Morimotoite

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Morimotoite
General and classification
other names

Fe melanite

chemical formula Ca 3 TiFe 2+ Si 3 O 12
Mineral class
(and possibly department) 		Silicates and Germanates
System no. to Strunz
and to Dana 		9.AD.25 ( 8th edition : 8 / A.08-125)
51.4.3c.3
Similar minerals Schorlomite , Ti- Andradite (Malanite), Schörl , Augite
Crystallographic Data
Crystal system cubic
Crystal class ; symbol cubic hexakisoctahedral; 4 / m  3  2 / m
Space group Ia 3 d (No. 230)Template: room group / 230
Lattice parameters a  = 12.162  Å
Formula units Z  = 8
Physical Properties
Mohs hardness 7.5
Density (g / cm 3 ) natural mixed crystal: measured: 3.75; calculated: 3.80
Cleavage not observed
colour black
Line color Please complete!
transparency Please complete!
shine Diamond luster
Crystal optics
Refractive index n  = 1.955 (natural mixed crystal)
Birefringence δ = -

The mineral morimotoite is a very rare island silicate from the upper group of the garnet and has the idealized chemical composition Ca 3 TiFe 2+ Si 3 O 12 . It crystallizes in the cubic crystal system with the structure of garnet.

Morimotoit forms black, rhombic dodecahedral crystals with a diamond luster , which can be 1–2 cm in size. Even crystals that appear microscopically uniform can be adhesions of two grenades that can be distinguished by radiography . Due to the resulting distortions of the crystal lattice, these garnets can show a slight birefringence .

Finds from basic magmaites, carbonatites as well as contact metamorphic calcareous silicate rocks and skarns have been described. In addition to its type locality , the Fuka mine near Bitchu-cho not far from Takahashi , Okayama Prefecture in the Chūgoku region on Honshū in Japan, there are only very few documented sites for morimotoite worldwide.

Etymology and history

For the description of iron-titanium grenades, Huckenholz introduced the hypothetical end link Fe-melanite with the composition 3CaO * FeO * TiO 2 * 3SiO 2 (Ca 3 TiFe 2+ Si 3 O 12 ), together with the Mg analogue Ca 3 TiMgSi 3 O 12 (Mg melanite).

It was not until 1992 that Morimotoite was described by a Japanese working group led by Chiyoko Henmi as a new mineral with this idealized composition and recognized by the IMA. They named the new Ti garnet after the then emeritus professor of mineralogy at Osaka University Dr. Nobuo Morimoto in recognition of his outstanding contributions to mineralogy and crystallography.

The status of morimotoite as a separate mineral is controversial. The chemical composition of both natural garnets from the type locality and synthetic morimotoite hardly differs from natural schorlomite . The decisive factor for differentiating between schorlomite and morimotoite is the oxidation state of iron and titanium and the position of these ions in the crystal lattice: Fe 3+ on the tetrahedral silicon position for schorlomite, Fe 2+ on the octahedral position for morimotoite and in both minerals Ti 4+ on the Octahedral position. Henmi and co-workers set the oxidation state of titanium to Ti 4+ and determined that of iron mathematically by normalizing the analyzes to 12 oxygen ions and 8 cations. The distribution of Ti and Fe on the various grid positions is based on comparisons with other grenades. There is no confirmation of these assumptions and results by independent methods.

In 2002, the synthesis of Fe-Ti garnets with the composition of almost pure morimotoite was achieved, thus demonstrating that morimotoite can be formed under geologically relevant conditions.

The working group around Grew restructured the garnet group in 2013 and established a uniform scheme for calculating the oxidation levels and distribution of the elements on the lattice positions of the garnet structure from chemical compositions only. In addition to the morimotoite from the type locality (Fuka mine, Japan), they also list grenades from five other deposits as morimotoite.

classification

The structural classification of the International Mineralogical Association (IMA) is one of the Morimotoit to Garnet supergroup, where he along with almandine , Andradite , Calderit , Eringait , Goldmanit , Grossular , Knorringit , majorite , Menzerit- (Y) , Momoiit , pyrope , Rubinit , Spessartine and Uvarowite form the garnet group with 12 positive charges on the tetrahedrally coordinated lattice position.

