Mohrite

Chemical formula: (N<sup>3-</sup>H<sub>4</sub>)<sub>2</sub>Fe<sup>2+</sup>(S<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>·6H<sub>2</sub>O

Mohrite is a hydrated iron(II) ammonium sulfate, a synthetic analogue of a natural mineral, valued for its well-formed crystals.

## Characteristics Mohrite is a mineral from the sulfate group, a member of the picromerite group. It is a hydrated iron(II) ammonium sulfate. It occurs extremely rarely in nature, and the vast majority of specimens, including those available on the collector's market, are synthetic crystals, grown under laboratory conditions. These crystals are characterized by high quality and often reach significant sizes. They typically form tabular or short prismatic shapes with well-defined, flat faces. ## Physical Properties Mohrite crystals are relatively soft, with a hardness of about 2-2.5 on the Mohs scale. They have a vitreous luster and are transparent to translucent. The mineral's density is approximately 1.86 g/cm³. It is soluble in water. ## Colors and Varieties Mohrite has a characteristic pale, greenish-blue or green color. This color is a result of the presence of iron(II) ions. There are no distinct color varieties or trade names, as its chemical composition is constant, and most material comes from controlled synthesis. ## History and Name The name "mohrite" refers to its chemical composition and its connection to Mohr's salt – a popular chemical reagent (ammonium iron(II) sulfate), named after the German chemist Karl Friedrich Mohr (1806-1879). The mineral was first described in 1964 based on material found in the geothermal area of Tuscany, Italy. ## Uses Natural mohrite has no industrial applications due to its extreme rarity. Synthetic Mohr's salt, however, is a widely used reagent in analytical chemistry, particularly in titrations. Collector's specimens are almost exclusively synthetic crystals, valued for their aesthetic qualities.

Properties

Mohs hardness
2-2.5
Luster
Vitreous
Streak
White
Density
1.862
Cleavage
Good on {001}
Fracture
Conchoidal
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Mohrite is relatively easy to identify based on its characteristic pale green or greenish-blue color, vitreous luster, and well-formed, tabular crystals. Key diagnostic features include its water solubility and low hardness. Most specimens on the market are synthetic crystals, which is also an important identifying piece of information. ## Distinguishing from Similar Minerals Mohrite can be confused with other green, water-soluble sulfates, such as melanterite. However, melanterite has a different chemical composition (iron(II) sulfate heptahydrate) and often forms efflorescences or fibrous aggregates, in contrast to the large, single crystals typical of synthetic mohrite. From other green minerals (like apophyllite or fluorite), it is primarily distinguished by its low hardness and water solubility. ## Crystal Forms Mohrite crystals crystallize in the monoclinic system. Synthetic specimens typically form well-developed, single crystals with a tabular or short prismatic habit, often with sharply terminated edges.

Geological environment

## Genesis Natural mohrite is a secondary mineral. It forms as a result of the evaporation of geothermal waters rich in sulfates, iron, and ammonia. Its occurrence is associated with fumarole activity and hot springs. ## Mineral Associations At the only known natural locality in Tuscany, mohrite co-occurred with sassoline (boric acid), larderellite, santite, and native sulfur. ## Localities The only confirmed natural locality for this mineral in the world is the Larderello geothermal area in the Val di Cornia valley, Pisa province, Tuscany, Italy. All other mohrite specimens available on the collector's market are synthetic products, manufactured in laboratories worldwide.

Rarity

Extremely rare

For collectors

## Quality Criteria The quality of collector's specimens of mohrite, which are almost exclusively synthetic, is assessed based on several criteria. The most important are the size and perfection of the crystal's form – the larger and more ideally formed, the more valuable. The intensity and uniformity of color, as well as transparency, are also highly prized. The absence of mechanical damage (chips, scratches) is crucial, especially considering the mineral's low hardness. ## Market Prices The prices of synthetic mohrite crystals are moderate. Small, single crystals (1-2 cm) can be acquired for several tens of Polish zlotys. Larger, well-formed specimens (over 5 cm) with good transparency and no damage can reach prices of several hundred Polish zlotys. ## Popular Localities Since natural mohrite is virtually unavailable, there are no "popular localities" in the geological sense. All collector's material comes from laboratory production, without attribution to a specific "producer" or "laboratory" as a distinguishing feature.

Care and storage

## Cleaning Mohrite is soluble in water, so it must absolutely not be cleaned wet. Any contact with water, even a damp cloth, will cause damage or complete dissolution of the crystal. Dust can be removed very carefully using a soft brush or compressed air from a distance. ## What to Avoid Avoid water and high humidity, which can cause superficial dissolution and loss of luster. The mineral is sensitive to heat and can dehydrate (lose water from its crystal lattice), leading to its destruction. It should be protected from direct sunlight, which can cause fading. Avoid contact with chemicals. ## Storage Mohrite specimens should be stored in a dry place, preferably in a sealed, closed box (a "box") away from sources of moisture and heat. The use of desiccants (e.g., silica gel) in the storage container can be considered. Due to its low hardness, it should be stored separately to avoid scratches from other minerals.

External references

Sources

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