Pyroaurite
Chemical formula: Mg<sub>6</sub>Fe<sup>3+</sup><sub>2</sub>(OH)<sub>16</sub>CO<sub>3</sub>·4H<sub>2</sub>O
Pyroaurite is a hydrated magnesium and iron hydroxycarbonate, forming soft, pearly aggregates with a golden or greenish hue.
Description
## Characteristics Pyroaurite is a mineral from the hydrotalcite group, a hydrated magnesium and iron hydroxycarbonate. It most often forms platy, scaly, or fibrous aggregates and crusts, which may resemble talc or micas in appearance. It rarely occurs as small, well-formed crystals with a tabular or rhombohedral habit. A characteristic feature is its flexibility and pliability in thin lamellae. ## Physical Properties This mineral is very soft, with a Mohs hardness of 2.5. On cleavage surfaces, it exhibits a strong, pearly luster, while on other surfaces or in compact aggregates, it is more waxy or dull. It is a light mineral, with a density of about 2.14 g/cm³. It is transparent to translucent. ## Colors and Varieties Pyroaurite occurs in various colors – from colorless and white, through yellowish, greenish-white, to brownish-yellow. Its surface may be covered with a coating of a golden or brown hue. No named commercial or color varieties are distinguished. ## History and Name The name pyroaurite comes from the Greek words *pyros* (fire) and *aurum* (gold), referring to its property of changing color to golden-brown when heated. The mineral was first described in 1865 by L.J. Igelström based on specimens found in the historic Långban mine in Sweden. ## Uses Pyroaurite has no significant industrial uses and is primarily an object of interest for mineral collectors.
Diagnostic features
## Identification Key diagnostic features of pyroaurite include its low hardness (about 2.5 on the Mohs scale), perfect cleavage in one direction, pearly luster on cleavage planes, and flexible but inelastic lamellae. Its occurrence in serpentine rocks is also characteristic. ## Distinguishing from Similar Minerals Pyroaurite is sometimes confused with talc, which is significantly softer (hardness 1) and feels greasy to the touch. It can also be mistaken for brucite, which has similar hardness but often a more waxy luster and a different crystal structure. Distinguishing it from other minerals in the hydrotalcite group often requires advanced studies, such as X-ray diffraction (XRD). ## Crystal Forms Most commonly found are platy, scaly (mica-like), and fibrous aggregates, as well as thin crusts. Well-formed, single crystals are rare and take the form of small, thin tablets with a hexagonal or rhombohedral outline.
Geological environment
## Genesis Pyroaurite is a secondary mineral, formed under low-temperature hydrothermal processes. It forms as a result of the alteration of magnesium-rich minerals (such as olivine or pyroxene) in ultramafic rocks, mainly serpentinites. It is also found in some dolomitic marbles and metamorphosed iron ore deposits. ## Mineral Associations This mineral often co-occurs with minerals typical of its formation environment, such as serpentine, brucite, magnesite, hydromagnesite, artinite, as well as calcite and dolomite. ## Localities Important global localities for pyroaurite include the historic site of Långban in Värmland (Sweden), Kraubath in Styria (Austria), Val Malenco in Italy, as well as numerous occurrences in serpentinites in Asbestos, Quebec (Canada), and in quarries in Pennsylvania and New Jersey (USA).
Rarity
Not very common
Collector aspects
## Quality Criteria The most prized specimens by collectors are those with well-formed, sharp crystals, which is a great rarity for this mineral. Samples with an intense, golden-yellow color and strong pearly luster are also highly valued. The aesthetic association with other minerals, for example, contrasting with a dark serpentine matrix, enhances the attractiveness of the specimen. ## Popular Localities The localities in Långban (Sweden) and Cedar Hill Quarry in Lancaster County, Pennsylvania (USA) are considered classic, providing the best specimens. Specimens from these localities are sought after by collectors due to their historical significance and quality.
Care and storage
## Cleaning Pyroaurite specimens are very soft and delicate, so they should be cleaned with great care. It is best to use a soft brush to remove dust or, if necessary, compressed air. Brief rinsing in distilled water is permissible, after which the specimen should be immediately and thoroughly dried. Ultrasonic cleaners should be avoided. ## What to Avoid The mineral is susceptible to mechanical damage, scratches, and abrasion. Due to its perfect cleavage, its lamellae can easily separate. As a carbonate, it reacts with acids, even weak ones. It should be protected from chemicals and prolonged exposure to moisture. ## Storage It is recommended to store pyroaurite specimens in separate, padded boxes to avoid contact with harder minerals that could scratch it. It should be displayed away from areas prone to dust and vibrations.