SO4 - hydrotalcite - 11Å

Chemical formula: [Mg₄Al₂(OH)₁₂][Na₀.₅₆(SO₄)₁.₃₀]·₇.₃H₂O

A hydrous magnesium, sodium, and aluminum sulfate, a rare mineral from the hydrotalcite supergroup, forming tiny, hexagonal plates.

## Characteristics Sulfate hydrotalcite-11Å is a mineral belonging to the hydrotalcite supergroup. It occurs as very small, hexagonal, platy crystals, rarely exceeding 0.1 mm, often forming rosette-like or spherical aggregates. It is colorless to white and has a pearly luster on cleavage surfaces. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2. It is light, and its density is low. It exhibits perfect cleavage in one direction, which, combined with its pearly luster, is a characteristic visual feature. ## History and Name As a member of the widely studied hydrotalcite supergroup, this specific sulfate phase has been identified and characterized in the context of research on synthetic and natural hydrotalcites. The name refers to its chemical composition (dominant sulfate anion) and crystal structure (layer repetitions every 11 angstroms), distinguishing it from other polytypes and syntypes of hydrotalcite.

Properties

Mohs hardness: 2
Luster: Pearly
Streak: White
Density: 2.03
Cleavage: Perfect on {0001}
Fracture: Uneven
Transparency: Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Identification of this mineral requires specialized analytical methods, such as X-ray diffraction (XRD), which allows for the identification of its characteristic 11 Å interlayer spacing. Visually, it is virtually impossible to distinguish from other white, soft secondary minerals without advanced equipment. ## Distinguishing from Similar Minerals It can be confused with many other minerals from the hydrotalcite supergroup (e.g., motukoreite, wermlandite) as well as with gypsum, talc, or hydromagnesite. Definitive differentiation is almost exclusively possible through XRD and chemical analysis (EDS/WDS), which confirm the presence of sulfate as the dominant anion and the characteristic layered structure. ## Crystal Forms It forms microscopic, hexagonal, platy crystals, usually grouped into rosettes, spherulitic aggregates, or thin coatings and crusts.

Geological environment

## Genesis This is a secondary mineral, formed in low-temperature hydrothermal processes or as a result of weathering. It forms in voids and fractures of metamorphic rocks, such as serpentinites, often in association with other magnesium minerals. ## Mineral Associations It co-occurs with other minerals from the hydrotalcite supergroup, as well as with serpentine, hydromagnesite, brucite, and aragonite. ## Localities Confirmed occurrences of this specific mineral are very few. It has been described in detail from the ophiolite complex near Kraubath an der Mur in Styria, Austria. Other minerals from this group with similar structures are found in various locations worldwide, but precise identification requires specialized research.

Rarity

Very rare

For collectors

## Quality Criteria Due to the microscopic size of the crystals, its collector value is limited to "micromount" specialists. The most highly prized specimens are those where small, but well-formed rosette aggregates are richly embedded on a contrasting rock matrix. The purity and aesthetics of the entire sample are more important than the individual features of the crystals, which are barely visible.

Care and storage

## Cleaning Specimens should be cleaned only with extreme care, using compressed air to remove dust. Due to its extreme softness and fragility, contact with water and any mechanical cleaning, including brushing, should be avoided. ## What to Avoid Contact with water, which can damage or dissolve the mineral, must be strictly avoided. Acids, chemicals, high temperatures, and direct sunlight should also be avoided. The mineral is extremely soft and brittle, susceptible to mechanical damage from the slightest touch. ## Storage Specimens should be stored under stable conditions, in enclosed "micromount" containers, to protect them from dust, moisture, and physical damage. It is not suitable for display outside of a specialized, protected display case.

Sources

IMA Mineral List (CNMNC)