Nosean

Chemical formula: Na₈(Si₆Al₆)O₂₄(S₆+O₄)·H₂O

A silicate from the feldspathoid group, most often forming small, hexagonal crystals of gray or bluish color.

## Characteristics Nosean is a mineral from the feldspathoid group, belonging to the sodalite group. It is a sodium and aluminum silicate with additional sulfate anions. It rarely forms well-developed crystals visible to the naked eye, which usually take the form of rhombic dodecahedra or cubes, often with rounded edges. Most commonly, it occurs as anhedral grains or rounded inclusions in the host rock, which are silica-poor alkaline volcanic rocks, such as phonolites. ## Physical Properties Nosean is characterized by a hardness of 5.5 to 6 on the Mohs scale. It has a vitreous, sometimes slightly greasy luster on fracture surfaces. It is a relatively light mineral, with a density of about 2.2-2.4 g/cm³. It is brittle, and its fracture is uneven to conchoidal. It is usually translucent, less often transparent or almost opaque. ## Colors and Varieties Typical colors of nosean include shades of gray, blue, and greenish. It can also be white, yellowish, brown, or colorless. There are no distinct commercial or gemological varieties, and its value is primarily scientific and collectible. ## History and Name The mineral's name, given in 1815 by Martin Heinrich Klaproth, honors the German mineralogist Karl Wilhelm Nose (1753–1835), who was the first to study this mineral in the volcanic rocks of the Rhineland in Germany. The type locality is Maria Laach in the Eifel mountains. ## Uses Nosean has no industrial significance. Its use is limited to objects of scientific interest and as a collector's mineral, especially in the form of well-formed microcrystals in the host rock.

Properties

Mohs hardness: 5.5-6
Luster: Vitreous to greasy
Streak: White
Density: 2.22-2.40
Cleavage: Indistinct on {110}
Fracture: Uneven to conchoidal
Transparency: Transparent to translucent
Crystal system: Cubic

Diagnostic features

## Identification Nosean is most often identified by its occurrence in a specific geological environment – alkaline volcanic rocks, such as phonolites. In granular form, it is difficult to identify without specialized examination. A characteristic feature is its reaction with hydrochloric acid, in which it gels. Under ultraviolet light, it may exhibit reddish-orange or pink fluorescence. ## Distinguishing from Similar Minerals Nosean is very difficult to distinguish from other minerals of the sodalite group (sodalite, haüyne, lazurite) based on visual characteristics. Haüyne is usually more intensely blue. Sodalite can also be blue but has a slightly different chemical composition. Definitive differentiation requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD). ## Crystal Forms Nosean crystals, though rare, have an isometric habit. Most often, they are rhombic dodecahedra, sometimes cubes, often with modified and rounded faces. It usually occurs as disseminated, rounded grains in the rock.

Geological environment

## Genesis Nosean is a magmatic mineral, crystallizing from lava and magma rich in sodium and other alkaline elements, and simultaneously poor in silica. It forms in the late stages of crystallization in volcanic and subvolcanic rocks, such as phonolites and nephelinites. ## Mineral Associations This mineral often co-occurs with other feldspathoids, such as leucite, nepheline, sodalite, and haüyne. It is also accompanied by sanidine, aegirine, eudialyte, and various minerals from the amphibole and pyroxene groups. ## Localities The most important and classic localities for nosean are in the Eifel volcanic complex in Germany (especially in the area of Lake Laacher See). It also occurs in the Mont-Dore and Cantal massifs in France, on Mount Vesuvius in Italy, in the Canary Islands (Spain), in the Ilimaussaq complex in Greenland, and in Colorado and Arkansas in the USA.

Rarity

Not very common

For collectors

## Quality Criteria The most prized by collectors are specimens with well-formed, sharp-edged crystals of intense, bluish color, set on a contrasting rock matrix. Due to the rarity of large crystals, even small but well-formed specimens are desirable. The presence of rare associated minerals enhances its attractiveness. ## Popular Localities Specimens considered the best in the world come from quarries in the Eifel volcanic region in Germany, especially from the vicinity of Mendig and Maria Laach. These are classic localities from which type specimens for this mineral originate.

Care and storage

## Cleaning Nosean specimens should be cleaned very carefully, using a soft brush to remove dust. For heavier soiling, a damp cloth can be used, but prolonged contact with water should be avoided. Ultrasonic cleaners or steam cleaners are not recommended. ## What to Avoid Nosean is sensitive to acids, which cause its decomposition with the release of hydrogen sulfide (rotten egg smell) and the formation of silica gel. It should be protected from contact with household and laboratory chemicals. Avoid sudden temperature changes and strong impacts. ## Storage Specimens should be stored in a dry place, away from direct sunlight, which, however, does not cause fading in its case. It is best to keep it in separate boxes or on stands to avoid scratching by harder minerals.