Povondraite

Chemical formula: NaFe₃+₃(Fe₃+₄Mg₂)(Si₆O₁₈)(BO₃)₃(OH)₃O

Povondraite is a very rare, black mineral from the tourmaline group, distinguished by its unique chemical composition rich in trivalent iron.

## Characteristics Povondraite is a rare member of the tourmaline supergroup, classified as an oxygen-bearing tourmaline. It forms black, less commonly dark brown, prismatic crystals with the characteristic triangular cross-section and rounded edges typical of tourmalines. It usually occurs as radial or tangled aggregates, less often as single, well-formed crystals. It is opaque, and only in very thin fragments can it be translucent in shades of brown. ## Physical Properties This mineral is characterized by a hardness of 7 on the Mohs scale, which is typical for tourmalines. It has a vitreous luster, sometimes transitioning to resinous. Its density ranges from 3.21-3.31 g/cm³. It does not exhibit cleavage, and its fracture is uneven to subconchoidal. ## Colors and Varieties Povondraite is a mineral of uniform, black or very dark brown color. It does not form color varieties or have special trade names. Its value and significance are purely scientific and collectible. ## History and Name The mineral was officially recognized by the IMA in 1993. Its name honors Professor Pavel Povondra (1924–2013), a Czech mineralogist from Charles University in Prague, who made significant contributions to the study of the chemistry and crystal chemistry of the tourmaline group. Type specimens come from Bolivia. ## Applications Due to its rarity, povondraite has no industrial applications. It is solely an object of scientific interest and a sought-after acquisition for advanced mineral collectors, especially those specializing in the tourmaline group.

Properties

Mohs hardness: 7
Luster: Vitreous
Streak: Brown
Density: 3.21-3.31
Cleavage: Indistinct
Fracture: Uneven
Transparency: Opaque to Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Povondraite, under collecting conditions, is practically impossible to distinguish from other black tourmalines, such as schorl, without advanced analysis. Macroscopic features, such as black color, prismatic crystal habit with a triangular cross-section, high hardness (scratches steel and glass), and lack of cleavage, only allow for the identification of the mineral as a member of the tourmaline group. ## Distinguishing from Similar Minerals Distinguishing it from schorl, the most common black tourmaline, requires chemical analysis (e.g., EDS) to confirm the dominance of trivalent iron (Fe³⁺) and high magnesium content, which is a defining characteristic of povondraite. Schorl is an iron-sodium tourmaline, where divalent iron (Fe²⁺) dominates. ## Crystal Forms Povondraite crystals are trigonal, with a prismatic habit, often vertically striated. A characteristic feature is their rounded cross-section in the shape of a spherical triangle. It often forms radial aggregates or disordered clusters of tangled crystals.

Geological environment

## Genesis Povondraite forms under specific geological conditions. Its main environment of formation is metamorphosed, magnesium- and boron-rich marine sediments. It also occurs in some granitic pegmatites, where it crystallizes from post-magmatic solutions. ## Mineral Associations In its type locality in Bolivia, povondraite coexists with quartz and albite. In other localities, it may be accompanied by schorl, elbaite, and other minerals typical of pegmatites and metamorphic rocks. ## Localities The most important and globally renowned locality for povondraite, from which the best specimens originate, is the San Francisco mine, located near Villa Tunari in the Chapare Province of Bolivia. This mineral has also been reported in the Czech Republic (Pikárec) and Italy (Elba Island).

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued povondraite specimens are those consisting of well-formed, single crystals with strong luster and a distinct, prismatic habit. Aesthetic radial aggregates are also highly valued. Specimens on a light rock matrix, such as quartz or albite, are particularly sought after due to their attractive contrast. ## Popular Localities The vast majority of the best collectible povondraite specimens come from its type locality – the San Francisco mine in Bolivia. Specimens from other localities are much rarer and usually have less aesthetic value.

Care and storage

## Cleaning Povondraite specimens are best cleaned with a soft brush and lukewarm distilled water. For heavier dirt, a small amount of mild soap can be used, followed by thorough rinsing. Strong detergents should be avoided. ## What to Avoid The mineral is sensitive to strong acids. It should not be subjected to sudden temperature changes or cleaned in ultrasonic cleaners, as internal stresses in the crystal can cause it to crack. ## Storage Povondraite is a relatively hard and durable mineral. It can be stored under standard conditions. To avoid scratching the surface and losing its luster, it should be protected from contact with harder minerals (e.g., quartz, corundum) and stored in separate boxes or on a soft surface.