Rosenhahnite

Chemical formula: Ca₃Si₃O₈(OH)₂

Rosenhahnite is a rare calcium silicate, forming colorless or white crystals, valued by collectors for its unique structure and rarity.

## Characteristics Rosenhahnite is a hydrated calcium silicate, typically occurring as small, tabular or prismatic crystals. It most often forms colorless or white, transparent to translucent aggregates. Specimens can be found in radial aggregates or rosettes, and individual crystals rarely exceed a few millimeters in length. Its unique crystal structure contains both silicate and hydroxyl groups. ## Physical Properties Rosenhahnite crystals are characterized by a vitreous luster. This mineral is relatively soft, with a Mohs hardness of approximately 5.5. It is brittle, and its density is slightly higher than that of quartz. ## Colors and Varieties Rosenhahnite is a mineral with a very limited color palette. It occurs almost exclusively as colorless or white. No color or commercial varieties are distinguished. ## History and Name The mineral was discovered in 1965 in serpentinites in the Russian River area of Mendocino County, California, USA. Its name honors Anna Rosenhahn (1920-2006) and Charles Rosenhahn (1918-2002), Californian mineral collectors who first drew attention to this unknown mineral. It was described and recognized as a new mineral species in 1967 by Adolf Pabst, Joseph A. Schalker, and Robert E. Wallace. ## Uses Due to its rarity and small crystal size, rosenhahnite has no industrial applications. It is solely a mineral of scientific and collector interest.

Properties

Mohs hardness: 5.5
Luster: Vitreous
Streak: White
Density: 2.76
Cleavage: Perfect on {100}, Good on {010} and {001}
Fracture: Uneven
Transparency: Transparent to translucent
Crystal system: Triclinic

Diagnostic features

## Identification Rosenhahnite is identified by its crystal habit – small, tabular crystals, often forming radial aggregates. Its vitreous luster and occurrence in a specific geological environment, most often in veins cutting serpentinites, are also characteristic. ## Distinguishing from Similar Minerals It can be confused with other white calcium silicates, such as xonotlite or tobermorite. Differentiation often requires advanced research methods, such as X-ray diffraction (XRD). Visually, the characteristic crystal habit and mineral paragenesis can be helpful in preliminary identification. ## Crystal Forms Rosenhahnite crystals are tabular, flattened parallel to (100) and elongated along the c-axis. They often form fan-shaped or radial aggregates, as well as rosettes and granular masses.

Geological environment

## Genesis Rosenhahnite is a low-temperature hydrothermal mineral. It forms in veins and fractures cutting serpentinite rocks, where it crystallizes from solutions rich in calcium and silica. Its formation is associated with the serpentinization processes of ultramafic rocks. ## Mineral Associations It most commonly co-occurs with other calcium silicates, such as xonotlite and tobermorite. Its paragenesis may also include calcite, aragonite, brucite, and minerals from the serpentine group (antigorite, lizardite). ## Localities The most important and classic locality, from which the best specimens originate, is in California (Mendocino, Sonoma, Santa Clara counties). This mineral has also been confirmed in several other places worldwide, including the Wessels Mine in South Africa, the Ilímaussaq intrusive complex in Greenland, and Jordan.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp, and transparent crystals, forming aesthetic, radial aggregates or rosettes. Crystal size is also important – specimens with crystals exceeding a few millimeters are rare. Contrast with the dark matrix rock (serpentinite) significantly enhances the visual appeal of the specimen. ## Popular Localities By far the most sought-after specimens come from localities in California, USA, especially Mendocino County, which is the type locality. Specimens from these localities are considered the global standard for this mineral.

Care and storage

## Cleaning Rosenhahnite specimens should be cleaned very carefully, using a soft brush to remove dust. If wet cleaning is necessary, distilled water is recommended, followed by gentle drying with compressed air. Ultrasonic cleaners should be avoided, as they can damage the brittle crystals. ## What to Avoid Contact with acids and other aggressive chemicals that may react with the mineral should be avoided. Rosenhahnite is sensitive to sudden temperature changes. It should not be exposed to prolonged strong sunlight. ## Storage Collector specimens of rosenhahnite should be stored in separate, padded boxes to prevent abrasion and mechanical damage. Due to its brittleness, it is not recommended to store it loosely with other, harder minerals.

Sources

IMA Mineral List (CNMNC)