Sjögrenite

Chemical formula: Mg<sub>6</sub>Fe<sup>3+</sup><sub>2</sub>(CO<sub>3</sub>)(OH)<sub>16</sub>·4H<sub>2</sub>O

Sjögrenite is a rare mineral from the hydrotalcite group, forming hexagonal, tabular crystals with a pearly luster, closely related to pyroaurite.

## Characteristics Sjögrenite is a hydrated basic magnesium and iron carbonate, belonging to the hydrotalcite group. It occurs as small, hexagonal, tabular crystals, often forming rosette-like or lamellar aggregates. Specimens typically have a waxy or pearly luster on cleavage surfaces. It is a polymorph of pyroaurite, meaning it has the same chemical composition but crystallizes in a different crystallographic system. ## Physical Properties This mineral is very soft, with a Mohs hardness of only 2. It is light, with a density of approximately 2.11 g/cm³. It exhibits perfect cleavage in one plane, which accounts for its lamellar appearance. The crystals are flexible but inelastic, meaning they can be bent but will not return to their original shape. ## Colors and Varieties Sjögrenite ranges in color from white, through yellowish, brownish, to greenish. Its color often depends on the degree of iron oxidation and the presence of impurities. There are no named varieties of this mineral. ## History and Name The mineral was named in honor of Hjalmar Sjögren (1856–1922), a Swedish mineralogist and professor of mineralogy at Uppsala University, who first described a similar mineral – pyroaurite. Sjögrenite was formally described as a new mineral species by Clifford Frondel in 1941 based on material from the Långban mine in Sweden. ## Uses Sjögrenite has no industrial applications. It is solely a mineral of scientific and collecting interest, valued for its rarity and association with classic mineralogical localities.

Properties

Mohs hardness
2
Luster
Pearly
Streak
White
Density
2.11
Cleavage
Perfect on {0001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Hexagonal

Diagnostic features

## Identification Key diagnostic features of sjögrenite include its hexagonal crystal habit, perfect cleavage in one plane, pearly luster on cleavage surfaces, and low hardness. It reacts with hydrochloric acid, releasing carbon dioxide bubbles. ## Distinguishing from Similar Minerals Sjögrenite is almost impossible to distinguish from pyroaurite without advanced studies (e.g., X-ray diffraction). Pyroaurite crystallizes in the trigonal system, and sjögrenite in the hexagonal, but visually their crystals can be identical. It can also be confused with other minerals from the hydrotalcite group, such as hydrotalcite or manasseite, from which differentiation also requires laboratory analysis. ## Crystal Forms Crystals are typically thin, tabular with a hexagonal outline. They often form lamellar, rosette-like, or foliated aggregates. They also occur as thin coatings and efflorescences.

Geological environment

## Genesis Sjögrenite is a secondary mineral, formed as a result of low-temperature hydrothermal processes or low-grade metamorphism. It forms in serpentinites and in altered iron and manganese ore deposits, often in rock fractures and fissures. ## Mineral Associations It most commonly co-occurs with pyroaurite, to which it is closely related. It is also accompanied by minerals such as calcite, dolomite, magnetite, brucite, serpentine, and minerals from the phlogopite group. ## Localities The most important and classic occurrences of this mineral are Långban and Nordmark in Värmland (Sweden). It is also known from the Sterling Hill mine in New Jersey (USA), from Mont Saint-Hilaire in Quebec (Canada), and from several localities in Norway (e.g., Snarum) and Russia (Urals).

Rarity

Rare

For collectors

## Quality Criteria Most valued by collectors are specimens with well-formed, sharp crystals with a distinct hexagonal outline. Rich rosette-like aggregates and contrasting associations with other minerals, such as calcite, enhance their appeal. Color intensity and good luster are also important. Due to its common co-occurrence with pyroaurite, specimens with analytically confirmed sjögrenite identity are particularly valuable. ## Popular Localities Classic specimens, considered the best, come from the Swedish deposits in Långban. High-quality collector material was also supplied by the Sterling Hill mine in the USA.

Care and storage

## Cleaning Sjögrenite specimens are extremely delicate and soft. They should be cleaned only very carefully, using a soft brush to remove dust. Avoid contact with water, which can damage the mineral. ## What to Avoid Absolutely avoid all acids and chemicals, which react violently with carbonates. The mineral is sensitive to high temperatures and humidity. It should not be cleaned in ultrasonic or steam cleaners. Due to its low hardness, it is very susceptible to scratching. ## Storage Sjögrenite specimens should be stored in separate, padded boxes to prevent abrasion and mechanical damage. They should be protected from dust, humidity, and direct sunlight.

Sources

Read more