The obsolete, but still used the eighth edition of the mineral classification by Strunz takes the Morimotoit along with almandine, Andradite, Calderit, Goldmanit, Grossular, Henritermierit , Hibschite , Holtstamit , Hydrougrandit , katoite , Knorringit, majorite, pyrope, Schorlomit , Spessartin, Uwarowit, Wadalite and Yamatoite (discredited because they are identical to Momoiite) in the "garnet group" with the system no. VIII / A.08 within the department of " Island Silicates (Nesosilicates)".

The 9th edition of Strunz's mineral system, which has been in effect since 2001, also counts the morimotoite to the "garnet group" with system no. 9.AD.25 within the department of "Island Silicates (Nesosilicates)". However, this is further subdivided according to the possible presence of further anions and the coordination of the cations involved , so that the mineral is classified according to its composition in the subsection “Island silicates without additional anions; Cations in octahedral [6] and usually greater coordination ”can be found.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , also assigns the morimotoite to the category of "island silicate minerals ". Here he is together with Kimzeyit and Schorlomit in the "Garnet group (Schorlomit-Kimzeyit-series)" with the system no. 51.04.03c within the subsection “ Island silicates: SiO 4 groups only with cations in [6] and> [6] coordination ”.

Chemism

Morimotoite with the idealized composition [X] Ca 3 [Y] (Ti 4+ Fe 2+ ) [Z] Si 3 O 12 is the Fe-Ti analogue of majorite ( [X] Ca 3 [Y] (Si 4+ Mg 2+ ) [Z] Si 3 O 12 ) and forms complex mixed crystals especially with andradite according to the exchange reaction

  • [Y] Ti 4+ + [Y] Fe 2+ = 2 [Y] Fe 3+ ,

Schorlomit corresponding to the exchange reaction

  • [Y] Fe 2+ + 2 [Z] Si 4+ = [Y] Ti 4+ + 2 [Z] Fe 3+

and the hypothetical Mg morimotoite about the reaction

  • [Y] Fe 2+ = [Y] Mg 2+ ,

where [X], [Y] and [Z] indicate the positions in the garnet structure.

The following composition is given for the morimotoite from the type locality:

  • [X] (Ca 2.89 Mg 0.11 ) [Y] (Ti 4+ 1.20 Fe 2+ 0.56 Fe 3+ 0.16 Zr 0.06 Mn 0.02 ) [Z] (Si 2 , 32 Fe 3+ 0.58 Al 0.10 ) O 12

Crystal structure

Morimotoite crystallizes with cubic symmetry in the space group Ia 3 d (space group no. 230) with 8 formula units per unit cell . The natural mixed crystal from the type locality has the lattice parameter a  = 12.163  Å . Template: room group / 230

The structure is that of garnet . Calcium (Ca 2+ ) occupies the dodecahedral X positions surrounded by 8 oxygen ions, titanium (Ti 4+ ) and iron (Fe 2+ ) in equal parts the octahedral Y position surrounded by 6 oxygen ions and the tetrahedral Z surrounded by 4 oxygen ions Position is exclusively occupied by silicon (Si 4+ ).

So much for the ideal cation distribution in pure morimotoite. Natural Fe-Ti garnets are complex mixed-circle crystals with a significantly more complicated crystal chemistry. There are different ideas about how the individual cations, especially Fe 2+ and aluminum (Al 3+ ) are distributed over the various grid positions.

The morimotoite-rich garnet from the Ice River carbonatite has been investigated by various working groups. This shows what influence the assumptions about the oxidation levels of iron and the intracrystalline distribution of the cations have on the mineral formulas and the names derived from them.

