Bentorite

Chemical formula: Ca<sub>6</sub>Cr<sup>3+</sup><sub>2</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>3</sub>(OH)<sub>12</sub>·26H<sub>2</sub>O

Bentonite is a rare, purple mineral of the ettringite group, discovered in the Hatrurim Formation in Israel.

## Characteristics Bentonite is a hydrated calcium chromium sulfate, belonging to the ettringite group. It occurs as small, hexagonal crystals, rarely exceeding 1 mm in length. It typically forms aggregates, veins, or coatings on other minerals. Its most characteristic feature is its intense, purple or lavender color. ## Physical Properties Bentonite crystals exhibit a vitreous luster. The mineral is relatively soft, with a Mohs hardness of 2. It is transparent to translucent. Due to the small size of the crystals, its density has been calculated as 1.79 g/cm³. ## Colors and Varieties Bentonite is an idiochromatic mineral, meaning its color comes from a key component in its chemical formula – in this case, from trivalent chromium (Cr³⁺). It occurs in shades from light purple to purple. No color varieties or commercial varieties are known. ## History and Name The mineral was described in 1980 by Shulamit Gross. The name commemorates Yaakov Ben-Tor (1910-2002), an Israeli geologist and petrologist, professor at the Hebrew University of Jerusalem and the University of California, San Diego, for his contributions to the study of the geology and mineralogy of Israel, and particularly the Hatrurim Formation, where bentonite was discovered. ## Uses Bentonite has no industrial application. Its significance is purely scientific and collectible due to its rarity, attractive color, and association with a unique type locality.

Properties

Mohs hardness
2
Luster
Vitreous
Streak
Light purple
Density
1.79
Cleavage
Perfect on {1010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification The key diagnostic features of bentonite are its characteristic purple color, hexagonal crystal habit, and occurrence in the specific geological environment of the Hatrurim Formation. Its low hardness (2 on the Mohs scale) is also helpful in identification. ## Distinguishing from Similar Minerals Purple sturmanite may appear similar, but typically forms larger and better-developed crystals. Charlesite, another mineral of the ettringite group, can be pale yellow or colorless. Final differentiation from other rare sulfates requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical analysis (EDS). ## Crystal Forms Bentonite forms hexagonal, prismatic crystals, often terminated by a hexagonal pyramid. Crystals are usually very small, less than 1 mm in length, and occur as single needles, rosette-like aggregates, or thin veins cutting through the host rock.

Geological environment

## Genesis Bentonite is a secondary mineral, formed by low-temperature hydrothermal processes. In its type locality (Hatrurim Formation), it formed in veins cutting larnite rocks. Its crystallization occurred in a calcium- and sulfate-rich environment, with the involvement of chromium released from primary minerals. ## Mineral Associations This mineral co-occurs with other rare minerals of the Hatrurim Formation, such as calcite, ettringite, truscottite, gypsum, as well as host rock minerals like larnite and brownmillerite. ## Localities The only confirmed and well-documented occurrence of bentonite worldwide is its type locality in the Hatrurim Formation, south of the Dead Sea in Israel. This is a unique complex of metamorphic rocks formed by the natural combustion of underground bitumen deposits.

Rarity

Very rare

For collectors

## Quality Criteria The most prized bentonite specimens are those with well-formed, sharply terminated crystals of an intense purple color, clearly contrasting with the lighter host rock. Due to the microscopic size of the crystals, specimens are evaluated under magnification. Rich aggregates forming druses or dense veins are more desirable than single, scattered crystals. ## Popular Localities The only source of collectible specimens is the Hatrurim Formation in Israel. Specimens from this locality are highly valued by collectors specializing in rare minerals and those from type localities.

Care and storage

## Cleaning Bentonite specimens are extremely delicate and sensitive to water. Mechanical cleaning is highly risky. If absolutely necessary, compressed air (from a safe distance) can be used to remove loose dust. Contact with water should be avoided, as it can dissolve or damage the fine crystals. ## What to Avoid Water, acids, solvents, and ultrasonics must be strictly avoided. The mineral is very soft (hardness 2), making it susceptible to scratches and mechanical damage. It should be protected from high temperatures and direct sunlight, which can lead to dehydration and loss of color. ## Storage It is recommended to store bentonite specimens in closed, stable containers (e.g., "micromount" type) to protect them from dust, moisture, and physical damage. It is best kept in a dry place with a constant temperature.

External references

Sources

Read more