Pigotite

Chemical formula: Al₄C₆H₅O₁₀·13H₂O (?)

Pigotite is a poorly understood, X-ray amorphous organic substance, likely an aluminum salt of mellitic acid, forming brown or black crusts.

## Characteristics Pigotite is an enigmatic, amorphous (non-crystalline) substance of organic origin, rather than a fully defined mineral species. Its status as a mineral is questioned by the International Mineralogical Association (IMA). It occurs as thin, resinous or earthy crusts and coatings on rocks, often near coal or peat outcrops. It typically ranges in color from yellowish-brown, through reddish-brown, to almost black. Due to its amorphous nature, it does not form crystals. ## Physical Properties As an amorphous substance, pigotite does not exhibit constant, measurable physical properties typical of crystalline minerals. It is very soft, with a hardness estimated at 1 to 2 on the Mohs scale. It has a dull, earthy, or slightly resinous luster. It is opaque. Its density is low but rarely measured. ## History and Name The name pigotite comes from the name of the Irish chemist and physician, Dr. Pigot of Dublin, who first described this substance in 1840. It was found in caves and on granite cliffs in Cornwall, United Kingdom. Chemical analyses have always been ambiguous, suggesting it is an organic aluminum salt, likely formed by the reaction of water seeping through peat with clay minerals in rocks.

Properties

Mohs hardness
1-2
Color
brown
Luster
Dull
Streak
yellow
Density
0
Fracture
Uneven
Transparency
Opaque

Diagnostic features

## Identification Pigotite is primarily recognized by its mode of occurrence – as thin, brown to black, earthy or resinous crusts on rocks, especially in association with organic matter (peat, coal). Its very low hardness and lack of crystalline structure are key features. It reacts with ammonia, dissolving in it. ## Distinguishing from Similar Substances Pigotite can be confused with other amorphous organic substances or manganese oxides. It is distinguished from manganese oxides (e.g., psilomelane, wad) by its significantly lower hardness and density. From other organic substances, such as amber or fossil resins, it differs by its lack of transparency and usually earthy appearance. Definitive identification would require advanced chemical analyses.

Geological environment

## Genesis Pigotite is a secondary substance of organic origin. It forms as a result of the reaction of solutions rich in humic acids (formed from the decomposition of plant matter, e.g., peat) with aluminum-containing minerals, such as feldspars or clay minerals in granitic rocks or gneisses. This process occurs at low temperatures, on or near the Earth's surface. ## Mineral Associations Due to its nature and genesis, pigotite does not form typical associations with crystalline minerals. It occurs on host rocks, such as granite, in association with organic matter (peat) and its weathering products. ## Localities Classic and historical localities from which pigotite was described are in Cornwall (United Kingdom), particularly on granite cliffs. Similar substances have also been reported in other places worldwide where similar geochemical processes occur, but they are rarely precisely identified.

Rarity

Very rare

For collectors

## Quality Criteria As a substance with unconfirmed mineral status, pigotite is primarily of historical and scientific significance. It is a rarity in collections. The value of a specimen is determined primarily by its historical origin (preferably from the type locality in Cornwall) and the quantity and quality of the crust's preservation on the original host rock.

Care and storage

## Cleaning Pigotite specimens should generally not be wet-cleaned. Due to its softness and potential water solubility, it is best to remove dust with a soft brush or a photographic blower. ## What to Avoid Contact with water, chemicals, and ultrasound should be strictly avoided. The substance is very soft and brittle, susceptible to scratches and mechanical damage. It should also be protected from high temperatures and direct sunlight, which can cause further changes in its structure. ## Storage Pigotite specimens are very delicate and should be stored under stable conditions, in closed boxes or display cases, away from dust, moisture, and temperature fluctuations. It is best to keep them in their original collector's box to prevent abrasion.

External references

Sources

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