Fluorcalciomicrolite

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Fluorcalciomicrolite
General and classification
other names

IMA 2012-036

chemical formula (Ca, Na, □) 2 Ta 2 O 6 F
Mineral class
(and possibly department) 		Oxides and hydroxides
System no. according to Strunz 4.DH.15
Crystallographic Data
Crystal system cubic
Crystal class ; symbol cubic hexakisoctahedral; 4 / m  3  2 / m
Space group Fd 3 m (No. 227)Template: room group / 227
Lattice parameters a  = 10.4191  Å
Formula units Z  = 8
Frequent crystal faces {111}, {110}
Twinning no
Physical Properties
Mohs hardness 4.5 to 5
Density (g / cm 3 ) 6,160 (calculated)
Cleavage no
Break ; Tenacity clamshell; brittle
colour colorless
Line color White
transparency translucent
shine Diamond to resin gloss
Crystal optics
Refractive index n  = 1.992 (calculated)
Birefringence optically isotropic

Fluorcalciomicrolite is a rarely occurring mineral from the mineral class of " oxides and hydroxides ". It crystallizes in the cubic crystal system with the idealized chemical composition (Ca, Na, □) 2 Ta 2 O 6 F, where □ represents a blank (vacancy) in the corresponding position. Fluorcalciomikrolith develops colorless crystals up to 1.5 mm in size with a diamond-like sheen . The main shape of the always idiomorphic, never twinned crystals is the octahedron , occasionally the crystal costume is complemented by surfaces of the rhombic dodecahedron .

Etymology and history

The mineral was discovered in the pegmatite of "Volta Grande" near Nazareno in the Sn-Ta-rich pegmatite province of São João del Rei, Minas Gerais , Brazil . The crystals of the Fluorcalciomikroliths come from a heavy mineral concentrate , so that the exact paragenetic relationships to the accompanying minerals remain unclear. The mineral was examined by a Brazilian research team led by Marcelo B. Andrade , Daniel Atencio , Aba IC Persiano and Javier Ellena . After it was recognized by the International Mineralogical Association (IMA) in 2012, the official first description followed a year later.

The name of the mineral was chosen in accordance with the redefinition of the nomenclature of the pyrochlore supergroup with the general composition A 2 B 2 X 6 Y. According to this, due to its chemical composition and the dominance of tantalum in the B position, the mineral is a member of the newly created microlith group . The name Fluorcalciomikrolith refers to the membership of the microlith group, the dominance of fluorine on the Y-position and the dominance of calcium on the A-position.

Type material of the mineral is in the collection of the "Museu de Geociências", Instituto de Geociências, Universidade de São Paulo , Brazil (reg. No. DR731).

classification

The current classification of the International Mineralogical Association (IMA) counts the Fluorcalciomikrolith to the pyrochlore upper group with the general formula A 2– m B 2 X 6– w Y 1– n , in which A , B , X and Y different positions in the structure the minerals of the pyrochlore upper group with A = Na, Ca, Sr, Pb 2+ , Sn 2+ , Sb 3+ , Y, U, □, or H 2 O; B = Ta, Nb, Ti, Sb 5+ or W; X = O, OH or F and Y = OH - , F, O, □, H 2 O or very large (>> 1.0 Å) monovalent cations such as K, Cs or Rb. To pyrochlore supergroup include not only Fluorcalciomikrolith still Fluornatromikrolith , Hydrokenomikrolith , Hydroxycalciomikrolith , Hydroxykenomikrolith , Kenoplumbomikrolith , Oxynatromikrolith , Oxystannomikrolith , Oxystibiomikrolith , Cesiokenopyrochlor , Fluorcalciopyrochlor , Fluornatropyrochlor , Hydrokenopyrochlor , Hydropyrochlor , Hydroxycalciopyrochlor , Hydroxykenopyrochlor , Hydroxymanganopyrochlor , Hydroxynatropyrochlor , Oxycalciopyrochlor , Fluorcalcioroméit , Hydroxycalcioroméit , Hydroxyferroroméit , Oxycalcioroméit , Oxyplumboroméite , Hydrokenoelsmoreit , Hydroxykenoelsmoreit , Fluornatrocoulsellit and Hydrokenoralstonit . Fluorocalciomicrolite, together with fluorine atromicrolite, hydrokenomicrolite, hydroxycalciomicrolite, hydroxykenomicrolite, kenoplumbomicrolite, oxynatromicrolite, oxystibiomicrolite and oxystannomicrolite, form the microlite group within the pyrochlore upper group .

