Eastonite

Chemical formula: KAlMg₂(Si₂Al₂)O₁₀(OH)₂

Eastonite is a rare mica group mineral, a magnesium end-member of the siderite series, characterized by its platy appearance.

## Characteristics Eastonite is a mineral from the mica group, belonging to the biotite series. It forms plates, flakes, and packets with a hexagonal outline, and also occurs as scaly and compact aggregates. Its appearance is typical of micas – flexible, elastic plates that are easily separated. It is usually opaque to translucent on thin edges. ## Physical Properties As a representative of micas, eastonite is characterized by low hardness, approximately 2.5-3 on the Mohs scale. Its plates are flexible and pliable. The luster is vitreous to pearly, especially visible on cleavage surfaces. ## Colors and Varieties Eastonite most often ranges in color from greenish, through brownish-green, to almost black. No distinct color or commercial varieties are distinguished. ## History and Name The mineral's name, given in 1886 by Carl Hermann Credner, comes from its discovery locality – the town of Easton in Northampton County, Pennsylvania, USA. It was found in a local limestone quarry (Sherrer Quarry). ## Uses Eastonite has no industrial significance. Its occurrence is rare enough that it is solely an object of interest for collectors specializing in mineral systematics, as well as scientists studying the mica group.

Properties

Mohs hardness
2.5-3
Luster
Vitreous
Streak
White
Density
0
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Eastonite can be identified by its typical mica-like, platy appearance, hexagonal crystal outline, perfect cleavage in one direction, and the flexibility of its plates. Its greenish or brownish color is also characteristic. ## Distinguishing from Similar Minerals It can be confused with other micas from the biotite group, such as phlogopite or annite. Distinguishing it from them often requires advanced chemical analysis (e.g., EDS), as their physical properties are very similar. Eastonite is distinguished by its specific chemical composition, rich in magnesium and aluminum in both tetrahedral and octahedral positions. ## Crystal Forms It most often forms tabular or platy crystals with a pseudohexagonal outline. It occurs as individual flakes, packets of plates, and radial or scaly aggregates.

Geological environment

## Genesis Eastonite is a metamorphic mineral. It forms under conditions of high-grade regional metamorphism, in the amphibolite and granulite facies. It occurs in aluminum-rich rocks such as gneisses, mica schists, and granulites. It can also form in skarns and metamorphosed limestones. ## Mineral Associations It co-occurs with minerals typical of high-grade metamorphic rocks, such as cordierite, sillimanite, garnets (especially pyrope and almandine), spinel, phlogopite, anorthite, and other potassium feldspars. ## Localities The most important localities worldwide include the historical type locality in Easton, Pennsylvania, USA. It is also known from Mont-Saint-Hilaire in Quebec, Canada, from some localities in the Alps (Italy, Switzerland), and from the Scandinavian Mountains region in Norway. Occurrences are primarily of scientific interest.

Rarity

Rare

For collectors

## Quality Criteria For collectors, the most important criteria are the form and size of the crystals. Well-formed, pseudohexagonal packets of plates, with distinct color and without mechanical damage, are most valued. The attractiveness of a specimen is enhanced by its association with other rare metamorphic minerals. ## Popular Localities Specimens from the type locality in Easton, Pennsylvania, have historical value. Material of good crystallographic quality also comes from Mont-Saint-Hilaire, Canada, known for the occurrence of many rare minerals.

Care and storage

## Cleaning Eastonite specimens should be cleaned very carefully, preferably using compressed air to remove dust. A soft brush can be used, but with great delicacy, so as not to damage the fragile plates. Avoid water and any liquids that could penetrate between the plates and cause delamination. ## What to Avoid Contact with water, acids, and other chemicals should be strictly avoided. The mineral is very soft and susceptible to mechanical damage – scratches, crushing, or separation of packets. It should not be heated or exposed to prolonged sunlight. ## Storage Eastonite specimens are best stored in closed, dry containers or display cases to protect them from dust and mechanical damage. They should be isolated from harder minerals that could scratch them.

External references

Sources

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