Althausite

Chemical formula: Mg<sub>4</sub>(PO<sub>4</sub>)<sub>2</sub>(OH,O)(F,&#9744;)

Althausite is a rare magnesium phosphate mineral, found in serpentinite rocks, valued by collectors for its unique origin.

## Characteristics Althausite is a rare magnesium phosphate, most often forming anhedral or subhedral grains up to several centimeters in size. It is less commonly found as small, prismatic crystals. It usually occurs as embedded aggregates within the rock, and its inconspicuous appearance makes it easy to overlook. ## Physical Properties The mineral's Mohs hardness is 4, and its density is approximately 3.13 g/cm³. It exhibits a vitreous luster, transitioning to greasy on some surfaces. It is a brittle mineral, translucent to opaque. ## Colors and Varieties Althausite occurs in shades of gray—from grayish-white, through light gray, to pale brownish-gray or pale bluish-gray. No color varieties or commercial varieties have been distinguished. ## History and Name The mineral was first described in 1975 by Gunnar Raade and Magne Tysseland. Its name comes from Egon Althaus (1933-2022), a German mineralogist and petrologist from the University of Karlsruhe, in recognition of his contributions to the study of minerals and metamorphic rocks. The type locality is the Tingelstadtjern quarry, Modum, Norway. ## Uses Due to its rarity and small accumulations, althausite has no industrial application. It is solely an object of scientific interest and a prized acquisition in advanced collections of systematic and regional minerals.

Properties

Mohs hardness
4
Luster
Vitreous to greasy
Streak
White
Density
3.13
Cleavage
Good on {010} and {100}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Field identification of althausite is difficult due to its uncharacteristic appearance. A key clue is its specific occurrence environment—serpentinites rich in magnesite. In a collection, it can be recognized by its gray color, vitreous-greasy luster, and relatively low hardness. However, definitive confirmation requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis. ## Distinguishing from Similar Minerals Althausite can be confused with other gray minerals found in similar environments, such as apatite, wagnerite, or some feldspars. It is distinguished from apatite by its lower hardness. Wagnerite, to which it is closely related, has very similar properties, and often the only way to differentiate them is through laboratory analysis. Unlike feldspars, althausite has a higher density. ## Crystal Forms Althausite crystals are rare, short, and prismatic. It usually forms granular, massive aggregates embedded in the host rock. These grains can reach sizes up to 15 cm.

Geological environment

## Genesis Althausite is a mineral of metamorphic origin. It forms in phlogopite-magnesite-serpentine lenses within serpentinized dunites. Its formation is a result of metamorphic processes acting on ultramafic rocks in the presence of phosphorus-rich fluids. ## Mineral Associations This mineral co-occurs with other minerals typical of its formation environment. The most common associations include magnesite, phlogopite, serpentine, apatite, enstatite, talc, wagnerite, xenotime-(Y), and holtedahlite. ## Localities The most important and historical occurrence of althausite is its type locality in Modum, Norway, from which the best-known specimens originate. Its presence has also been confirmed in several other locations worldwide, including the tin ore deposit in Ehrenfriedersdorf, Saxony (Germany). However, all its occurrences are very limited and rare.

Rarity

Very rare

For collectors

## Quality Criteria The collector's value of althausite is primarily determined by its rarity. Specimens with well-formed, even if small, prismatic crystals are the most desirable. Rich granular aggregates, clearly visible in the host rock (matrix), are also highly valued. An additional advantage is its co-occurrence with other rare minerals from the same locality, such as holtedahlite. ## Popular Localities By far the most prized and sought-after specimens by collectors come from the type locality—the Tingelstadtjern quarry in Modum, Norway. Material from this locality is considered classic for this species.

Care and storage

## Cleaning Althausite specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, it can be rinsed with distilled water and then thoroughly dried. Ultrasonic cleaners should be avoided, as they can damage the brittle mineral. ## What to Avoid Althausite, as a phosphate, is sensitive to acids, which can damage it. Contact with all household and laboratory chemicals should be avoided. The mineral should also be protected from high temperatures and sudden temperature changes. ## Storage It is recommended to store specimens in separate, closed collector boxes, lined with soft material, to prevent abrasion and mechanical damage. A label with the name and locality is essential due to the mineral's inconspicuous appearance.

External references

Sources

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