Preiswerkite

Chemical formula: NaAlMg₂(Si₂Al₂)O₁₀(OH)₂

A rare mineral from the mica group, a member of the isomorphic series with phengite, occurring as fine flakes and lamellae.

## Characteristics Preiswerkite is a mineral belonging to the mica group, being the sodium and magnesium analogue of phengite. It usually occurs as very fine, scaly or platy crystals, rarely exceeding 1 mm. It forms scaly aggregates, rosettes, or appears as disseminated flakes within the host rock. Due to the small size of the crystals, its macroscopic features are difficult to observe without magnification. ## Physical Properties Preiswerkite is characterized by a Mohs hardness of 2.5-3. It has perfect cleavage in one direction, typical of micas, allowing it to split into thin, flexible lamellae. The luster is vitreous to pearly on cleavage surfaces. The mineral's density is approximately 3.07 g/cm³. ## Colors and Varieties This mineral is usually colorless, white, or has a pale greenish tint. There are no known colored or commercial varieties. ## History and Name Preiswerkite was first described in 1979 by Jan C. Hunziker, A.W.F. Schleicher, and E. Kubler. The name honors Heinrich Preiswerk (1876-1940), a Swiss geologist and professor of mineralogy at the University of Basel, who was the first to study rocks from the mineral's type locality. ## Uses Preiswerkite has no industrial or commercial significance. Its occurrence is solely of scientific importance and is of interest to collectors specializing in rare or systematic minerals.

Properties

Mohs hardness: 2.5-3
Luster: Vitreous to pearly
Streak: White
Density: 3.07
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Identifying preiswerkite based on macroscopic features is practically impossible and requires advanced analytical techniques such as X-ray diffraction (XRD) or chemical analysis (EDS/WDS). In collections, identification relies on a label from a confirmed locality. A characteristic feature, visible under a microscope, is the habit of small, hexagonal flakes and perfect mica cleavage. ## Distinguishing from Similar Minerals Preiswerkite is indistinguishable to the naked eye from other fine-grained micas, such as muscovite, phengite, paragonite, or talc. All these minerals form similar aggregates and have a comparable appearance. Definitive differentiation is only possible through chemical analysis, which will show high sodium and magnesium content with a simultaneous absence of potassium, which is crucial for preiswerkite identification. ## Crystal Forms Preiswerkite crystals are very small, usually less than 1 mm in diameter. They take the form of hexagonal (pseudohexagonal) tablets and flakes. They occur as single, disseminated crystals in the rock or form dense, felted aggregates and rosettes.

Geological environment

## Genesis Preiswerkite is a metamorphic mineral, forming under conditions of high-pressure and relatively low-temperature metamorphism. It forms in rocks rich in aluminum, sodium, and magnesium, such as talc schists, serpentinites, or amphibolites, which have undergone transformations in the amphibolite or eclogite facies. ## Mineral Associations This mineral co-occurs with other metamorphic minerals, such as talc, chlorite, amphiboles (e.g., glaucophane, hornblende), kyanite, staurolite, garnet (almandine-pyrope), phengite, paragonite, as well as dolomite and quartz. ## Localities The most important and classic occurrences of preiswerkite are in the Alps, from which the type locality originates – Geisspfad in the Binn Valley (Valais) in Switzerland. It is also known from other Alpine localities in Italy (Aosta Valley, Western Alps) and Austria. Outside the Alps, its occurrence has been noted, among others, in Spain, Greece, Siberia in Russia, and Japan.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a preiswerkite specimen is determined primarily by the richness of its occurrence and its association with other minerals. Since the crystals are always small, their size is not a key factor. The most desirable specimens are those where well-formed, though fine, rosettes or dense aggregates of flakes are visible against a contrasting background of the host rock. Co-occurrence with other rare or aesthetic minerals from the same paragenesis enhances its attractiveness. ## Popular Localities The most prized specimens by collectors come from classic Alpine localities, especially from the type locality in Geisspfad, Switzerland, and from the Aosta Valley in Italy. Specimens from these locations are considered reference material for this mineral species.

Care and storage

## Cleaning Preiswerkite specimens, due to their delicacy and small size, should be cleaned with utmost care. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, distilled water and a very soft brush can be used, avoiding disturbance of the delicate lamellae. Ultrasonic cleaners should be avoided. ## What to Avoid Avoid contact with acids and strong chemicals, which can damage the mineral. The lamellae are very soft and susceptible to scratching and mechanical damage. The mineral is sensitive to high temperatures. ## Storage Specimens containing preiswerkite are best stored in closed collector's boxes (micromount type) to protect them from dust and mechanical damage. Due to the small size of the crystals, display requires magnification.

Sources

IMA Mineral List (CNMNC)