Melanophlogite
Encyclopedia
Melanophlogite is a rare silicate mineral
and a polymorph
of silica
(SiO2). It has a zeolite
-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".
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
, and thus the mineral was called Girghenti, an old name for Agrigento
town in Sicily. The name was officially changed to melanophlogite in 1927.
-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 for non-spherical guests like tetrahydrofuran
or tetrahydrothiopene. 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
after heating at temperatures above 900 °C.
, Torino
, Caltanissetta
and Livorno
provinces of Italy; also in several mines of California
in the US, in Crimea
(Ukraine) and Pardubice Region
(Czech Republic
).
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...
and a polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
of silica
Silicon dioxide
The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
(SiO2). It has a zeolite
Zeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
-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
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
or calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
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 LasaulxArnold von Lasaulx
Arnold Constantin Peter Franz von Lasaulx was a German mineralogist and petrographer.He was born at Kastellaun near Coblenz, and educated at the University of Berlin, where he took his Ph. D. in 1868. In 1871 he became professor of mineralogy at Breslau, and in 1880 professor of mineralogy and...
in 1876 although G. Alessi had described a very similar mineral as early as in 1827. The mineral had a cubic
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
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
Sicily
Sicily is a region of Italy, and is the largest island in the Mediterranean Sea. Along with the surrounding minor islands, it constitutes an autonomous region of Italy, the Regione Autonoma Siciliana Sicily has a rich and unique culture, especially with regard to the arts, music, literature,...
, and thus the mineral was called Girghenti, an old name for Agrigento
Agrigento
Agrigento , is a city on the southern coast of Sicily, Italy, and capital of the province of Agrigento. It is renowned as the site of the ancient Greek city of Akragas , one of the leading cities of Magna Graecia during the golden...
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 zeoliteZeolite
Zeolites are microporous, aluminosilicate minerals commonly used as commercial adsorbents. The term zeolite was originally coined in 1756 by Swedish mineralogist Axel Fredrik Cronstedt, who observed that upon rapidly heating the material stilbite, it produced large amounts of steam from water that...
-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
Cubic crystal system
In crystallography, the cubic crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals....
and the symmetry lowers to tetragonal for non-spherical guests like tetrahydrofuran
Tetrahydrofuran
Tetrahydrofuran is a colorless, water-miscible organic liquid with low viscosity at standard temperature and pressure. This heterocyclic compound has the chemical formula 4O. As one of the most polar ethers with a wide liquid range, it is a useful solvent. Its main use, however, is as a precursor...
or tetrahydrothiopene. 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
The mineral cristobalite is a high-temperature polymorph of silica, meaning that it has the same chemical formula, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group which also include coesite, tridymite and stishovite...
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 crystalites a few millimeters in size. It is found in ParmaProvince of Parma
The Province of Parma is a province in the Emilia-Romagna region of Italy. Its capital is the city of Parma.It has an area of 3,449 km², and a total population of 413,198...
, Torino
Province of Turin
The Province of Turin is a province in the Piedmont region of Italy. Its capital is the city of Turin.It has an area of 6,830 km², and a total population of 2,277,686 . There are 315 comuni in the province – the most of any province in Italy...
, Caltanissetta
Province of Caltanissetta
The Province of Caltanissetta is a province in the southern part of Sicily, Italy...
and Livorno
Province of Livorno
The Province of Livorno or Leghorn is a province in the Tuscany region of Italy. It includes several islands of the Tuscan Archipelago, including Elba and Capraia. Its capital is the city of Livorno....
provinces of Italy; also in several mines of California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
in the US, in Crimea
Crimea
Crimea , or the Autonomous Republic of Crimea , is a sub-national unit, an autonomous republic, of Ukraine. It is located on the northern coast of the Black Sea, occupying a peninsula of the same name...
(Ukraine) and Pardubice Region
Pardubice Region
Pardubice Region is an administrative unit of the Czech Republic, located mainly in the eastern part of its historical region of Bohemia, with a small part in northwestern Moravia. It is named after its capital Pardubice. "There are a total of 452 municipalities in the region...
(Czech Republic
Czech Republic
The Czech Republic is a landlocked country in Central Europe. The country is bordered by Poland to the northeast, Slovakia to the east, Austria to the south, and Germany to the west and northwest....
).
External links
- Spectroscopic data on Melanophlogite - zeolite properties of melanophlogite