Noelbensonite

Chemical formula: BaMn³⁺₂Si₂O₇(OH)₂·H₂O

Noelbensonite is a very rare, brownish-red mineral from the sorosilicate group, occurring as small, tabular crystals.

## Characteristics Noelbensonite is a hydrated barium and manganese sorosilicate, belonging to the lawsonite group. It forms very small, tabular or bladed crystals, rarely exceeding 1 mm in length. Most often, it occurs in the form of radial or randomly oriented aggregates, as well as single crystals dispersed in the host rock. Its habit is typically tabular, flattened along one of the crystallographic axes. ## Physical Properties Noelbensonite crystals exhibit a vitreous luster. This mineral is brittle. Due to the small size of the crystals, precise determination of many physical properties is difficult. ## Colors and Varieties Noelbensonite has a characteristic, intense color ranging from brownish-red to reddish-brown. It exhibits strong pleochroism, changing hue from reddish-brown to yellowish-brown depending on the observation direction in polarized light. No varieties have been distinguished. ## History and Name The mineral was discovered in mines near the towns of Oacoma and Big Creek in Fresno County, California (USA). It was described in 1991 by J.T. Sullwold, D.F. Siems, P.J. Dunn, and R.A. Ayuso. The name "noelbensonite" honors Professor William Noel Benson (1885–1957), a New Zealand geologist and petrologist, for his contributions to the study of manganese-bearing metamorphic rocks, in which this mineral occurs.

Properties

Mohs hardness
4
Color
Dark brown
Luster
Earthy to brilliantly vitreous
Streak
Yellow-brown
Density
0
Cleavage
Good on {001}
Fracture
Irregular/Uneven
Transparency
Translucent
Crystal system
Orthorhombic

Diagnostic features

## Identification Noelbensonite is identified by its unique brownish-red color, tabular crystal habit, and occurrence in a characteristic geological environment – in veins cutting metamorphosed siliceous rocks (radiolarites). Strong pleochroism is a key diagnostic feature in microscopic studies. ## Distinguishing from Similar Minerals It can be confused with other manganese silicates of similar color, such as piemontite. Differentiation requires advanced analytical methods, such as chemical analysis (EDS) or X-ray diffraction (XRD), due to the small crystal sizes. ## Crystal Forms Crystals are tabular, flattened, often forming radial or disordered aggregates. They also occur as single, dispersed flakes in the rock.

Geological environment

## Genesis Noelbensonite is a low-temperature hydrothermal mineral. It forms in veins and fractures cutting metamorphosed, manganese-rich sedimentary rocks, such as radiolarites (a type of siliceous rock). Its formation is associated with zeolite facies or prehnite-pumpellyite facies metamorphism. ## Mineral Associations It most commonly co-occurs with quartz, braunite, calcite, rhodochrosite, piemontite, bementite, harmotome, and other manganese minerals. ## Localities This is an extremely rare mineral. Besides the type locality in California (Oacoma Mine, Big Creek), its occurrence has been confirmed in only a few places worldwide, including Woods Mine in New South Wales (Australia) and the Sugashima and Shiozaki mines on the Shima Peninsula in Japan.

Rarity

Very rare

For collectors

## Quality Criteria Due to its extreme rarity, every confirmed noelbensonite specimen is valuable to specialized collectors. The most highly prized specimens are those that display well-formed, even if microscopic, crystals of intense color, set against a contrasting host rock background (e.g., on white quartz). The size of aggregates and the richness of occurrence on the matrix are key factors influencing value. ## Popular Localities The most well-known specimens, though still microscopic in size, come from the type locality in California, USA. Specimens from Japan are also occasionally available on the specialized market.

Care and storage

## Cleaning Due to its extreme rarity and small crystal size, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used to remove dust. Avoid contact with water and chemicals. ## What to Avoid Avoid all chemicals, acids, and ultrasonics. The mineral is brittle, so protect it from impacts and vibrations. Store away from moisture. ## Storage Noelbensonite specimens should be stored under stable conditions, preferably in a closed, padded display box, to protect them from mechanical damage, dust, and moisture.

External references

Sources

Read more