Dorrite

Chemical formula: Ca₄(Mg₃Fe³⁺₉)O₄[Si₃Al₈Fe³⁺O₃₆]

Dorrite is a rare, opaque mineral with a submetallic luster, occurring as tabular crystals and granular aggregates.

## Characteristics Dorrite is a complex silicate from the rhonite group, characterized by its dark color and submetallic luster. It most commonly occurs as small, tabular crystals up to 1 mm in size, forming granular or massive aggregates. Its appearance is inconspicuous, and identification requires specialized analysis. ## Physical Properties This mineral is opaque. Its Mohs hardness is 5.5, placing it in the medium range of mineral hardness. The luster is described as submetallic, and the fracture is uneven. ## Colors and Varieties Dorrite ranges in color from reddish-brown, through dark brown, to brownish-black. The streak, which is the color of the powdered mineral, is grayish-brown. No distinct color or commercial varieties are recognized. ## History and Name The mineral is named in honor of John A. "Jack" Dorr Jr. (1922-1986), a professor of geology at the University of Michigan, in recognition of his contributions to geological research in Wyoming. The mineral was described and approved as a new species in 1987, discovered at a paralava occurrence at Durham Ranch in Wyoming, USA.

Properties

Mohs hardness: 5.5
Luster: Sub-metallic
Streak: brownish grey
Density: 0
Cleavage: Good on {010} and {001}.
Fracture: Irregular/Uneven
Transparency: Opaque
Crystal system: Triclinic

Diagnostic features

## Identification Macroscopic identification of dorrite is very difficult due to its small size and inconspicuous appearance. Identification relies on chemical (EDS) and structural (XRD) analysis. Auxiliary features may include: dark color, submetallic luster, occurrence in a specific environment (paralava), and association with other rare minerals. ## Distinguishing from Similar Minerals Dorrite can be confused with other dark minerals from the rhonite group or the aenigmatite group, such as rhonite, aenigmatite, or welanite. Distinguishing them from each other is practically impossible without advanced analytical techniques. ## Crystal Forms Dorrite crystals are poorly formed, tabular, and reach sizes up to 1 mm. They typically form granular aggregates or massive aggregates within the host rock.

Geological environment

## Genesis Dorrite is a mineral formed as a result of pyrometamorphic processes. It forms in paralavas – rocks created by the melting of sedimentary rocks due to underground coal combustion. It occurs in cavities and fissures within these specific, high-temperature rock formations. ## Mineral Associations This mineral coexists with other products of pyrometamorphism. At its type locality, it was found in association with hematite, magnetite, cordierite, mullite, tridymite, diopside, anorthite, osumilite, and cristobalite. ## Localities The most important and type locality for dorrite is Durham Ranch near Reno Junction, Campbell County, Wyoming, USA. This is the only confirmed and thoroughly described locality for this mineral worldwide.

Rarity

Very rare

For collectors

## Quality Criteria As a microscopic mineral, dorrite is not evaluated according to typical aesthetic criteria such as color or clarity. The collector's value of a specimen primarily depends on the confirmation of its identity through analysis and the richness of aggregates on the rock matrix. Specimens from the type locality with well-documented chemical analysis are most desirable.

Care and storage

## Cleaning Due to its rarity and potential fragility, only gentle cleaning with compressed air or a very soft brush is recommended to remove dust. Avoid contact with water and chemical agents. ## What to Avoid Specimens should be protected from moisture, chemicals, acids, and ultrasonic cleaners. Abrupt temperature changes can be harmful. As an opaque mineral, it is not sensitive to sunlight, but extreme conditions should be avoided. ## Storage Dorrite specimens, typically small and embedded in a rock matrix, should be stored under stable conditions in sealed display boxes to protect them from dust and mechanical damage. Avoid storage in humid environments.