Messelite
Chemical formula: Ca<sub>2</sub>Fe<sup>2+</sup>(PO<sub>4</sub>)<sub>2</sub>·2H<sub>2</sub>O
Messelite is a hydrated calcium iron phosphate, forming characteristic radial aggregates and spherical clusters of tiny crystals.
Description
## Characteristics Messelite is a hydrated phosphate belonging to the fairfieldite group. It forms tabular or prismatic crystals, most commonly occurring as radial, fan-shaped aggregates. It often forms spherical or hemispherical clusters, and sometimes appears as fibrous aggregates or compact masses. It is a relatively soft and brittle mineral. ## Physical Properties Its Mohs hardness is 3.5, and its density is approximately 3.16 g/cm³. Crystals exhibit a vitreous luster, and a pearly luster on cleavage surfaces. It is transparent to translucent. It shows perfect cleavage in one direction. ## Colors and Varieties Messelite is most commonly colorless, white, grayish, or has a greenish-white to pale yellow hue. Due to the oxidation of its iron content, it can transform into other minerals, such as anapaite, which is often associated with a change in color to greenish or brownish. No commercial varieties are distinguished. ## History and Name The name messelite comes from its discovery site and first description – the Messel oil shale open-pit mine (Grube Messel) near Darmstadt in Hesse, Germany. The mineral was first described in 1890 by the German chemist Wilhelm Muthmann. ## Applications Messelite has no industrial applications. It is valued solely as a collector's mineral, sought after for its characteristic forms and its occurrence in paragenesis with other rare phosphates.
Diagnostic features
## Identification The most characteristic feature of messelite is its mode of occurrence – radial, fan-shaped aggregates and spherical clusters. Its relatively low hardness, pearly luster on cleavage planes, and occurrence environment (late-stage pegmatite zones or organic-rich sediments) are also helpful in identification. ## Distinguishing from Similar Minerals It can be confused with other phosphates from the fairfieldite group, such as fairfieldite (with manganese instead of iron) or collinsite (with magnesium). Definitive differentiation of these minerals often requires chemical analysis. Messelite is also a precursor to anapaite, into which it can transform; however, anapaite usually has an intense green color and often forms crystals filling fossil shells. ## Crystal Forms Crystals are tabular or short-prismatic, but rarely occur as single, well-formed individuals. Most often, they form fan-shaped, radial aggregates, spherulitic (spherical) clusters, and fibrous masses.
Geological environment
## Genesis Messelite is a secondary mineral, forming at low temperatures. It forms in two main types of environments. The first are granitic pegmatites, where it crystallizes in the late stages of hydrothermal processes. The second, characteristic of the type locality, are sedimentary deposits, especially oil shales rich in organic matter and phosphorus derived from the decomposition of organic remains. ## Mineral Associations In granitic pegmatites, messelite coexists with minerals such as triphylite, especially in zones of its alteration, as well as with ludlamite, vivianite, and manganese phosphates. In clay sediments and shales, its associated minerals are often siderite, pyrite, and anapaite, which forms as a result of its alteration. ## Localities Important localities for this mineral worldwide include the Messel mine in Germany (type locality), numerous pegmatites in the Hagendorf area in Bavaria (Germany), mines in the Custer area in South Dakota (USA), such as the Tip Top Mine, and the famous Rapid Creek locality in Yukon Territory, Canada, which yields some of the best specimens in the world.
Rarity
Not very common
Collector aspects
## Quality Criteria The most highly prized messelite specimens are those that form well-defined, spherical or fan-shaped aggregates with sharp, undamaged crystals. The size of the clusters and their placement on an aesthetic rock matrix are also important. Transparent specimens, with a pure color (white or colorless), without traces of alteration into other minerals, are more sought after. Associations with other rare phosphates also have high value. ## Popular Localities The best specimens in the world are considered to be those from the Rapid Creek locality in Yukon (Canada), which are distinguished by sharp crystals and attractive forms. Classic, though often less visually striking, are specimens from the type locality in Messel (Germany). Specimens from pegmatites in South Dakota (USA) are also highly valued.
Care and storage
## Cleaning Messelite specimens should be cleaned very carefully, using a soft brush and distilled water. Avoid strong water jets, which could damage delicate aggregates. Ultrasonic cleaners are not recommended. ## What to Avoid The mineral is sensitive to acids, which can damage it. As a hydrated mineral, it is also susceptible to structural changes under high temperatures, which lead to water loss. It should be protected from scratching by harder minerals. ## Storage It is recommended to store specimens in a dry place, in closed boxes or display cases, to protect them from dust and mechanical damage. It is best to store it separately from other, harder minerals.