Pyrosmalite-(Fe)

Chemical formula: Fe₂+₈Si₆O₁₅(OH)₁₀

An iron silicate with platy, hexagonal crystals and a characteristic odor emitted when heated.

## Characteristics Pyrosmalite-(Fe) is a mineral from the phyllosilicate group, being the iron analogue of pyrosmalite-(Mn). It forms characteristic, well-developed, platy crystals with a hexagonal outline. It also occurs as lamellar, scaly aggregates or in massive forms. Its color ranges from pale yellow, brown, and grayish-green to almost black. It exhibits a pearly luster on cleavage planes, while on other surfaces, it is greasy or vitreous. ## Physical Properties This mineral is characterized by a Mohs hardness of 4.5 and a density ranging from 3.06-3.19 g/cm³. It possesses perfect cleavage in one plane, making it easily separable into thin lamellae. It is brittle, and its fracture is uneven. It is usually translucent to opaque. ## Colors and Varieties Pyrosmalite-(Fe) is a member of an isomorphic series with pyrosmalite-(Mn), in which iron predominates over manganese. Its coloration is directly related to its iron content and includes shades of brown, green, and gray. No named commercial or colored varieties are distinguished. ## History and Name The mineral's name, given in 1808 by Friedrich Hausmann, comes from the Greek words *pyros* (fire) and *osme* (smell). It refers to the specific, pungent odor emitted by the mineral when heated in a flame. The suffix "-(Fe)" was added in 1987 by the International Mineralogical Association (IMA) to distinguish it from its manganese counterpart within the pyrosmalite group. ## Uses Pyrosmalite-(Fe) has no industrial applications. However, it is a mineral valued by collectors and holds scientific significance in studies of metamorphic processes in iron and manganese ore deposits.

Properties

Mohs hardness: 4.5
Luster: Pearly to Greasy
Streak: Pale yellowish brown
Density: 3.06 - 3.19
Cleavage: Perfect on {0001}
Fracture: Uneven
Transparency: Translucent to Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Key identifying features of pyrosmalite-(Fe) are its hexagonal crystal habit, perfect basal cleavage, and pearly luster on cleavage planes. Its characteristic odor when heated is also diagnostic, although this is a destructive test. Occurrence in paragenesis with iron and manganese ore minerals is a strong diagnostic indicator. ## Distinguishing from Similar Minerals Pyrosmalite-(Fe) is sometimes confused with biotite or stilpnomelane. It is distinguished from biotite by its greater hardness and lack of elasticity of the lamellae. Stilpnomelane more often forms radial aggregates and has a different luster hue. Differentiation from pyrosmalite-(Mn) is usually impossible without chemical analysis (EDS, WDS). ## Crystal Forms It most commonly forms thick or thin platy crystals with a hexagonal outline. It also occurs as lamellar, scaly, foliated aggregates, and as massive or granular aggregates.

Geological environment

## Genesis Pyrosmalite-(Fe) is a mineral typical of metamorphosed, stratiform iron and manganese ore deposits. It also forms in calc-iron-manganese skarns. Less commonly, it is found as a product of hydrothermal alteration in volcanic and subvolcanic rocks. ## Mineral Associations It co-occurs with minerals such as magnetite, hematite, grunerite, dannemorite, rhodonite, tephroite, stilpnomelane, as well as with sulfides, e.g., pyrite and chalcopyrite. ## Localities The most important localities worldwide include the historical type locality at the Bjelke mine in Nordmark (Sweden) and the Broken Hill deposit in New South Wales (Australia), from which large and well-formed crystals originate. Other known occurrences include Sterling Hill in New Jersey (USA), as well as localities in Japan, China, and Kazakhstan.

Rarity

Rare

For collectors

## Quality Criteria The most highly valued specimens are those with sharp, well-formed, hexagonal crystals, especially of significant size and thickness. Intense, uniform color (e.g., deep brown) and transparency enhance its attractiveness. Specimens on matrix, especially in association with contrasting minerals (e.g., magnetite), are particularly sought after. ## Popular Localities The localities in Nordmark (Sweden) and Broken Hill (Australia) are considered classic and provide the best specimens. Specimens from these localities are the standard for this mineral and fetch the highest prices in the collector's market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, preferably with a soft brush dry. If necessary, distilled water can be used, but prolonged soaking should be avoided, and the specimen should be dried immediately. Due to its perfect cleavage, mechanical cleaning must be exceptionally gentle. ## What to Avoid Ultrasonic cleaners must be absolutely avoided, as they can cause the specimen to disintegrate along cleavage planes. The mineral is sensitive to acids and other strong chemicals. It should be protected from impacts and sudden temperature changes. ## Storage It is recommended to store specimens in separate, padded boxes to protect them from mechanical damage. Avoid areas with high humidity and direct exposure to sunlight, which can affect its color over time.

Sources

IMA Mineral List (CNMNC)