Norbergite

Chemical formula: Mg<sub>3</sub>SiO<sub>4</sub>F<sub>2</sub>

Norbergite is a magnesium silicate from the humite group, most often forming granular aggregates or poorly developed crystals ranging in color from yellow to orange.

## Characteristics Norbergite is a mineral belonging to the humite group, relatively rarely found in the form of well-developed crystals. It usually occurs as rounded grains or granular aggregates embedded in rock. Well-formed crystals are most often short-prismatic, tabular, or bipyramidal. Its appearance can be inconspicuous, and identification without laboratory tests can be difficult. ## Physical Properties This mineral is characterized by a hardness of 6-6.5 on the Mohs scale, which makes it relatively resistant to scratching. The luster is vitreous to resinous. It is a brittle mineral with an uneven or conchoidal fracture. The density of norbergite is approximately 3.15-3.18 g/cm³. ## Colors and Varieties Norbergite most often occurs in shades of yellow, from light yellow, through honey-yellow, to orange and brownish. Colorless, white, gray, or pink specimens are rarer. There are no named commercial or color varieties. ## History and Name Norbergite was first described in 1926 by Per Johan Geijer. The mineral's name comes from its discovery location – the Östanmossa mine in the municipality of Norberg, Västmanland region, Sweden. This is the type locality for this mineral. ## Uses Norbergite has no industrial significance. However, its occurrence is scientifically interesting as an indicator of contact metamorphism conditions. It is solely an object of interest for collectors specializing in rare minerals or minerals from the humite group.

Properties

Mohs hardness
6-6.5
Luster
Vitreous to resinous
Streak
White
Density
3.15-3.18
Cleavage
Poor on {001}
Fracture
Uneven to conchoidal
Transparency
Transparent to translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Norbergite is most often identified by its characteristic yellow or orange color, vitreous luster, and occurrence as rounded grains in marbles or calc-silicate rocks. Its hardness (6-6.5) helps distinguish it from many softer minerals. It often exhibits yellow or orange fluorescence under ultraviolet light (UV-SW). ## Distinguishing from Similar Minerals Norbergite is difficult to distinguish from other minerals of the humite group (humite, clinohumite, chondrodite) without advanced studies (e.g., XRD). Chondrodite and clinohumite usually have more intense, more orange or brown colors. It can also be confused with olivine, which, however, forms under different geological conditions (igneous rocks) and rarely occurs in marbles. Grossular (garnet) can be similar but is harder (approx. 7) and crystallizes in the isometric system. ## Crystal Forms Norbergite crystals are rare. If they occur, they take the form of short, thick columns, often with an octagonal cross-section. However, granular, irregular forms embedded in the host rock (marble) dominate.

Geological environment

## Genesis Norbergite is a typical mineral of contact metamorphism. It forms in contact zones between magmatic intrusions (mainly granitoids) and magnesium-rich carbonate rocks, such as dolomites and dolomitic limestones. This process, called metasomatism, requires high temperatures and the presence of volatile components, especially fluorine, which is an essential element in its chemical structure. ## Mineral Associations This mineral often co-occurs with other minerals typical of calc-silicate rocks (skarns). Its most common associates include calcite, dolomite, fluorite, spinel, phlogopite, diopside, tremolite, and other minerals from the humite group, such as chondrodite and clinohumite. ## Localities The most important and classic norbergite localities are in Sweden (Norberg and Värmland) and Italy (Somma-Vesuvius volcanic complex). Significant specimens also come from the USA, especially from Franklin and Sterling Hill in New Jersey, where they occur in famous zinc deposits. Other known localities include Tajikistan (Pamir), Russia (Pribaikalia), and Scotland (Loch Ness).

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, and transparent norbergite crystals, which are very rare. Specimens where intensely colored (e.g., orange) crystals contrast with a white, marble matrix are also highly rated. The size of the crystals and the absence of mechanical damage are important. Specimens showing strong fluorescence are an additional asset. ## Popular Localities Classic and most sought-after specimens come from the type locality in Norberg, Sweden, and from the Franklin mine in New Jersey, USA. Specimens from Franklin are particularly prized for their intense fluorescence. Good quality crystals are also found in Tajikistan.

Care and storage

## Cleaning Norbergite specimens should be cleaned carefully, using a soft brush and distilled water. A neutral pH soap can be used, but it must be thoroughly rinsed off. Due to the possible presence of fractures, ultrasonic cleaners are not recommended. ## What to Avoid Norbergite is sensitive to strong acids, especially sulfuric acid, in which it decomposes. Contact with household and laboratory chemicals should be avoided. It should not be subjected to sudden temperature changes. ## Storage Norbergite specimens, especially those with well-developed crystals, are best stored in separate, padded boxes to prevent chipping and scratching by other, harder minerals. It is stable under household conditions and does not require special protection from light or humidity.

External references

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