Melanophlogite (MEP) is a rare
silicate mineral
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, silica (silicon dioxide, ) is usually consid ...
and a
polymorph of
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
(SiO
2). It has a
zeolite
Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These p ...
-like porous structure which results in relatively low and not well-defined values of its density and refractive index. Melanophlogite often overgrows crystals of
sulfur or
calcite and typically contains a few percent of organic and sulfur compounds. Darkening of organics in melanophlogite upon heating is a possible origin of its name, which comes from the Greek for "black" and "to be burned".
[
]
History
Melanophlogite was identified and named by Arnold von Lasaulx in 1876 although G. Alessi had described a very similar mineral as early as in 1827. The mineral had a cubic crystal structure; chemical analysis revealed that it is mainly composed of SiO2, but also contains up to 12% of carbon and sulfur. It was suggested that the decomposition of organic matter (carbon) in the mineral was responsible for its blackening upon heating. All studied samples originated from Sicily
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, and thus the mineral was called Girghenti, an old name for Agrigento
Agrigento (; scn, Girgenti or ; grc, Ἀκράγας, translit=Akrágas; la, Agrigentum or ; ar, كركنت, Kirkant, or ''Jirjant'') is a city on the southern coast of Sicily, Italy and capital of the province of Agrigento. It was one o ...
town in Sicily. The name was officially changed to melanophlogite in 1927.[
]
Synthesis and properties
Melanophlogite can be grown synthetically at low temperatures and elevated pressures (e.g. 160 °C and 60 bar). It has a zeolite
Zeolites are microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula ・y where is either a metal ion or H+. These p ...
-like porous structure composed of Si5O10 and Si6O12 rings. Its crystalline symmetry depends on the content of its voids: crystals with spherical guest molecules or atoms (e.g. CH4, Xe, Kr) are cubic and the symmetry lowers to tetragonal
In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a squar ...
for non-spherical guests like tetrahydrofuran
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
or tetrahydrothiophene.[ Since many molecules form unstable guests, the symmetry of melanophlogite can change between cubic and tetragonal upon mild heating (<100 °C).]
Even the cubic melanophlogite often shows anisotropic optical properties. They were attributed not to tetragonal fragments but to the organic film in the mineral which could be removed by low-temperature annealing (~400 °C). Otherwise, melanophlogite is thermally stable and its physical properties do not change upon 20-day annealing at 800 °C, but it converts to cristobalite
Cristobalite is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the ...
after heating at temperatures above 900 °C.[
]
Occurrence
Melanophlogite is a rare mineral which usually forms round drops (see infobox) or complex intertwinned overgrowth structures over sulfur or calcite crystals. Rarely, it occurs as individual cubic crystallites a few millimeters in size.[ It is found in ]Parma
Parma (; egl, Pärma, ) is a city in the northern Italian region of Emilia-Romagna known for its architecture, music, art, prosciutto (ham), cheese and surrounding countryside. With a population of 198,292 inhabitants, Parma is the second mos ...
, Torino
Turin ( , Piedmontese: ; it, Torino ) is a city and an important business and cultural centre in Northern Italy. It is the capital city of Piedmont and of the Metropolitan City of Turin, and was the first Italian capital from 1861 to 1865. T ...
, Caltanissetta
Caltanissetta (; scn, Nissa or ) is a ''comune'' in the central interior of Sicily, Italy, and the capital of the Province of Caltanissetta. Its inhabitants are called ''Nisseni''.
In 2017, the city had a population of 62,797. It is the 14th ...
and Livorno provinces of Italy; also in several mines of California
California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...
in the US, in Crimea
Crimea, crh, Къырым, Qırım, grc, Κιμμερία / Ταυρική, translit=Kimmería / Taurikḗ ( ) is a peninsula in Ukraine, on the northern coast of the Black Sea, that has been occupied by Russia since 2014. It has a pop ...
(Ukraine) and Pardubice Region
Pardubice Region ( cs, Pardubický kraj; , ; pl, Kraj pardubicki) is an administrative unit ( cs, kraj) of the Czech Republic, located mainly in the eastern part of its historical region of Bohemia, with a small part in northwestern Moravia. It ...
(Czech Republic
The Czech Republic, or simply Czechia, is a landlocked country in Central Europe. Historically known as Bohemia, it is bordered by Austria to the south, Germany to the west, Poland to the northeast, and Slovakia to the southeast. The ...
).[
]
References
External links
Spectroscopic data on Melanophlogite
*{{cite book, author=Rosemarie Szostak, year=1998, title=Molecular sieves: Principles of Synthesis and Identification, publisher=Springer, isbn=0-7514-0480-2, url=https://books.google.com/books?id=lteintjA2-MC - zeolite properties of melanophlogite
Silica polymorphs