Sekaninaite ((Fe
+2,Mg)
2Al
4Si
5O
18) is a
silicate mineral, the iron-rich analogue of
cordierite.
It was first described in 1968 for an occurrence in Dolní Bory, Vysočina Region,
Moravia
Moravia ( , also , ; cs, Morava ; german: link=yes, Mähren ; pl, Morawy ; szl, Morawa; la, Moravia) is a historical region in the east of the Czech Republic and one of three historical Czech lands, with Bohemia and Czech Silesia.
The me ...
,
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 ...
, and is now known also from Ireland, Japan, and Sweden. It was named after a
Czech mineralogist,
Josef Sekanina (1901–1986).
[ In Brockley on ]Rathlin Island
Rathlin Island ( ga, Reachlainn, ; Local Irish dialect: ''Reachraidh'', ; Scots: ''Racherie'') is an island and civil parish off the coast of County Antrim (of which it is part) in Northern Ireland. It is Northern Ireland's northernmost point. ...
, Ireland
Ireland ( ; ga, Éire ; Ulster Scots dialect, Ulster-Scots: ) is an island in the Atlantic Ocean, North Atlantic Ocean, in Northwestern Europe, north-western Europe. It is separated from Great Britain to its east by the North Channel (Grea ...
sekaninaite occurs in bauxitic clay within the contact aureole of a diabase
Diabase (), also called dolerite () or microgabbro,
is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. Diabase dikes and sills are typically shallow intrusive bodies and often exhibit fine-graine ...
intrusive plug.[
]
Structure and composition
The chemical formula of sekaninaite is: (Fe^2+, Mg^2+)2 l4Si5O18\mathitH2O. Grapes et al. (2010) calculated the percentage weights of the sample from Dolni Bory,
This compound exists in nature in the form of two polymorphs: one having a disordered hexagonal structure and the other arranged in an ordered orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...
structure. As an aluminosilicate, the repeated and ordered structure is based on polymerization of one or the other's tetrahedral framework of Si, Al tetrahedra (Yakubovich et al., 2003). Nearly all analyses show excess of Al and deficiency in Si with respect to tetrahedral components. The overall substitution of alkali
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
s causes excess in cations found in (K2O, Na2O, CaO), implying that sekaninaite is essentially anhydrous (Grapes et al., 2010).
The atomic structures of cordierites are interpreted as a continuous series of structures that vary based on the content of octahedrally coordinated Mg and Fe cations. The varying content of atoms in the octahedral M position has an effect on the orthorhombic unit cell's parameters. The wide range of isomorphism
In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word is ...
of Mg and Fe(4-96%) suggest the existence of a continuous isomorphic
In mathematics, an isomorphism is a structure-preserving mapping between two structures of the same type that can be reversed by an inverse mapping. Two mathematical structures are isomorphic if an isomorphism exists between them. The word is ...
series cordierite (Mg,Fe)2 l4Si4O18\mathitH2O-sekaninaite (Fe,Mg)2 l4Si4O18\mathitH2O. It is shown via crystallographic data that a shift in the iron content leads to a corresponding variance in a and b unit cell parameters (Yakubovich et al., 2003). As an aluminosilicate/cyclosilicate, the octahedral M-O distances consist of 5 independent tetrahedra form a 3-dimensional anionic framework of ordered and distributed Al3+ and Si4+ cations. One independent AlO4 and two SiO4 vortex-sharing tetrahedra share oxygen atoms to form six-member rings along the c axis of the unit cell. Mg, Fe octahedra share edges with SiO4 to form rings from alternating octahedra and tetrahedra. Thus, the framework can be described as a semi-layered structure formed of layers of tetrahedra linked into rings by sharing vertices and octahedra and tetrahedra sharing edges, alternating along the c axis. The distortion of the orthorhombic unit cell is determined by the chemical composition rather than the degree of ordering in the tetrahedral framework (Yakubovich et al., 2003). The temperature at which the liquidous phases crystallize in a sequence: mullite + tridymite, followed by sekaninaite and finally fayalite + clinoferrosilite (Grapes et al., 2010). Similar trends are observed for amphibole
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...
s, clinopyroxenes, olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
s, and others. The increase in the Fe mole fraction of minerals was not related with iron input, but was caused by its redistribution during contact metamorphism
Metamorphism is the transformation of existing rock (the protolith) to rock with a different mineral composition or texture. Metamorphism takes place at temperatures in excess of , and often also at elevated pressure or in the presence of chem ...
