibolite and granulite facies metamorphism). It forms in rocks rich in magnesium and aluminum, but poor in silica, often with the involvement of boron. ## Mineral Associations It most commonly co-occurs with minerals such as cordierite, anthophyllite, phlogopite, spinels, corundum, sapphirine, gedrite, and tourmalines. ## Localities Important localities worldwide include: the type locality in Waldheim, Saxony (Germany); the area around Port Shepstone in the Republic of South Africa; the Lac-Sainte-Marie region in Quebec (Canada); the Strangways Range area in the Northern Territory (Australia); and the vicinity of Caseville in New York State (USA).

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp, and undamaged crystals that clearly stand out from the host rock (matrix). Large crystals are sought after, although they are extremely rare. Transparency is not a key criterion, as this mineral is usually opaque. ## Popular Localities Specimens from the classic locality in Waldheim, Germany, have historical value. Good quality crystals, though usually small, also come from Canada (Quebec) and the USA (New York).

Care and storage

## Cleaning Prismatine specimens can be cleaned with lukewarm water and a soft brush to remove dust and loose debris. Strong detergents should be avoided. ## What to Avoid The mineral is sensitive to strong acids, especially hydrofluoric acid. It should be protected from sudden temperature changes and strong impacts, which can cause fractures. ## Storage It is recommended to store specimens in individual, padded boxes or on stands to prevent scratching by harder minerals. It does not require special conditions regarding light or humidity.