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Formula: C2H17O5Si46O92
Tectosilicate (framework silicate)
Crystal System: Tetragonal
Specific gravity: 1.99 to 2.11 measured, 1.98 calculated
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).
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