Gedrite

Chemical formula: &#9744;Mg<sub>2</sub>(Mg<sub>3</sub>Al<sub>2</sub>)(Si<sub>6</sub>Al<sub>2</sub>)O<sub>22</sub>(OH)<sub>2</sub>

Gedrite is a rare amphibole from the anthophyllite series, forming prismatic or fibrous crystals ranging in color from greenish-gray to brownish.

## Characteristics Gedrite is a mineral from the amphibole group, belonging to the isomorphic series with anthophyllite. It forms elongated, prismatic, or bladed crystals, often aggregated into radial, fibrous, or lamellar masses. It rarely occurs as well-formed, single crystals. Its appearance is typical of many amphiboles, with a distinct luster on cleavage surfaces. ## Physical Properties This mineral is characterized by a hardness of 5.5-6 on the Mohs scale and a density ranging from 3.1 to 3.5 g/cm³, depending on its chemical composition, especially iron content. It has a vitreous luster, and sometimes a silky luster on fibrous aggregate surfaces. It is translucent to opaque. It exhibits perfect cleavage in two directions, intersecting at an angle of approximately 120°, which is a characteristic feature of amphiboles. ## Colors and Varieties The color of gedrite is variable, most commonly including shades of grayish-green, greenish-brown, brown, and even almost black in iron-rich varieties. Some specimens may exhibit weak pleochroism in yellowish, greenish, or gray hues. There are no named commercial or gemological varieties, and classification is primarily based on chemical composition within the amphibole group. ## History and Name The mineral's name comes from the location of its first description – the Gèdre Valley in the Pyrenees, France. It was first described in 1836. Gedrite is formally recognized as the magnesium end-member in the sodium-calcium amphibole subgroup, although its definition and position in classification have evolved over the years with the development of analytical methods. ## Applications Due to its rarity and generally unattractive form, gedrite has no industrial or jewelry applications. It is solely an object of interest for collectors specializing in rock-forming minerals and mineral systematics.

Properties

Mohs hardness
5.5-6
Luster
Vitreous
Streak
Gray to brownish
Density
3.1-3.5
Cleavage
Perfect on {110}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Orthorhombic

Diagnostic features

## Identification Gedrite is most easily identified by its habit – elongated, prismatic, or fibrous crystals, often forming radial aggregates. A key feature is the cleavage angle (approximately 120°), visible on cross-sections of crystals as a network of intersecting fractures. Its color, from greenish to brownish, combined with a vitreous luster, is also helpful. ## Distinguishing from Similar Minerals Gedrite is difficult to distinguish from other amphiboles, especially anthophyllite, with which it forms a series. Macroscopically, they are practically indistinguishable and require chemical analyses (EDS, WDS) or detailed optical studies. It can also be confused with tourmaline, which, however, has a different, triangular crystal cross-section and lacks distinct cleavage. It is distinguished from pyroxenes by its cleavage angle (for pyroxenes, it is approximately 90°). ## Crystal Forms Gedrite crystals are prismatic, elongated along the crystallographic c-axis. They typically form subparallel, bladed, fibrous, or radial aggregates. Single, well-formed crystals with distinct faces are rarely found.

Geological environment

## Genesis Gedrite is a metamorphic mineral, typical of medium- to high-grade metamorphic rocks such as amphibolites, gneisses, and cordierite schists. It forms under conditions of regional metamorphism, in environments rich in magnesium and aluminum, and poor in calcium. It can also occur in some igneous rocks. ## Mineral Associations This mineral often co-occurs with other metamorphic minerals, such as cordierite, anthophyllite, staurolite, sillimanite, kyanite, garnets (mainly almandine and pyrope), as well as quartz, biotite, and plagioclase. ## Localities Key gedrite localities worldwide include the Gèdre Valley in the Pyrenees (France - type locality), the Bamble and Kongsberg regions (Norway), Falun (Sweden), Orijärvi (Finland). It also occurs in the USA in New Hampshire, Massachusetts, and Colorado. It is also known from Greenland and various localities in Canada.

Rarity

Not very common

For collectors

## Quality Criteria For collectors, the most desirable specimens are those with well-formed, large prismatic crystals, which is a rare occurrence. Rich, radial aggregates with intense color and distinct luster are also highly valued. Specimens where gedrite occurs in association with other rare metamorphic minerals, such as cordierite or staurolite, forming aesthetic compositions, are of particular value. ## Popular Localities Classic and most prized specimens come from historical localities in Scandinavia, such as Bamble in Norway or Falun in Sweden. Specimens from these locations often serve as reference examples for this mineral in systematic collections.

Care and storage

## Cleaning Gedrite specimens can be cleaned with a soft, dry brush to remove dust. For more stubborn dirt, distilled water and a soft brush are permissible. Ultrasonic cleaners should be avoided, as they can damage fragile fibrous aggregates. ## What to Avoid Avoid contact with strong acids and bases, which can react with the mineral. Gedrite is stable under normal conditions, but it should be protected from sudden temperature changes and mechanical damage, to which it is susceptible due to its cleavage. ## Storage Collector's specimens of gedrite, especially those with delicate, fibrous structures, are best stored in sealed boxes or display cases to protect them from dust and damage. Avoid placing heavier minerals on gedrite specimens.

External references

Sources

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