Oxyphlogopite

Chemical formula: K(Mg,Ti₄+,Fe₃+,Fe₂+)₃[(Si,Al)₄O₁₀](O,F)₂

Oxyphlogopite is a rare mica group mineral, rich in titanium and iron, distinguished by its dark color and platy habit.

## Characteristics Oxyphlogopite is a mineral from the mica group, belonging to the phyllosilicates. It is an oxygen analog of phlogopite, where oxygen replaces fluorine and the hydroxyl group in the structure. It forms crystals with a tabular or platy habit, often with a pseudohexagonal outline. It typically occurs as disseminated flakes or scaly aggregates within the rock. Its appearance is typical of micas – with a perfectly developed, flat basal surface and a characteristic luster. ## Physical Properties This mineral is characterized by a hardness ranging from 2-3 on the Mohs scale, meaning it is relatively soft. Its lamellae are elastic and flexible. It exhibits perfect, unidirectional cleavage, allowing for easy separation of thin, transparent sheets. The luster on cleavage surfaces is vitreous to pearly. The density is approximately 2.86 g/cm³. ## Colors and Varieties Oxyphlogopite ranges in color from reddish-brown to almost black, which is associated with the high content of titanium and iron in its chemical composition. No named color varieties or commercial varieties are distinguished. ## History and Name The name "oxyphlogopite" refers to its chemical composition – it is a phlogopite in which oxygen (O²⁻) is the dominant anion in the hydroxyl group. It was formally described as a distinct mineral species and approved by the International Mineralogical Association (IMA) in 2006. Its discovery is linked to research on micas from unusual volcanic environments.

Properties

Mohs hardness: 2-3
Luster: Vitreous to pearly
Streak: Light brown
Density: 2.86
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The key diagnostic feature of oxyphlogopite is its micaceous character: perfect, unidirectional cleavage allowing for the separation of thin, flexible lamellae. It is distinguished from other micas by its dark, reddish-brown to black color and specific occurrence environment. It is opaque or translucent only on the thinnest edges. ## Distinguishing from Similar Minerals Oxyphlogopite can be confused with biotite or phlogopite. It is usually darker than phlogopite. Final differentiation from biotite and other dark micas often requires advanced chemical analyses (e.g., electron microprobe analysis) to determine the content of titanium, iron, and the ratio of oxygen to fluorine and OH groups. ## Crystal Forms This mineral forms hexagonal, tabular, or platy crystals. It most commonly occurs as scaly and platy aggregates, disseminated in the host rock.

Geological environment

## Genesis Oxyphlogopite is a high-temperature mineral, typical of certain volcanic rocks. It forms under low-pressure and high-temperature conditions, in an environment poor in water and fluorine. It crystallizes directly from magma or through metasomatic processes in the contact zones of alkaline intrusions. ## Mineral Associations It co-occurs with minerals typical of volcanic and metasomatic rocks, such as diopside, sanidine, leucite, apatite, magnetite, and other mica group minerals. ## Localities The most important and well-documented occurrences of oxyphlogopite are in Italy, in the volcanic complexes of Lazio (e.g., in volcanic tuffs in the Alban Hills and Monti Sabatini areas) and in the Vesuvius massif. It has also been reported in Germany (Eifel) and Russia.

Rarity

Very rare

For collectors

## Quality Criteria For collectors, the most valuable specimens are those with well-formed, sharp crystals with a pseudohexagonal outline. Rich aggregates of lamellae contrasting with light host rock are also prized. Luster, crystal size, and the absence of mechanical damage significantly increase the attractiveness of a specimen. ## Popular Localities The most well-known collector specimens come from localities in Italy, especially from the Vesuvius and Lazio regions, where this mineral has been extensively studied.

Care and storage

## Cleaning Oxyphlogopite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. If necessary, the specimen can be rinsed with distilled water and then thoroughly dried, avoiding leaving moisture between the lamellae. ## What to Avoid Avoid contact with chemicals, especially acids, which can damage the mineral's structure. The lamellae are soft and susceptible to scratches and mechanical damage, such as bending or breaking. Do not heat the specimen or expose it to sudden temperature changes. ## Storage Specimens should be stored in individual, padded boxes to protect them from scratching by harder minerals. Avoid storage in humid environments, as water can penetrate between the lamellae and cause delamination.

Sources

IMA Mineral List (CNMNC)