Montdorite

Chemical formula: KFe²⁺₁.₅Mn²⁺₀.₅Mg₀.₅Si₄O₁₀(F,OH)₂

Montdorite is a rare mica group mineral, closely related to biotite, typically characterized by a reddish-brown color and a platy structure.

## Characteristics Montdorite is a mica group mineral belonging to the layered silicates. Its name comes from its discovery locality – the Puy de Dôme volcano in France. It is a member of the biotite group, to which it is closely related. It forms tabular or platy crystals, often aggregated into packets or clusters. Its appearance is typical for micas – a platy structure with perfect cleavage in one direction is visible. ## Physical Properties This mineral is characterized by a hardness of approximately 2.5 on the Mohs scale, which is typical for micas. It has a vitreous luster, and a pearly luster on cleavage surfaces. It is translucent to opaque. Its density is approximately 2.84 g/cm³. ## Colors and Varieties Montdorite occurs in a characteristic reddish-brown color. No color or commercial varieties are distinguished. ## History and Name The mineral's name, given in 1979 by Robert C. Rousseau, comes from the volcanic region of Monts Dore in France, where it was first identified in trachytes of the Puy de Dôme volcano. It was approved by the IMA in 1979. ## Uses Montdorite has no industrial significance. It is solely an object of interest for collectors of rare minerals and scientists studying the mica group.

Properties

Mohs hardness
2.5
Color
Green to brownish-green
Luster
Vitreous, Pearly
Streak
Light brown
Density
3.15
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Montdorite can be identified by its characteristic reddish-brown color, platy structure, and perfect cleavage in one direction, typical of micas. The luster is vitreous to pearly on cleavage planes. Low hardness (approximately 2.5) is also a diagnostic feature. ## Distinguishing from Similar Minerals Montdorite is difficult to distinguish from other reddish-brown micas, such as biotite or phlogopite, without advanced chemical analyses (EDS/WDS). Identification usually relies on knowledge of co-occurring minerals and locality. It differs from biotite by its specific chemical composition, including manganese content. ## Crystal Forms Montdorite crystals are tabular, with a pseudohexagonal outline. It occurs as scattered flakes or small, radial aggregates in the host rock.

Geological environment

## Genesis Montdorite is a mineral of magmatic origin. It forms in alkali-rich, silica-undersaturated volcanic rocks, such as trachytes. It crystallizes under conditions of high temperature and low pressure during lava cooling. ## Mineral Associations This mineral co-occurs with other minerals typical of its formation environment, such as sanidine, arfvedsonite, aegirine, fayalite, quartz, and iron and titanium oxides. ## Localities The most important and type locality for montdorite is the Monts Dore massif in Auvergne, France, particularly the Puy de Dôme volcano. It is also known from the Ilímaussaq igneous complex in Greenland and the Strange Lake massif in Canada (Quebec/Newfoundland and Labrador).

Rarity

Very rare

For collectors

## Quality Criteria The collector's appeal of montdorite primarily depends on its rarity and locality. The most prized specimens are those with well-formed, distinct lamellae of intense, reddish-brown color, embedded in a contrasting rock matrix. Crystal size is also important, although they rarely exceed a few millimeters. ## Popular Localities Undoubtedly, the most classic and desired locality for collectors is the type locality – Puy de Dôme in France. Specimens from Greenland and Canada are also valued due to their rarity.

Care and storage

## Cleaning Montdorite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its platy structure and low hardness, washing in water should be avoided, as water could penetrate between the lamellae and cause them to delaminate. If a liquid is necessary, alcohol, which evaporates quickly, is recommended. ## What to Avoid Contact with chemicals, especially acids, which can damage the mineral, should be avoided. It is susceptible to mechanical damage – it scratches and crumbles easily. The lamellae can easily separate, so it should be handled gently. ## Storage Montdorite specimens are best stored in closed display boxes to protect them from dust and mechanical damage. Other minerals should not be placed on top of them. It is stable under normal household conditions and is not sensitive to light.

External references

Sources

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