Chesterite
Chemical formula: Mg<sub>17</sub>Si<sub>20</sub>O<sub>54</sub>(OH)<sub>6</sub>
Chesterite is a rare magnesium silicate, closely related to anthophyllite and cummingtonite, occurring as fibrous or acicular aggregates.
Properties
- Mohs hardness
- 5
- Luster
- Vitreous
- Streak
- White
- Density
- 2.96-3.01
- Cleavage
- Perfect on {110}, good on {010}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Identifying chesterite is extremely difficult without advanced analytical methods. In the field and in a collection, it is practically impossible to distinguish from similar amphiboles and pyroxenes based on visual characteristics. Its fibrous or acicular aggregates of light color are characteristic. Definitive identification requires X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS). ## Distinguishing from Similar Minerals Chesterite is most often confused with anthophyllite, cummingtonite, tremolite, and talc. It is distinguished from talc by its higher hardness. Differentiation from anthophyllite and cummingtonite, with which it forms intergrowths at a submicroscopic level, is almost exclusively possible using transmission electron microscopy (TEM) or electron diffraction. ## Crystal Forms It forms elongated, bladed, or acicular crystals, often gathered into radial, fibrous, or tangled aggregates. Crystals are usually small, reaching lengths of up to several millimeters.
Geological environment
## Genesis Chesterite is a mineral formed during regional metamorphic processes in rocks rich in magnesium and poor in calcium. It forms under amphibolite facies conditions. It is a product of the transformation of other magnesium silicates, such as pyroxenes and amphiboles. ## Mineral Associations It most often co-occurs with anthophyllite, cummingtonite, talc, chlorite, magnetite, pyroxenes (clinopyroxene, orthopyroxene), and other amphiboles. It often forms very fine-grained, difficult-to-separate intergrowths with them. ## Localities The most important and classic occurrence of chesterite is its type locality: the Carlton talc mine in Chester, Windsor County, Vermont, USA. It has also been identified in several other locations worldwide, including the actinolite mine in Nagasaki Prefecture, Japan, and in some ultramafic rocks in the Alps.
Rarity
Very rare
For collectors
## Quality Criteria Chesterite's collector appeal is low due to its inconspicuous appearance. Specimens from the type locality (Chester, Vermont) are most valued, especially if they form well-developed, rich acicular aggregates on matrix. A key value factor is reliable analytical confirmation of the mineral's identity and a well-documented association with other minerals, such as anthophyllite. ## Popular Localities The only locality of significance to collectors is the historic type locality in Chester, Vermont, USA. Specimens from this location are rare on the market and sought primarily by specialized collectors.
Care and storage
## Cleaning Chesterite specimens, especially those with a fibrous structure, are brittle and delicate. They should only be cleaned using a soft brush or compressed air to remove dust. Avoid contact with water, which can penetrate the aggregate structure and weaken it. ## What to Avoid Avoid ultrasonic cleaners, chemical agents, and sudden temperature changes. The fibrous structure is susceptible to mechanical damage, so specimens should be handled with great care. ## Storage It is recommended to store chesterite specimens in closed boxes or display cases to protect them from dust and mechanical damage. Avoid placing heavier minerals on delicate, fibrous aggregates.