Tweddillite

Chemical formula: CaSrMn³⁺₂Al(Si₂O₇)(SiO₄)O(OH)

Tweddleite is a very rare mineral from the epidote group, forming dark brown to black, tabular crystals, discovered in the Wessels Mine in South Africa.

## Characteristics Tweddleite is a very rare mineral belonging to the epidote group. It occurs as single, tabular or bladed crystals, reaching sizes up to 0.5 mm. Crystals are usually well-formed and grown on other minerals. Its color ranges from dark brown to black, and the edges of thinner crystals may be translucent reddish-brown. ## Physical Properties This mineral is characterized by a hardness of approximately 6 on the Mohs scale. It has a vitreous to metallic luster and is opaque, with the exception of thin edges, which can be translucent. The calculated density is approximately 3.98 g/cm³. ## History and Name The mineral's name honors Douglas "Dougie" Tweddle (born 1957), a South African mineral collector and co-owner of a mineral shop in Johannesburg, in recognition of his contribution to the mineralogy of the Kalahari region. Tweddleite was approved as a new mineral species by the International Mineralogical Association (IMA) in 2015 (IMA2015-082).

Properties

Mohs hardness
6-7
Color
Deep red
Luster
vitreous
Streak
Brownish red
Density
0
Cleavage
perfect after {001}
Fracture
Sub-Conchoidal
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of tweddleite is based on its characteristic appearance – small, black, tabular crystals with a vitreous or metallic luster. The key factors are its locality (Wessels Mine) and co-occurrence with other minerals such as sugilite or hennomartinite. Final confirmation requires advanced analytical methods, such as Raman spectroscopy or chemical analysis. ## Distinguishing from Similar Minerals It can be confused with other dark minerals from the epidote group, such as piemontite or andradite. Visual distinction is practically impossible without chemical analysis, which will reveal the unique combination of calcium, strontium, manganese, and aluminum characteristic of tweddleite. ## Crystal Forms Crystals are typically tabular, flattened parallel to (100), and exhibit a bladed habit. They occur as single, well-formed crystals grown on a substrate.

Geological environment

## Genesis Tweddleite forms in manganese deposits within metamorphic rocks. It is a product of hydrothermal processes occurring within these deposits. It occurs in small fissures and mineral vugs. ## Mineral Associations This mineral co-occurs with sugilite, hennomartinite, quartz, and other manganese oxides. ## Localities The only confirmed locality for tweddleite worldwide (type locality) is the Wessels Mine, located near Hotazel in the Kalahari Manganese Field in the Northern Cape Province, Republic of South Africa.

Rarity

Extremely rare

For collectors

## Quality Criteria As a "micromount" type mineral, specimens with sharp, well-formed, undamaged crystals that stand out clearly from the matrix are most highly valued. Color contrast, for example, black crystals on purple sugilite, significantly enhances aesthetic and collector value. Crystal size, even if measured in fractions of a millimeter, is also an important factor. ## Popular Localities All known specimens originate exclusively from the Wessels Mine in South Africa, which is the sole source of this mineral.

Care and storage

## Cleaning Due to the extreme rarity and small size of the crystals, mechanical cleaning is not recommended. If absolutely necessary, compressed air can be used to remove loose dust particles. Avoid contact with water and chemicals. ## What to Avoid Avoid ultrasonic cleaners, steam cleaners, acids, solvents, and all chemical agents. The mineral should be protected from high temperatures, sudden thermal changes, and direct sunlight, which can cause fading. ## Storage Tweddleite specimens should be stored in stable conditions, preferably in a sealed "micromount" box, to protect them from dust, mechanical damage, and moisture. Avoid storage in areas with high humidity.

External references

Sources

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