Beidellite

Chemical formula: (Na,Ca)<sub>0.3</sub>Al<sub>2</sub>(Si,Al)<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub>·nH<sub>2</sub>O

Beidellite is a common clay mineral of the smectite group, forming soft, waxy, or earthy aggregates, resulting from the weathering of volcanic rocks.

## Characteristics Beidellite is a clay mineral belonging to the smectite group. It rarely forms crystals visible to the naked eye, most often occurring as compact, plastic, or earthy masses. These aggregates can have a platy or scaly structure. In appearance, it resembles wax or dried clay. It is very soft and light, and feels slightly greasy or waxy to the touch. ## Physical Properties Its Mohs hardness is only 1.5 to 2, meaning it can be scratched with a fingernail. It has a waxy or pearly luster on cleavage surfaces, or is completely dull in earthy aggregates. It is translucent to opaque. Density is variable due to differing water content in its structure, ranging approximately from 2.2 to 2.7 g/cm³. ## Colors and Varieties Beidellite is most often white, gray, yellowish, greenish, or reddish-brown. Its color depends on chemical impurities. There are no named commercial or color varieties. ## History and Name The mineral's name comes from its type locality – Beidell in Saguache County, Colorado, USA. It was first described in 1925 by mineralogists Esper S. Larsen and Edgar T. Wherry. ## Applications As an essential component of bentonites, beidellite finds application in industry. Bentonites are used, among other things, in the production of drilling muds, as sealing agents, absorbents, and as catalysts in chemical processes. It is rarely used as a pure mineral, more often as part of a mixture with other clay minerals, mainly montmorillonite.

Properties

Mohs hardness
1.5-2
Luster
Waxy, Pearly, Dull
Streak
White
Density
2.2-2.7
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Translucent to Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Field identification of beidellite is very difficult and often impossible without specialized testing. Key features include extreme softness (scratchable with a fingernail), waxy or dull appearance, and a greasy feel to the touch. Its swelling upon contact with water is characteristic, but this test can destroy the specimen. The most reliable identification method is X-ray diffraction (XRD). ## Distinguishing from Similar Minerals Beidellite is almost impossible to distinguish with the naked eye from other smectite group minerals, such as montmorillonite or nontronite. It can also be confused with kaolinite, which, however, does not swell in water. From talc, which is also very soft, it is distinguished by a lesser sense of "soapiness" to the touch. ## Crystal Forms Beidellite crystals are microscopic and appear as pseudohexagonal plates or laths. On a macroscopic scale, it forms compact, earthy, scaly, or platy aggregates.

Geological environment

## Genesis Beidellite is a product of low-temperature hydrothermal processes or weathering. It forms from the alteration of aluminum-rich minerals and rocks, especially feldspars, volcanic glass, tuffs, and volcanic ashes. It is a common component of soils, sediments, and bentonite deposits. ## Mineral Associations It often co-occurs with other clay minerals, such as montmorillonite, nontronite, kaolinite, and illite. It is also accompanied by quartz, calcite, feldspars, and zeolites. ## Localities As a common clay mineral, beidellite occurs worldwide. The type locality, from which it takes its name, is Beidell, Colorado (USA). Significant occurrences are found in bentonite deposits, including Wyoming (USA), Italy, Germany, Russia, and Japan.

Rarity

Common

For collectors

## Quality Criteria Beidellite is not a mineral valued by typical collectors due to its unattractive appearance and difficulties in preservation. Its value is primarily scientific. For systematic collectors or micromounters, the purity of the material and a precisely documented locality are important. Specimens with well-formed microcrystals visible under magnification may be more desirable. ## Popular Localities The most significant historical and scientific importance lies with specimens from the type locality in Beidell, Colorado. Other localities are important mainly from a geological and industrial perspective, rather than for collecting.

Care and storage

## Cleaning Beidellite specimens should only be dry-cleaned, using a soft brush to remove dust. Contact with water is highly inadvisable, as the mineral swells and may disintegrate. ## What to Avoid Water and other liquids, as well as high humidity and its fluctuations, should be strictly avoided. Changes in humidity cause the mineral to swell and shrink, leading to its physical degradation. As a very soft mineral, it is susceptible to scratches and mechanical damage. ## Storage Beidellite requires storage in a dry, stable environment. It is best kept in a closed, padded box to protect it from moisture, dust, and mechanical damage. Each specimen should be properly labeled.

Sources

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