Siderophyllite

Chemical formula: KFe²⁺₂Al(Si₂Al₂)O₁₀(OH)₂

Siderophyllite is an iron-rich mineral from the biotite group, forming dark green to black, platy crystals with perfect cleavage.

## Characteristics Siderophyllite is a mineral from the mica group, belonging to the biotite series, in which iron predominates. Its name accurately reflects its chemical composition (iron) and habit (foliated, platy). Typical specimens consist of platy or scaly aggregates that can be easily separated into thin, flexible sheets. Crystals, though rare, take on a tabular form with a pseudohexagonal outline. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5-3. It is characterized by a vitreous, pearly, or slightly metallic luster, especially visible on cleavage planes. It is translucent, and in very thin sheets, it can be almost transparent. Its density is approximately 3.1 g/cm³. ## Colors and Varieties Siderophyllite occurs in shades of dark green, brown, and black. The color is directly related to its high iron content. There are no named commercial or gemological varieties. ## History and Name The name siderophyllite comes from the Greek words *sideros* (σίδηρος) meaning "iron" and *phyllon* (φύλλον) meaning "leaf," referring to its chemical composition and platy structure. The mineral was first described in 1880 based on specimens found in the Pikes Peak region of El Paso County, Colorado, USA, which is its type locality. ## Uses Siderophyllite has no significant industrial application. However, it is an interesting mineral for collectors specializing in micas, petrologists, and researchers of rock-forming minerals.

Properties

Mohs hardness: 2.5-3
Luster: Vitreous, Pearly
Streak: Gray, Greenish
Density: 3.1
Cleavage: {001}
Fracture: Uneven
Transparency: Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Siderophyllite is identified by its dark, greenish-black or brown color, platy structure, and perfect cleavage in one direction, allowing for the separation of thin, flexible sheets. The pearly luster on cleavage planes is also characteristic. ## Distinguishing from similar minerals It can be confused with other dark micas from the biotite group, such as annite or phlogopite. Distinguishing it from annite (which is the purely iron end-member of the series) is practically impossible without advanced chemical analyses (EDS, WDS). From dark phlogopite (an iron-rich variety), it usually differs in darker color and higher iron content relative to magnesium. ## Crystal forms It most often occurs as platy, scaly, or tabular aggregates. Well-formed crystals are rare and take the form of hexagonal, tabular monoclinic crystals with a pseudohexagonal outline.

Geological environment

## Genesis Siderophyllite is a rock-forming mineral. It forms mainly in acidic igneous rocks, such as granites, syenites, and granitic pegmatites. It also occurs in some metamorphic rocks, for example, in skarns and gneisses that have undergone iron metasomatism. ## Mineral associations It often co-occurs with other minerals typical of pegmatites and granites, such as quartz, feldspars (especially microcline and albite), other micas (muscovite), as well as topaz, fluorite, cassiterite, and minerals from the tourmaline group. ## Localities Important worldwide localities include: the type locality at Pikes Peak in Colorado (USA), where it occurs in pegmatites; Mont Saint-Hilaire in Quebec (Canada) in nepheline syenites; the Langesundsfjord Islands in Norway; the Kola Peninsula in Russia; and various localities in Finland and Sweden.

Rarity

Not very common

For collectors

## Quality criteria The most prized specimens by collectors are those with well-formed, sharp crystals with a pseudohexagonal outline. The size of the crystals, their luster, and the absence of mechanical damage are also important. Well-defined associations with other minerals, such as smoky quartz, microcline, or fluorite, forming aesthetic compositions, are also attractive. ## Popular localities Specimens of collector's significance mainly come from classic pegmatite localities. Those from the Pikes Peak region in Colorado (USA) and Mont Saint-Hilaire in Canada, where well-formed crystals originate, are particularly sought after.

Care and storage

## Cleaning Siderophyllite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, compressed air can be used. Avoid washing with water, which can penetrate between the lamellae and cause delamination or corrosion of inclusions. ## What to avoid Avoid contact with chemicals, especially acids, which can damage the mineral. As a soft and brittle mineral, it is susceptible to scratches and mechanical damage. The lamellae easily bend and break. It should be protected from high temperatures. ## Storage Siderophyllite specimens are best stored in closed display boxes to protect them from dust and mechanical damage. Due to its fragility, it should not be stored in contact with harder minerals.