Since the fluorocalciomicrolite was only recognized as an independent mineral by the International Mineralogical Association (IMA) in 2012 and the discovery was only published in 2013, it is not listed in the Strunz classification of minerals (8th edition), which has been outdated since 2001 . Only in the “Lapis Mineralienverzeichnis” published in 2014, which, out of consideration for private collectors and institutional collections, is still based on the classic systematics of Karl Hugo Strunz , was the mineral given system no. IV / C.13-22.

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), classifies the fluoroccalciomicrolite in the category of "Oxides with a metal: oxygen ratio = 1: 2 and comparable". This is further subdivided according to the relative size of the cations involved and the crystal structure, so that the mineral is classified according to its composition and structure in the subsection “With large (± medium-sized) cations; Layers of edge-linked octahedra ”can be found, where together with all representatives of the pyrochlore, microlith, Betafit, Roméit and Elsmoreit groups, the pyrochlore supergroup with the system no. 4.DH.15 forms. Fluorcalciomikrolith is with Fluornatromikrolith , Hydrokenomikrolith , Hydromikrolith , Hydroxykenomikrolith , Kenoplumbomikrolith , Oxycalciomikrolith , Oxystannomikrolith and Oxystibiomikrolith found in the Mikrolithgruppe.

Fluorcalciomicrolite is also not yet included in the systematics of minerals according to Dana , which is mainly used in the English-speaking area . The mineral is, however, a close relative of fluorine atromicrolite , which can be found in the mineral class of "oxides and hydroxides" and there in the department of "multiple oxides with Nb, Ta and Ti", where it is found together with microlite, bariomicrolite, plumbomicrolite, Uranium microlite, bismuth microlite, stannomicrolite and stibiomicrolite the "pyrochlore group (microlite subgroup; Ta> Nb; (Ta + Nb)> 2 (Ti))" with system no. 02/08/02 within the subdivision of " Multiple oxides with Nb, Ta and Ti with the formula A 2 (B 2 O 6 ) (O, OH, F) ". Due to its chemical and structural relationship, fluorocalciomicrolite will probably also be classified there.

Chemism

Six microprobe analyzes on fluorocalciomicrolite from the pegmatite of “Volta Grande” yielded mean values ​​of 4.68% Na 2 O; 11.24% CaO; 0.01% MnO; 0.04% SrO; 0.02% BaO; 0.63% SnO 2 ; 0.02% UO 2 ; 3.47% Nb 2 O 5 ; 76.02% Ta 2 O 5 ; 0.48% H 2 O (calculated for the charge balance) and 2.80% F and [(O ≡ F) -1.18%, total = 98.23%]. On the basis of two cations on the B position, the empirical formula (Ca 1.07 Na 0.81 □ 0.12 ) Σ = 2.00 (Ta 1.84 Nb 0.14 Sn 0.02 ) Σ = 2.00 [O 5.93 (OH) 0.07 ] 6.00 [F 0.79 (OH) 0.21 ], which was simplified to (Ca, Na, □) 2 Ta 2 O 6 F.

Within the pyrochlore upper group there are theoretically a multitude of substitution possibilities due to the four different positions to be occupied. Fluorocalciomicrolite is the Ca-dominant analogue of the Na-dominated fluorine atomic microlite and the F-dominant analogue of the OH-dominated hydroxycalciomicrolite. With fluorine atomic microlite, fluorine calciomicrolite also forms a mixed crystal row . Across all subgroups, fluorocalciomicrolite is the Ta-dominant analogue of the Nb-dominated fluoroccalciopyrochlore and the Sb 5+ -dominated fluoroccalcioroméite.

The minerals ferrous , (Ca, Ce, Na) (Nb, Ta, Ti) 2 (O, OH, F) 6 , oxynatromicrolite, (Na, Ca, U) 2 (Ta, Nb) 2 O 6, are chemically similar (O, F), and “Samiresite”, (U, Ca, Ce) 2 (Nb, Ta) 2 O 6 (OH, F), a mineral previously described as a lead-containing variety of “uranium pyrochlore”.

