SO4 - hydrotalcite - 8.8Å

Chemical formula: Mg₄Al₂(OH)₁₂(SO₄)·₃H₂O

Hydrotalcite is a mineral from the hydrotalcite supergroup, forming soft, waxy or pearly aggregates of white or yellowish color, often in the form of lamellar aggregates.

## Characteristics Hydrotalcite is a hydrated magnesium aluminum carbonate belonging to the hydrotalcite supergroup. It typically occurs as fine-grained, earthy, or fibrous aggregates, as well as massive aggregates with a lamellar or layered structure. It rarely forms discernible, tabular crystals with a hexagonal outline. Its appearance is characteristic – it often resembles wax or talc, and is sometimes described as chalky white. ## Physical Properties This mineral is very soft, with a Mohs hardness of approximately 2. It is light, with a density close to 2.03-2.09 g/cm³. Crystals exhibit perfect cleavage in one direction, consistent with their layered structure. The luster is usually waxy, pearly on cleavage surfaces, or dull in the case of earthy aggregates. It is translucent to opaque. ## Colors and Varieties Hydrotalcite is most often white, but can take on yellowish, greenish, or gray hues, which is a result of impurities. No named color varieties are distinguished. The name "hydrotalcite" is often used commercially to refer to the entire group of minerals, but in mineralogy, it refers to a specific species. ## History and Name The name "hydrotalcite" was given in 1842 by Carl Hochstetter. It refers to its chemical composition (hydration - "hydro") and its resemblance to talc ("talc"). The mineral was known from deposits in Snarum, Norway, and the Ural region in Russia. ## Applications Synthetic hydrotalcite is produced on a large scale and is used as an antacid in pharmaceuticals, a stabilizer in plastic production (e.g., PVC), and as an absorbent for pollutants and a catalyst in the chemical industry.

Properties

Mohs hardness: 2
Luster: Pearly, Waxy, Greasy
Streak: White
Density: 2.03-2.09
Cleavage: Perfect on {0001}
Fracture: Uneven
Transparency: Translucent
Crystal system: Trigonal

Diagnostic features

## Identification The mineral can be identified by its exceptional softness (it can be scratched with a fingernail), waxy or pearly luster, and often lamellar form. A characteristic reaction with acids (e.g., dilute HCl) is also observed, during which it effervesces vigorously, distinguishing it from visually similar talc or brucite. ## Distinguishing from Similar Minerals - **Talc**: It is equally soft but feels more soapy or greasy to the touch and does not react with acids. - **Brucite**: It can form similar lamellae but is slightly harder (2.5 on the Mohs scale) and does not react with hydrochloric acid. - **Gypsum (especially selenite)**: It is also soft but has a different luster (vitreous to pearly) and different crystal forms. ## Crystal Forms Hydrotalcite crystallizes in the trigonal system. It is most commonly found as flexible but inelastic lamellae and plates. It forms fibrous, scaly, rosette-like, and massive, earthy aggregates.

Geological environment

## Genesis It is a secondary mineral, formed by low-temperature hydrothermal processes or the weathering of magnesium-rich rocks such as serpentinites, peridotites, or dolomites. It can form in veins cutting through these rocks. ## Mineral Associations It often co-occurs with minerals such as serpentine, brucite, hydromagnesite, aragonite, dolomite, calcite, and magnesite. ## Localities Significant deposits and classic localities from which well-formed specimens originate include Snarum in Norway, the Ural region in Russia, the Shetland Islands in Scotland, as well as various locations in the USA (e.g., Lancaster County in Pennsylvania, Texas). It also occurs in many other places worldwide in serpentinite rocks.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with well-formed, even if small, tabular crystals. Aggregates with a distinct lamellar or fibrous structure are also valued, as are clean specimens with a white color and pearly luster. It is important that the specimen is well-mounted on a rock matrix, which emphasizes its form. ## Popular Localities Classic specimens, prized by collectors, come from historical localities in Snarum (Norway) and the Urals (Russia). Specimens from these locations have historical significance and are sought after in the market.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. They can be rinsed with distilled water, but prolonged soaking should be avoided. Ultrasonic cleaners or pressure washing should not be used. ## What to Avoid Hydrotalcite is very soft and susceptible to mechanical damage. It readily reacts with acids, even weak ones, dissolving with the release of carbon dioxide. It should be protected from high temperatures, which can cause water loss and structural damage. ## Storage It is recommended to store specimens in separate, padded boxes to avoid scratches from harder minerals. They should be protected from dust, moisture, and direct sunlight.