CO3-SO4 - hydrotalcite - 18.5Å

Chemical formula: Mg₈Al₄(OH)₂₄·Na₀.₅(SO₄)₁.₂₅CO₃·₉H₂O

This is a hydrated magnesium, sodium, and aluminum sulfate-carbonate of the hydrotalcite group, forming extremely rare, microscopic, hexagonal crystals.

## Characteristics This mineral is a complex, hydrated sulfate-carbonate belonging to the hydrotalcite supergroup. It occurs as extremely small, hexagonal, platy crystals, rarely exceeding 20 micrometers in diameter and 5 micrometers in thickness. These crystals form rosette-like aggregates or appear as single, dispersed flakes. Due to its microscopic size, its visual features are not discernible to the naked eye. ## Physical Properties The crystals are too small to determine their hardness, density, or other physical properties in a standard way. The luster is described as pearly on the basal surfaces {0001}. The mineral is transparent and colorless. ## History and Name The mineral was first described in 2012 by Anthony R. Kamp and co-workers. The name refers to its chemical composition (dominant carbonate and sulfate anions), its belonging to the hydrotalcite group, and a characteristic interlayer spacing of 18.5 Å (angstroms), which is a key identifying feature distinguishing it from other hydrotalcite polytypes.

Properties

Mohs hardness: 1.5-2
Luster: Pearly
Streak: White
Density: 2.03
Cleavage: Perfect on {0001}
Fracture: Micaceous
Transparency: Transparent
Crystal system: Trigonal

Diagnostic features

## Identification Identification of this mineral is impossible without advanced laboratory techniques. A key method is powder X-ray diffraction (PXRD), which allows measurement of the characteristic 18.5 Å interlayer spacing. Chemical analysis (e.g., using an electron microprobe) is also necessary to confirm the presence of magnesium, aluminum, sodium, sulfur, and carbon in appropriate proportions. ## Distinguishing from similar minerals This mineral is one of many polytypes within the hydrotalcite supergroup. It can be distinguished from other minerals in this group (such as hydrotalcite, manasseite, or motukoreite) only on the basis of detailed X-ray diffraction and chemical studies, which reveal differences in crystal structure (interlayer spacings) and chemical composition. ## Crystal forms It forms exclusively microscopic, hexagonal, platy crystals, often grouped into small, rosette-like aggregates.

Geological environment

## Genesis This mineral forms as a result of low-temperature hydrothermal processes. In the type locality (Palabora mine), it crystallized in voids and fractures within carbonatite (sovite) that had undergone secondary alteration. ## Mineral associations It occurs in association with minerals such as calcite, dolomite, phlogopite, magnetron, clinochlore, apatite, zircon, and other rare minerals from the hydrotalcite group. ## Localities The only confirmed occurrence of this mineral in the world is the Palabora mine, located in the Olifants River valley in Limpopo Province, Republic of South Africa. This is its type locality.

Rarity

Extremely rare

For collectors

## Quality criteria As a microscopic mineral, it is not traded on the typical collector's market. Its value is purely scientific. For research institutions and specialized micromounters, the most valuable samples are those where crystals are well-formed, create distinct aggregates, and occur in association with other rare minerals. Precise documentation confirming identification by analytical methods is crucial.

Care and storage

## Cleaning Specimens of this mineral, due to their microscopic nature and fragility, should not be subjected to any mechanical or chemical cleaning methods. Any attempts may destroy the crystals. ## What to avoid Avoid contact with any liquid, including water, which can dissolve or damage the delicate crystals. Also avoid changes in temperature, humidity, and exposure to ultrasound. ## Storage Specimens should be stored only under stable conditions, in specialized, tightly sealed containers (such as "micromount boxes"), to protect them from dust, moisture, and mechanical damage. Observation is only possible under a microscope.

Sources

IMA Mineral List (CNMNC)