Crystal structure

Fluorcalciomicrolite crystallizes in the cubic crystal system in the space group Fd 3 m (space group no. 227) , with the lattice parameter a  = 10.4191  Å and eight formula units per unit cell . Template: room group / 227

As with all representatives of the pyrochlore upper group, the crystal structure of the fluorocalciomicrolite consists of - in this case - TaO 6 - octahedra with common corners, which form parallel layers of three and six rings [110]. Tantalum is coordinated by six equivalent oxygen atoms that form an almost ideal octahedron. In the layers mentioned there are channels in direction 110, which take up the oxygen atoms and the atoms such as Ca and Na in the A position. Again, these atoms form (Ca, Na, □) O 6 F 2 - polyhedron , which are joined together by common edges.

Fluoroccalciomicrolite is isotypic (isostructural) to all other representatives of the pyrochlore upper group that crystallize in space group Fd 3 m (space group no. 227) . Template: room group / 227

properties

morphology

Fluorcalciomikrolith forms crystals up to 1.5 mm in size with the octahedron {111} as the shape that determines the costume. Occasionally the crystal costume is supplemented by the blunt edges of the rhombic dodecahedron. The crystals of the Fluorcalciomikroliths are generally not twinned.

physical and chemical properties

The crystals of the Fluorcalciomikroliths are colorless, the line color of the translucent mineral, however, is always white. The fluoroccalciomicrolite crystals have a diamond to resin-like sheen . The mineral shows no cleavage and , due to its brittleness, breaks like quartz or glass , whereby the fracture surfaces are formed like mussels. With a Mohs hardness of 4.5–5, fluorocalciomicrolite is one of the medium-hard minerals that are slightly easier to scratch with a pocket knife than the reference mineral apatite . The calculated density of the mineral is a maximum of 6.160 g / cm³.

Modifications and varieties

Fluorocalciomicrolite is the calcium-dominant analogue of the sodium- dominated fluorous atomic microlite . There are also titanium and uranium-containing varieties of fluorocalciomicrolite, which are described by the formula (Ca, Na, U) 2 (Ta, Ti, Nb) 2 O 6 (F, OH) can. It has also been found that the largest part of the former “tantalum betafite” is fluorocalciomicrolites rich in titanium and uranium.

Education and Locations

Fluorcalciomikrolith is known only from heavy mineral concentrates because of its type locality, a granite pegmatite sitting in amphibolite schist with exceptionally high contents of lithium and rubidium . Accompanying minerals are microcline , albite , quartz , muscovite , spodumene , lepidolite , cassiterite , tantalite (Mn) , monazite (Ce) , fluorite , apatite , beryl , garnet , epidote , magnetite , gahnite , zircon , tourmaline , bityite , hydrokenomicrolite and further representatives of the Mikrolith group. Some of these crystals are characterized by the association of hydrokenomicrolite with fluorocalciomicrolite, which could indicate that hydrokenomicrolite can be interpreted as an alteration product of fluorocalciomicrolite.

As a seldom occurring mineral formation, fluorocalciomicrolite has only been observed from a few sites and there only in small amounts. So far (as of 2016) about 10 locations for the mineral are known.

In addition to its type locality , the pegmatite from Volta Grande near Nazareno in the Sn-Ta-rich pegmatite province of São João del Rei, fluoroccalciomicrolite in the Brazilian state of Minas Gerais was also observed in the Ipê mine in the valley of the Rio Doce near Governador Valadares . It is also from the pegmatites "Viitaniemi", Eräjärvi area near Orivesi , and "Kiimassuo" near Forssa in Finland , a Li / Rb / Cs-rich pegmatite near Nová Ves u Brloha not far from Český Krumlov , South Bohemian region in the Czech Republic , the granite pegmatite "Jezuitské Lesy", Devín near Bratislava in Slovakia , and the "Bagoly Hill" near Bükkszentkereszt in the Bükk Mountains, Borsod-Abaúj-Zemplén County , Hungary . Other sites are the lithium-rich, lepidolite- petalite- containing Red Cross Lake pegmatites on the lake of the same name in the Canadian province of Manitoba , the pegmatite dike No. 309 in the pegmatite field of Guanpo near the city of the same name in Lushi County , Sanmenxia , Henan Province , China , as well as the lithium-rich pegmatites from Nagatare in Fukuoka Prefecture , Kyūshū , Japan . Locations in Germany , Austria and Switzerland are not known.

use

Fluorcalciomicrolite is economically insignificant due to its rarity and is only a coveted mineral for collectors.

