Almarudite
Chemical formula: K(☐,Na)<sub>2</sub>(Mn<sup>2+</sup>,Fe<sup>2+</sup>,Mg)<sub>2</sub>[(Be,Al)<sub>3</sub>Si<sub>12</sub>]O<sub>30</sub>
Almarudite is an extremely rare mineral of the osumilite group, forming small, hexagonal, yellowish crystals found in volcanic rocks of the Eifel region in Germany.
Properties
- Mohs hardness
- 6
- Luster
- Vitreous
- Streak
- White
- Density
- 2.75
- Cleavage
- Poor
- Fracture
- Conchoidal to uneven
- Transparency
- Transparent to translucent
- Crystal system
- Hexagonal
Diagnostic features
## Identification Helpful features for identifying almarudite include its characteristic hexagonal crystal form, yellowish or brownish color, and vitreous luster. However, the geological context is crucial – its occurrence in sanidinite xenoliths within volcanics from the Eifel region. Definitive identification is only possible using advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS). ## Distinguishing from Similar Minerals Almarudite can be confused with other rare minerals of the osumilite group found in the same locality, such as osumilite or eifelite. However, osumilite is usually blue, gray, or brown, while eifelite is colorless. It is distinguished from other similar minerals from Eifel (e.g., roedderite) by its chemical composition, which requires specialized analysis. ## Crystal Forms Almarudite forms hexagonal crystals with a tabular or short prismatic habit. They usually occur as single, well-formed crystals in small cavities within the host rock.
Geological environment
## Genesis Almarudite forms under specific conditions of contact metamorphism, at high temperature and low pressure. It crystallizes in xenoliths of silica- and aluminum-rich rocks (sanidinites) that have been entrained and transported to the surface by volcanic lava. ## Mineral Associations This mineral coexists with other rare minerals typical of the Eifel volcanic environment. Its most common associations include sanidine, tridymite, quartz, hematite, as well as members of the osumilite group, such as osumilite, eifelite, and roedderite. ## Localities The type locality and main source of the best almarudite specimens is the Caspar quarry on the Bellerberg volcano near Ettringen and Mayen, in the Eifel region, Rhineland-Palatinate, Germany. Beyond this locality, its occurrence has only been confirmed in a few other places worldwide, including Jordan.
Rarity
Extremely rare
For collectors
## Quality Criteria The collector's value of almarudite is primarily determined by its extreme rarity. The most desirable specimens are those with well-formed, sharp, hexagonal crystals, even if they are microscopic in size. Additional advantages include an intense, yellow or orange-brown color, transparency, and the aesthetic placement of the crystal on a contrasting rock matrix, often accompanied by other rare minerals. ## Popular Localities The only place in the world from which almarudite specimens of collector's significance originate is the Eifel volcanic region in Germany, and specifically the Caspar quarry on Bellerberg.
Care and storage
## Cleaning Almarudite specimens, typically micromounts, should be cleaned with utmost care. For dust removal, it is best to use compressed air from a safe distance or a very soft brush. If necessary, the specimen can be rinsed in distilled water and then thoroughly dried. The use of ultrasonic cleaners is strongly discouraged. ## What to Avoid Avoid contact of the mineral with acids and strong chemicals. Despite its relatively high hardness, the crystals are brittle and susceptible to mechanical damage. They should also be protected from sudden temperature changes. ## Storage The safest way to store almarudite is to place it in a specialized, sealed micromount box. This protects the small and valuable specimen from dust, moisture, and, most importantly, from accidental damage or loss.