Tibiscumite

Chemical formula: (Ca,Na,K)<sub>0.7</sub>(Al,Fe,Mg)<sub>2</sub>(Si,Al)<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub>·0.8H<sub>2</sub>O

Tibiscumite is a very rare mineral from the illite group, a hydrated aluminum, calcium, sodium, and potassium silicate, discovered in Romania.

## Characteristics Tibiscumite is a hydrated aluminosilicate belonging to the illite group, closely related to brammallite. It occurs as extremely fine-grained, earthy, or compact aggregates. Individual crystals are microscopic in size and appear as very small, hexagonal platelets, visible only under a scanning electron microscope (SEM). Due to its nature, it does not form visible crystals macroscopically and most often resembles clay in appearance. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It has a dull or earthy luster and is opaque. Its calculated density is 2.75 g/cm³. ## Colors and Varieties Tibiscumite is observed in white color. No varieties have been distinguished. ## History and Name The mineral's name comes from the ancient Roman name of the Timiș River in Romania - "Tibiscum". It was first described by I. Mârza, V. Ghergari, and L. Ionescu in 1995, and approved by the International Mineralogical Association (IMA) in the same year (IMA1995-010). The type specimen comes from the Tincova quarry in Romania. ## Applications Due to its extreme rarity and microscopic size, tibiscumite has no industrial applications. It is of purely scientific importance and is of interest to specialized collectors of rare minerals.

Properties

Mohs hardness
~2
Luster
Dull
Streak
White
Density
2.75
Cleavage
Perfect on {001}
Transparency
Opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of tibiscumite is impossible without advanced laboratory techniques, such as X-ray diffraction (XRD) and chemical analysis (e.g., EDS/WDS). It is unrecognizable under collector and field conditions. Macroscopically, it appears as a white, earthy, or clay-like coating. ## Distinguishing from Similar Minerals It can be confused with many other clay minerals, such as kaolinite, illite, montmorillonite, or halloysite. Differentiation from these common minerals requires specialized instrumental analysis. ## Crystal Forms Crystals are microscopic, pseudohexagonal platelets up to 5 micrometers in size. They form rosette-like or spherulitic aggregates, as well as coatings and earthy masses.

Geological environment

## Genesis Tibiscumite is a secondary mineral, formed as a result of low-temperature hydrothermal processes. In its type locality (Tincova, Romania), it formed through the alteration of plagioclase (oligoclase) within granitic pegmatites. ## Mineral Associations In its type locality, it co-occurs with quartz, albite, microcline, muscovite, biotite, chlorite, calcite, laumontite, and stilbite-(Ca). ## Localities The only confirmed and verified locality of tibiscumite in the world is the Tincova quarry, in the valley of the stream of the same name, in the Poiana Ruscă Mountains (Caraș-Severin County, Romania).

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of a tibiscumite specimen depends solely on its analytical confirmation and the abundance of its occurrence on the host rock (matrix). Since the mineral is microscopic, specimens are evaluated based on the aesthetics of the accompanying rock and documentation confirming its identity. It has no macroscopic aesthetic value. ## Popular Localities The only source of specimens is the type locality in Tincova, Romania. Any specimen originating from this location is potentially valuable for specialized collectors.

Care and storage

## Cleaning Tibiscumite specimens, which are typically microscopic aggregates on a host rock, should not be cleaned mechanically or chemically. Any attempts at cleaning may destroy the delicate, earthy coatings. Only very careful dust removal using a photographic blower brush is permissible. ## What to Avoid Contact with water, acids, detergents, and other chemicals must be strictly avoided. The mineral is very soft and brittle, susceptible to any mechanical damage. It should be protected from vibrations, shocks, and abrasion. ## Storage Specimens should be stored under stable conditions, in a closed, padded "micromount" box, to protect them from dust, moisture, and physical damage. Do not expose it to direct sunlight or sudden temperature changes.

Sources

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