(Korchak et al., 2010).
Physical properties
Stanek and Miskovsky (1975) first identified and diagnosed sekaninaite as a new mineral in the cordierite series. They sampled the poorly developed crystals of the Dolni Bory region, Czechoslovakia, where specimen did not exceed 70 cm. Dolni Bory samples are very different from samples found in the Kuznetsk
Kuznetsk (russian: Кузне́цк) is a town in Penza Oblast, Russia, located in the foothills of the Volga Upland, mainly on the left bank of the Truyov River. Population:
Administrative and municipal status
Within Russia's framework of a ...
paralavas. They are very close analogues with respect to Mg/Fe ratios but vastly different a-, b- and c- parameters (Grapes et al., 2010). Grapes and colleagues calculated cell dimension to be a 17.230(5), b 9.835(3), c 9.314(3) A. The colour of sekaninaite is bright blue and distinctly pleochroic
Pleochroism (from Greek πλέων, ''pléōn'', "more" and χρῶμα, ''khrôma'', "color") is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light.
Backgroun ...
with X = colourless; Y = blue; Z = pale blue; absorption occurs in the sequence Y > Z > X. Sekaninaite has a hardness of 7-7.5; it cleaves imperfectly along and exhibits parting on (Fleischer and Jambor, 1977). Majority of crystals show zonation (Fe increasing from core to rim). It common twinned on and , simulating hexagonal symmetry. Sekaninaite is classified under the space group Cccm; it is an orthorhombic
In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...
crystal that is found in series with cordierite (Stanek et al., 1975).
Geologic occurrence and location
Sekaninaite was first discovered in the Dolni Bory region of the Czech Republic. Its occurrence is in the albite
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. It represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula . It is a tectosilicate. I ...
zone of pegmatite
A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...
in granulites and gneiss
Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures an ...
es (Fleischer and Jambor, 1977). Sekaninaite is found in pyrometamorphic rocks, extensively rocks formed via process of ancient combustion metamorphism; paralavas, clinkers and buchites. These combustion metamorphic rocks occur in clinker beds and breccia
Breccia () is a rock composed of large angular broken fragments of minerals or rocks cemented together by a fine-grained matrix.
The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of di ...
s of vitrified sandstone-siltstone clinker fragments cemented by paralava. These partially baked and oxidized
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...
psammitic-pelitic
A pelite (Greek: ''pelos'', "clay") or metapelite is a metamorphosed fine-grained sedimentary rock, i.e. mudstone or siltstone. The term was earlier used by geologists to describe a clay-rich, fine-grained clastic sediment or sedimentary rock, ...
sediments are associated with burnt coal seams, belonging to places like the Kuznetsk coal basin
The Kuznetsk Basin (russian: Кузнецкий угольный бассейн, Кузбасс; often abbreviated as Kuzbass or Kuzbas) in southwestern Siberia, Russia, is one of the largest coal mining areas in Russia, covering an area of aroun ...
, Siberia
Siberia ( ; rus, Сибирь, r=Sibir', p=sʲɪˈbʲirʲ, a=Ru-Сибирь.ogg) is an extensive geographical region, constituting all of North Asia, from the Ural Mountains in the west to the Pacific Ocean in the east. It has been a part of ...
(Grapes et al., 2010). Sekaninaite-Fe-cordierite exists in series and is largely dependent upon variations in solid solution. These minerals are more prevalent in paralavas found in: Power River, Wyoming
Wyoming () is a U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the south ...
