Melanophlogite

melanophlogite

cristobalite

marcasite

opal

Images

Formula: C2H17O5Si46O92
Tectosilicate (framework silicate)
Specific gravity: 1.99 to 2.11
Hardness: 6½ to 7
Streak: White
Colour: Colourless, white, water-clear (pale yellow to deep red brown with impurities)
Luminescence: Fluorescent, weak grey-white in short wave UV, grey-white in long wave UV
Solubility: Insoluble in acids
Common impurities: ORG,C,H,S
Environments

Volcanic igneous environments
Carbonatites
Metamorphic environments
Hydrothermal environments

Melanophlogite is a late-stage, low-temperature mineral with large, clathrate* voids which host CH4, CO2 and N2. It is stable below 40-65oC (Webmin). Associated minerals include sulphur, calcite, opal, cristobalite, quartz, dolomite, celestine, pyrite, marcasite, rhodochrosite and magnesite (HOM).
*Clathrate: A texture found chiefly in leucite rocks, in which the leucite crystals are surrounded by tangential augite crystals in such a way as to suggest a net or a section of a sponge, the felted mass of augite prismoids representing the threads or walls, and the clear, round leucite crystals, the holes (Mindat))

Localities

At Chvaletice, Pardubice District, Pardubice Region, Czech Republic, melanophlogite occurs in low-temperature hydrothermal veins associated with metamorphosed sedimentary manganese deposits (HOM).

At the Racalmuto sulphur mines, Racalmuto, Agrigento Province, Sicily, Italy, melanophlogite is a late-stage, low-temperature mineral in sulphur deposits (HOM).
Melanophlogite occurs as regular cubes with a maximum size of 2 mm on edge, encrusting sulphur, calcite and celestine crystals. The melanophlogite is sometimes, but not always, separated from the sulphur and calcite crystals by a thin film of opaline silica. In many instances it has grown on a nucleus of quartz or opal. Unaltered melanophlogite cubes range from colourless through pale yellow to a deep red-brown. The sulphur and calcite crystals were the earliest minerals, and evidence indicates that the opaline silica had ceased depositing before any melanophlogite formed, but the sulphur crystals were still growing when melanophlogite started forming. Quartz pseudomorphs after melanophlogite are common, and fresh, unaltered crystals of melanophlogite are rare (AM 48.854-867).

At the type locality, the Giona mine, Milena, Caltanissetta Province, Sicily, Italy, melanophlogite occurs as light brownish cubes to about 1 mm and aggregates forming chains and shells. Associated minerals include sulphur, quartz, celestine and calcite (Mindat).

At Fortullino, Rosignano Marittimo, Livorno Province, Tuscany, Italy, melanophlogite occurs in carbonate-bearing serpentinites (HOM).

Ol Doinyo Lengai, Ngorongoro District, Arusha region, Tanzania, is the world's only active carbonatite volcano, with natrocarbonatite, calcite-carbonatite, jacupirangite, nepheline syenite and other rocks. The lava is extremely alkali-rich, with 30% Na2O (Mindat).
Melanophlogite occurs as part of a tuffaceous layer within a sample of the 2006 natrocarbonatite lava. This is the first reported occurrence of a clathrate* in an igneous carbonatite. It occurs in an ash pellet-rich layer within the natrocarbonatite lava, as abundant groundmass crystals and as cores of individual ash pellets, with numerous inclusions of nepheline laths (AM 98.1998-2006).
*Clathrate: A texture found chiefly in leucite rocks, in which the leucite crystals are surrounded by tangential augite crystals in such a way as to suggest a net or a section of a sponge, the felted mass of augite prismoids representing the threads or walls, and the clear, round leucite crystals, the holes (Mindat))

At the Mount Hamilton Melanophlogite locality, Sugarloaf Mountain, Black Wonder Mining District, Santa Clara county, California, USA, melanophlogite occurs in carbonate-bearing serpentinites (HOM).
The contact zone exposed by the roadcut is located between serpentine and a partially silicified arkosic sandstone. This zone has been subjected to several periods of hydrothermal activity and fracturing after its initial formation, producing brecciated rock composed of rounded masses of silicified serpentine surrounded by quartz and dolomite seams. Small quantities of pyrite, magnesite and calcite were the only accessory minerals noted in the seams.
After the breccia had developed it was again subjected to folding and transverse fracturing. Low-temperature and pressure silica-bearing solutions entered the fractures and formed cristobalite and melanophlogite. These solutions were probably sulphur-free and low in organic content (AM 57.1494-1504).

Back to Minerals