Melanophlogite

Chemical formula: C₂H₁₇O₅·Si₄₆O₉₂

Melanophlogite is a rare, low-temperature polymorphic variety of silica that darkens when heated but regains its original color upon cooling.

## Characteristics Melanophlogite is one of the polymorphic varieties of silica (SiO₂), similar to quartz, cristobalite, or tridymite. It forms small, usually spherical or cubic crystals, which can occur as radial aggregates or crusts. It is typically colorless or white, sometimes with a yellowish or grayish tint. Its most characteristic feature is the phenomenon of reversible darkening – upon heating, it becomes black and opaque, only to return to its original state after cooling. This phenomenon is associated with the release and re-incorporation of organic matter trapped within its clathrate structure. ## Physical Properties Melanophlogite crystals exhibit a hardness in the range of 6.5-7 on the Mohs scale, similar to quartz. They have a vitreous luster, and their transparency ranges from transparent to translucent. The mineral's density is low, around 2.04 g/cm³, which is lower than that of quartz. ## Colors and Varieties This mineral is most often colorless, white, pale yellow, or grayish. No named color varieties or commercial varieties are distinguished. Darkening under temperature is a physical characteristic, not a permanent variety. ## History and Name The name melanophlogite comes from the Greek words *melas* (μέλας) meaning "black" and *phlogos* (φλογός) meaning "to burn", which is a direct reference to its unique property of darkening upon heating. The mineral was first described in 1876 by Arnold von Lasaulx based on specimens found in Sicily, Italy. ## Uses Melanophlogite has no industrial application. Due to its rarity and unique physical properties, it is a mineral valued exclusively by collectors and scientific institutions.

Properties

Mohs hardness: 6.5-7
Luster: Vitreous
Streak: White
Density: 2.04
Cleavage: None
Fracture: Conchoidal
Transparency: Transparent to Translucent
Crystal system: Tetragonal

Diagnostic features

## Identification The most characteristic diagnostic feature of melanophlogite is its reversible darkening (to black) when heated in a flame or hot air. In field conditions and in a collection, it can be identified by its typical spherical or cubic crystals with a vitreous luster, often forming crusts on sulfur or calcite. ## Distinguishing from Similar Minerals Melanophlogite is sometimes confused with other colorless or white siliceous minerals, such as chalcedony, opal-CT (lussatite), or quartz. It differs from quartz by its lower density and often spherical crystal habit. It differs from opal and chalcedony by its fully crystalline structure (though visible only under high magnification) and the heating test. Hyalite (a variety of opal) can form similar spherical aggregates but does not reversibly darken when heated. ## Crystal Forms Melanophlogite crystallizes in the tetragonal system (at low temperatures) or the cubic system (at high temperatures). It most often forms microscopic crystals with a cubic or octahedral habit. It occurs as small, spherical aggregates (spherulites), radial groupings, and thin crusts on other minerals.

Geological environment

## Genesis Melanophlogite is a low-temperature mineral, forming under hydrothermal conditions or as a result of volcanic exhalations. It forms in voids and fissures of sedimentary rocks, especially in sulfur and bitumen deposits, where the slow decomposition of organic substances provides methane and other hydrocarbons necessary for the stabilization of its clathrate structure. It can also crystallize in serpentinites. ## Mineral Associations This mineral most commonly co-occurs with native sulfur, calcite, celestine, aragonite, and quartz. In serpentinites, it is associated with magnesite, opal, and chalcedony. ## Localities The most important and classic localities for melanophlogite are in Italy, in the sulfur mines of Sicily (e.g., Racalmuto, Agrigento). It is also known from Livorno in Tuscany and Fortunago in Lombardy. Other reported localities include Chvaletice in the Czech Republic, Kaltan in Russia (Gornaya Shoria), and several places in the USA, including Santa Clara County in California and Mount Hamilton in the Diablo Range.

Rarity

Rare

For collectors

## Quality Criteria The most prized by collectors are specimens with well-formed, sharp crystals of cubic habit, which are rare. Rich crusts composed of numerous, shiny spherulites are also highly valued, especially on a contrasting matrix, such as yellow sulfur or white calcite. Crystal size is key – the larger and more transparent they are, the more attractive the specimen. The aesthetics of the composition and the absence of damage are also important. ## Popular Localities Classic and most sought-after melanophlogite specimens come from historical localities in Sicily, Italy. Specimens from the sulfur mines around Agrigento and Racalmuto are considered exemplary for this mineral.

Care and storage

## Cleaning Melanophlogite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. If necessary, the specimen can be rinsed in distilled water and then thoroughly dried with compressed air or left to air dry at room temperature. ## What to Avoid Contact with strong chemicals, especially hydrofluoric acid, which etches silica, must be strictly avoided. Specimens should not be subjected to sudden temperature changes. Although its darkening is reversible, repeated and intense heating can lead to microfractures and permanent structural damage. ## Storage Collector's specimens of melanophlogite, often found on delicate rock matrix, should be stored in closed boxes or display cases to protect them from dust and mechanical damage. It is not sensitive to sunlight.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FMelanophlogite