, Ravat area, Tajikistan
Tajikistan (, ; tg, Тоҷикистон, Tojikiston; russian: Таджикистан, Tadzhikistan), officially the Republic of Tajikistan ( tg, Ҷумҳурии Тоҷикистон, Jumhurii Tojikiston), is a landlocked country in Centr ...
, Kenderlyk Basin, eastern Kazakhstan
Kazakhstan, officially the Republic of Kazakhstan, is a transcontinental country located mainly in Central Asia and partly in Eastern Europe. It borders Russia to the north and west, China to the east, Kyrgyzstan to the southeast, Uzbeki ...
and the Djhar basin in India; each differ in sedimentary mineral assemblage and results depend on high-temperature fusion of mixtures of sandstone-siltstone and minor ferruginous components (Grapes et al., 2010). These Fe-rich paralavas are composed of Fe-olivine, esseneite
Esseneite is a relative rare mineral of the pyroxene group, with formula CaFeAlSiO6. It is the ferric-iron-dominant member. Esseneite is an iron-analogue of other pyroxene-group members, davisite, grossmanite, and kushiroite. It is a metamorphic ...
, dorite, melilite, Fe-cordierite, anorthite
Anorthite is the calcium endmember of the plagioclase feldspar mineral series. The chemical formula of pure anorthite is Ca Al2 Si2O8. Anorthite is found in mafic igneous rocks. Anorthite is rare on the Earth but abundant on the Moon.
Mineralo ...
, spinel
Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals.
Properties
S ...
, tridymite, fayalite, magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
, quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
etc. (Novikova, 2009).
See also
* List of minerals
This is a list of minerals for which there are articles on Wikipedia.
Minerals are distinguished by various chemical and physical properties. Differences in chemical composition and crystal structure distinguish the various ''species''. Within a m ...
* List of minerals named after people
References
{{Reflist
* Anthony, John W., Bideaux, Richard A., Bladh, Kenneth W., and Nichols, Monte C., Eds. (2003). Handbook of mineralogy, Mineralogical Society of America, Chantilly, VA 20151-1110, USA. http://www.handbookofmineralogy.org.
* Fleischer, M., Jambor, J. (1977). American Mineralogist, 62, 195–397.
* Geiger, C.A., Voigtlander, H., (2000). Contributions to mineral petrology. The heat capacity of synthetic anhydrous Mg and Fe cordierite. Springer-Verlag, Kiel, 46–50.
* Grapes, R., Korzhova, S., Sokol, E., Seryotkin, Y. (2010). Paragenesis of unusual Fe-cordierite (sekaninaite)-bearing paralava and clinker from the Kuznetsk coal basin, Siberia, Russia. Contributions of Mineral Petrology, 162, 253–273.
* Korchak, Yu. A., Men’shikov, Yu.P., Pakhomovskii, Ya. A., Yakovenchuk, V.N., Ivanyuk, G.Yu. (2011). Trap formation of the Kola Peninsula. Petrologiya, 19 (1), 89–103.
* Miskovsky, J., Stanek, J. (1975). Sekaninaite, a new mineral of the cordierite series from Dolni Bory, Czechoslovakia. Scr. Fac. Sci. Nat. Ujep. Brun. Geol. 1(5), 21–30.
* Novikova, S. A. (2009). Fayalite from Fe-rich Paralavas of ancient coal fires in the Kuzbass, Russia. Geology of Ore Deposits, 51 (8), 800–811. https://doi.org/10.1134/S1075701509080133
* Yakubovich, O. V., Massa, V., Pekov, I. V., Gavrilenko, P. G., Chukanov, N. V. (2004). Crystallography Reports, 49 (6), 953–963.
Iron(II) minerals
Magnesium minerals
Aluminium minerals
Cyclosilicates
Orthorhombic minerals
Minerals in space group 66