See also

literature

  • Marcelo B. Andrade, Daniel Atencio, Aba IC Persiano and Javier Ellena (2013): Fluorcalciomicrolite, (Ca, Na, □) 2 Ta 2 O 6 F, a new microlite-group mineral from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. In: Mineralogical Magazine , Volume 77, pp. 2989-2996.

Web links

Individual evidence

  1. ↑ a b c d e f g h i j k l Marcelo B. Andrade, Daniel Atencio, Aba IC Persiano and Javier Ellena: Fluorcalciomicrolite, (Ca, Na, □) 2 Ta 2 O 6 F, a new microlite-group mineral from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil . In: Mineralogical Magazine . tape 77 , no. 7 , 2013, p. 2989–2996 , doi : 10.1180 / minmag.2013.077.7.08 (English).
  2. ^ A b c Daniel Atencio, Marcelo B. Andrade, Andrew G. Christy, Reto Gieré, Pavel M. Kartashov: The Pyrochlore supergroup of minerals: Nomenclature . In: The Canadian Mineralogist . tape 48 , 2010, p. 673–698 , doi : 10.3749 / canmin.48.3.673 (English, rruff.info [PDF; 1,4 MB ; accessed on August 30, 2018]).
  3. ^ Andy G. Christy, Daniel Atencio: Clarification of the status of species in the pyrochlore supergroup . In: Mineralogical Magazine . tape 77 , 2013, p. 13–20 ( main.jp [PDF; 85 kB ; accessed on November 17, 2018]).
  4. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 6th completely revised and supplemented edition. Weise, Munich 2014, ISBN 978-3-921656-80-8 .
  5. ↑ a b Thomas Witzke , Manfred stone, Thomas Doering, Walter Schuckmann, Reinhard Wegner, Herbert Pöllmann: Fluornatromicrolite, (Na, Ca, Bi) 2 Ta 2 O 6 F, a new mineral species from Quixaba, Paraíba, Brazil . In: The Canadian Mineralogist . tape  49 , 2011, p. 1105–1110 , doi : 10.3749 / canmin.49.4.1105 (English).
  6. ↑ Marcelo B. Andrade, Hexiong Yang, Daniel Atencio, Robert T. Downs, Nikita V. Chukanov, Marie-Hélène Lemée-Cailleau, Aba Israel Cohen Persiano, Andrés E. Goeta, Javier Ellena: Hydroxycalciomicrolite, Ca 1.5 Ta 2 O 6 (OH), a new member of the microlite group from Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil . In: Mineralogical Magazine . tape 81 , no. 3 , 2017, p. 555–564 , doi : 10.1180 / minmag.2016.080.116 (English).
  7. ↑ Li Guowu, Yang Guangming, Lu Fude, Xiong Ming, Ge Xiangkun, Pan Baoming, Jeffrey de Fourestier: Fluorcalciopyrochlor, a new mineral species from Bayan Obo, Inner Mongolia, PR China . In: The Canadian Mineralogist . tape 54 , no. 5 , 2016, p. 1285–1291 , doi : 10.3749 / canmin.1500042 (English).
  8. ^ Daniel Atencio, Marcelo B. Andrade, Marco E. Ciriotti: Fluorcalcioroméite, (Ca, Na) 2 Sb 5+ 2 (O, OH) 6 F, a new roméite-group mineral from Starlera mine, Ferrera, Grischun, Switzerland: description and crystal structure . In: Mineralogical Magazine . tape 77 , no. 4 , 2013, p. 467–473 , doi : 10.1180 / minmag.2013.077.4.06 (English).
  9. ↑ a b Mindat - Fluorcalciomicrolite
  10. ^ A b Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  222-223 .
  11. ^ AP Kalita, AV Bykova: Tantalum betafite from the Ladoga pegmatites . In: Trudy Inst. Mineral. Geokhimii Kristallokhimii Redkikh Elementov . tape 7 , 1961, pp. 104–107 (Russian, with English abstract).
  12. ↑ Mindat - Number of localities for Fluorcalciomikrolith
  13. ↑ a b List of sites for fluoroccalciomicrolite in the Mineralienatlas and Mindat