Brinrobertsite

Chemical formula: (Na,K,Ca)<sub>0.3</sub>(Al,Fe<sup>3+</sup>,Mg)<sub>4</sub>(Si,Al)<sub>8</sub>O<sub>20</sub>(OH)<sub>4</sub>·3.5H<sub>2</sub>O

Brinrobertsite is a very rare mineral from the smectite group, a hydrated silicate with a complex composition, occurring as microscopic plates.

## Characteristics Brinrobertsite is a mineral from the smectite group, belonging to the silicate class. Due to its occurrence as extremely small, microscopic plates, less than 2 micrometers in size, its visual features cannot be determined with the naked eye. Observations using an electron microscope reveal aggregates composed of very fine, poorly formed, platy crystals. ## Physical Properties Due to the microscopic size of the crystals, most physical properties, such as hardness, luster, or density, have not been precisely determined for macroscopic samples. The mineral is soft and has a low density, typical for clay minerals. ## Colors and Varieties Observed brinrobertsite aggregates are white to grayish-white. No varieties of this mineral have been distinguished. ## History and Name The mineral was approved by the International Mineralogical Association (IMA) in 1993. Its name honors Dr. Brinley "Brin" Roberts (1930-2021), a geologist from Birkbeck College at the University of London, for his contributions to the study of clay minerals in Wales. ## Uses Brinrobertsite has no industrial or commercial applications. Its significance is purely scientific and is limited to the interest of specialized micromineral collectors.

Properties

Mohs hardness
1
Luster
Pearly
Streak
White
Density
2.23
Cleavage
Perfect on {001}
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identification of brinrobertsite is impossible without advanced laboratory techniques. It requires chemical analysis (EDS) and X-ray powder diffraction (XRD) to confirm its crystal structure and composition. Visually, it is indistinguishable from other white or gray clay minerals. ## Distinguishing from Similar Minerals It can be confused with many other clay minerals, such as kaolinite, illite, or other smectites. Definitive differentiation is possible only through instrumental analysis. ## Crystal Forms Brinrobertsite forms very fine, poorly formed crystals with a platy or lamellar habit, less than 2 micrometers in size. These crystals combine into rosette-like or spherulitic aggregates.

Geological environment

## Genesis Brinrobertsite is a secondary mineral, formed by low-temperature hydrothermal processes. It forms in cavities and fissures of volcanic rocks, such as basalts, as a result of the alteration of other minerals. ## Mineral Associations This mineral co-occurs with other secondary minerals such as calcite, saponite, analcime, thomsonite, chabazite, and other zeolites. ## Localities The type locality, from which brinrobertsite was first described, is the Hafotty quarry in Nant Ffrancon, Gwynedd, Wales (UK). Its occurrence has also been confirmed in basalts in County Antrim, Northern Ireland.

Rarity

Extremely rare

For collectors

## Quality Criteria As a micromineral, brinrobertsite is not evaluated according to typical collecting criteria such as the size or color of individual crystals. The value of a specimen lies in the confirmed presence of the mineral on the host rock, the abundance of aggregates visible under the microscope, and its origin from the type locality (Hafotty). The most valuable samples are those with well-documented analysis confirming the mineral's identity.

Care and storage

## Cleaning Due to its microscopic nature and occurrence as coatings or within rock mass, individual crystals are not isolated or cleaned. Rock specimens containing brinrobertsite should be handled with great care. ## What to Avoid Avoid contact with water and chemicals that could damage the delicate structure of the aggregates. As a hydrated mineral, it is sensitive to temperature changes, which can lead to dehydration and structural destruction. It should be protected from ultrasound and any mechanical stress. ## Storage Specimens should be stored in stable conditions, in closed containers protecting them from dust, moisture, and physical damage. It is best to store them in their original host rock.

Sources

Read more