Desautelsite

Chemical formula: Mg<sub>6</sub>Mn<sup>3+</sup><sub>2</sub>(OH)<sub>16</sub>(CO<sub>3</sub>)·4H<sub>2</sub>O

Desautelsite is a rare mineral from the hydrotalcite group, forming characteristic orange-red, rosette-like crystal aggregates.

## Characteristics Desautelsite is a hydrated magnesium and manganese carbonate belonging to the hydrotalcite group. It occurs as small but well-formed, hexagonal tabular crystals. These crystals almost always form compact, rosette-like or radial aggregates, which are its most characteristic feature. Specimens typically have an intense, bright color. ## Physical Properties This mineral is very soft, with a Mohs hardness of just 2. The crystals are flexible but not elastic, meaning they can be deformed without returning to their original shape. The luster is pearly on cleavage surfaces and vitreous on fracture surfaces. It is lightweight, with a density of approximately 2.33 g/cm³. ## Colors and Varieties Desautelsite is prized for its striking color. The most common shades range from bright orange-red to deep red. It is less frequently orange-yellow. No distinct color or commercial varieties are recognized. ## History and Name The mineral was described in 1979 by Pete J. Dunn, Donald R. Peacor, and Palmer D. Sturm. Its name honors Paul E. Desautels (1920-1991), a long-time curator of the mineral and gem department at the Smithsonian Institution in Washington, D.C., in recognition of his contributions to mineralogy. ## Uses Due to its rarity, small crystal size, and low hardness, desautelsite has no industrial applications. It is solely a mineral of scientific and collector interest.

Properties

Mohs hardness
2
Luster
Pearly, Vitreous
Streak
Light orange
Density
2.33
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification The most important diagnostic feature of desautelsite is its mode of occurrence – characteristic, radial or rosette-like aggregates of small, tabular crystals. The intense orange-red color combined with pearly luster and very low hardness are also key. A vigorous reaction with acids is a confirming characteristic. ## Distinguishing from Similar Minerals Desautelsite is sometimes confused with other minerals from the hydrotalcite group, especially with the related pyroaurite, which, however, usually has a color ranging from white through yellow to greenish. From other red or orange minerals (e.g., crocoite, vanadinite), it is distinguished by its extremely low hardness, crystal form, and environment of occurrence. ## Crystal Forms It forms thin, tabular crystals with a hexagonal outline. They almost always occur as radial, spherical, or rosette-like aggregates, less commonly as isolated flakes or scales.

Geological environment

## Genesis Desautelsite is a secondary mineral formed at low temperatures during the hydrothermal alteration or weathering of ultramafic rocks, such as serpentinites and peridotites. It forms in veins and fractures within these rocks, often in association with other magnesium minerals. ## Mineral Associations It most commonly co-occurs with other minerals from the hydrotalcite group (pyroaurite, stichtite, reevesite), as well as with brucite, hydromagnesite, aragonite, dolomite, magnesite, chromite, and calcite. ## Localities The type locality, from which the best-known specimens originate, is the Cedar Hill Quarry in Lancaster County, Pennsylvania, and nearby quarries in Cecil County, Maryland, USA. It is also known from the ophiolite complex in Kōchi Prefecture, Japan, and from several other, limited localities worldwide where serpentinized ultramafic rocks are exposed.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, distinct crystal rosettes of intense, orange-red color are most valued by collectors. High contrast between desautelsite and the host rock (most often dark serpentinite) is important. The size of the aggregates, their completeness (lack of damage), and aesthetic arrangement on the matrix significantly increase the specimen's value. ## Popular Localities By far the most sought-after and classic specimens come from the Cedar Hill and Wood's Chrome Mine quarries in Pennsylvania and Maryland, USA. These are considered the global standard for this mineral.

Care and storage

## Cleaning Desautelsite is extremely soft and delicate. It should be cleaned only with great care, using compressed air to remove dust. Any contact with water or other liquids is inadvisable, as the mineral may be damaged by them. Avoid ultrasonic and steam cleaners. ## What to Avoid The mineral is sensitive to acids, which cause it to dissolve rapidly with the release of carbon dioxide. Contact with chemicals, high temperatures, and prolonged exposure to direct sunlight, which can cause fading, should be avoided. Due to its perfect cleavage and low hardness, it is very susceptible to mechanical damage (scratches, chipping). ## Storage Desautelsite specimens should be stored in separate, padded boxes to avoid contact with harder minerals. It is best to display them in enclosed cabinets, protecting them from dust and physical damage.

Sources

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