Charlesite

Chemical formula: Ca<sub>6</sub>Al<sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>B(OH)<sub>4</sub>(OH,O)<sub>12</sub>·26H<sub>2</sub>O

Charlesite is a rare, hydrated calcium and aluminum sulfate and borate, forming colorless or white, hexagonal crystals.

## Characteristics Charlesite is a complex, hydrated mineral from the ettringite group, crystallizing in the trigonal system. It occurs as well-formed, hexagonal crystals with a prismatic or tabular habit, reaching up to 1 cm in length. It is most often colorless or white, less frequently acquiring a pale yellow hue. Its crystals are usually transparent to translucent, with a vitreous luster. ## Physical Properties This mineral is characterized by relatively low hardness, approximately 2.5 on the Mohs scale, meaning it can be scratched with a fingernail. It has a low density, about 1.79 g/cm³, which is typical for minerals with such a high degree of hydration. It exhibits perfect cleavage in one direction, parallel to the crystal base. ## Colors and Varieties Charlesite is usually colorless or white. Pale yellow coloration is less common. No named varieties of this mineral are distinguished. ## History and Name The mineral was first described in 1983 by Pete J. Dunn, Donald R. Peacor, Paul B. Leavens, and Robert A. Ramik. The name "charlesite" was given in honor of Charles Palache (1869–1954), a distinguished American mineralogist and crystallographer, professor at Harvard University, in recognition of his contributions to mineralogy, including his research on minerals from Franklin, New Jersey, where charlesite was discovered. ## Uses Charlesite has no industrial applications. It is solely a mineral of scientific and collector's interest, sought after for its rarity and origin from a classic mineralogical locality.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
White
Density
1.79
Cleavage
Perfect on {0001}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Charlesite is most easily identified by its characteristic, hexagonal crystals with a vitreous luster. Its low hardness (approx. 2.5 on the Mohs scale), low density, and occurrence in a specific geological environment (metamorphic rocks in zinc deposits) are key diagnostic features. Under ultraviolet light, it exhibits weak, bluish-white fluorescence (SW-UV) or is inert. ## Distinguishing from Similar Minerals Charlesite can be confused with other minerals from the ettringite group, such as ettringite itself or sturmanite. Ettringite usually has a yellow color, while sturmanite is intensely yellow to almost black. Charlesite stands out from them by its most often colorless or white form. Definitive differentiation requires advanced chemical (EDS/WDS) or X-ray (XRD) analyses. ## Crystal Forms Charlesite crystals have a habit of hexagonal prisms or thicker tabular crystals, often terminated by a flat base (pinacoid). They occur as single, overgrown crystals or in small groups on the surface of the host rock.

Geological environment

## Genesis Charlesite is a hydrothermal mineral, forming in the late stages of crystallization in veins cutting metamorphosed zinc and manganese ore deposits. It forms at relatively low temperatures as a result of the interaction of boron- and sulfate-rich solutions with earlier minerals. ## Mineral Associations This mineral coexists with many other rare species, typical of the Franklin deposit. The most common associated minerals include: willemite, franklinite, andradite, rhodochrosite, barite, as well as rare arsenates like allanite-(As) or johnbaumite. ## Localities The most important and classic locality for charlesite, from which the best specimens originate, is the Franklin Mine in Sussex County, New Jersey, USA. This is the so-called type locality. This mineral has also been identified in the Kombat and Tsumeb mines in Namibia and the N'Chwaning Mine in South Africa, but specimens from these locations are much rarer on the collector's market.

Rarity

Very rare

For collectors

## Quality Criteria The most prized charlesite specimens are characterized by sharply terminated, well-formed crystals with high transparency and vitreous luster. An ideal specimen is a single, colorless crystal or an aesthetic group of them, contrastingly set on a matrix of willemite or franklinite. Crystal size is a key factor – specimens exceeding 5 mm are already considered significant, and those approaching 1 cm are exceptionally rare and sought after. ## Popular Localities By far the most desired and known source of charlesite is its type locality – the Franklin Mine in New Jersey, USA. Specimens from Franklin set the standard against which other finds are compared. Material from Namibia or South Africa is encountered sporadically and is mainly of scientific importance.

Care and storage

## Cleaning Charlesite specimens should be cleaned with the utmost care, using only a soft brush to remove dust. Due to its low hardness and solubility, contact with water, especially hot water, which can damage it, should be avoided. Ultrasonic cleaning is absolutely forbidden. ## What to Avoid Charlesite is sensitive to heat, which can cause dehydration and structural damage. It should be protected from direct sunlight, high temperatures, and all chemicals, including acids and detergents. It is very soft and brittle, susceptible to scratches and mechanical damage. ## Storage Specimens should be stored in stable conditions, in a dry place, away from sources of heat and light. It is best to keep it in a separate, padded collector's box to avoid abrasions and contact with harder minerals.

External references

Sources

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