Based on their interpretation of Mössbauer spectra , supported by infrared spectra , Locock and co-workers conclude that Fe 2+ is preferably incorporated in the tetrahedral position and assign aluminum (Al 3+ ) to the octahedral position. Logically, they refer to the garnet as schorlomite:

  • Schorlomite with [Z] Fe 2+ : [X] (Ca 2.866 Mn 0.019 Mg 0.080 Na 0.038 ) [Y] (Ti 4+ 1.058 Fe 3+ 0.631 Al 0.137 Fe 2+ 0.057 Mg 0.055 Zr 0.039 V 3+ 0.014 Mn 0.013 ) [Z] (Si 2.348 Fe 3+ 0.339 Fe 2+ 0.311 4 [H] 0.005 ) O 12

Anato calculates the cation distribution only on the basis of the composition with the calculation scheme for grenades presented by Locock 2008 and thus comes to an exclusive incorporation of Fe 2+ on the octahedron position and Al 3+ on the tetrahedron position. Just as logically they refer to the same garnet as morimotoite:

  • Morimotoite with [Y] Fe 2+ : [X] (Ca 2.91 Mn 0.03 Mg 2+ 0.05 ) [Y] (Ti 4+ 1.09 Fe 3+ 0.46 Fe 2+ 0.37 Mg 0.08 ) [Z] (Si 2.36 Fe 3+ 0.51 Al 0.14 ) O 12

Education and Locations

Garnets rich in morimotoite form either igneously in alkaline-rich, basic to ultra-basic igneous rocks under reducing conditions or when such magmas interact with calcareous silicate rocks such as B. Marl . There are very few confirmed sites worldwide.

Skarne and contaminated magmas

The type locality is a skarn at the contact with the quartz - monzonite - passages from the Fuka mine at Bitchu-cho near Takahashi , Okayama Prefecture in the region Chūgoku on Honshu in Japan . Morimotoite crystallized here from magmas that intruded the surrounding limestone and changed its composition (contaminated magmas). Accompanying minerals are clinopyroxene , feldspar , Vesuvian , andradite-grossular mixed crystals, wollastonite , prehnite and, in small amounts, perovskite , apatite , titanite , biotite , epidote , hematite , zirconium , baddeleyite and calcirtite .

Carbonatite complexes

In Melteigit the Ice River Alkaline Complex in British Columbia , Canada occurs morimotoitreicher garnet with diopside, calcite , nepheline , apatite and pyrite on.

In the silicocarbonatite of the Afrikanda Complex on the Kola Peninsula in Murmansk Oblast of the Northwestern Federal District , Russia , garnet rich in morimotoite occurs together with magnesiohastingsite , calcite, magnetite , perovskite, titanite, clinochlor and various Zr minerals.

In the small Sung Valley carbonatite complex in the East Khasi Hills in Meghalaya , India , garnet rich in morimotoite occurs in ijolite along with nepheline , clinopyroxene, apatite, titanite and magnetite.

Other confirmed sites are the carbonatite complexes of Iivaara (Ijola) near Kuusamo in northeast Finland and Rusinga Island in Kenya . Infrared spectra of the Rusinga grenade indicate that Fe 2+ is incorporated in the tetrahedron position, as is the case with the grenades of the Ice River alkali complex. The classification of this grenade as morimotoite is therefore questionable.

See also

Web links

Individual evidence

  1. ^ A b HG Huckenholz: Synthesis and stability of Ti-andradite . In: American Journal of Science . 267-A, 1969, pp. 209–232 ( yale.edu [PDF; 941 kB ; accessed on January 17, 2018]).
  2. ↑ a b c d e f g h i j k l m n Chiyoko Henmi, Isao Kusachi and Kitinosuke Henmi: Morimotoite, Ca3TiFe 2+ Si3012, a new titanian garnet from Fuka, Okayama Prefecture, Japan . In: Mineralogical Magazine . tape 59 , 1995, pp. 115–120 ( rruff.info [PDF; 163 kB ; accessed on January 14, 2018]).
  3. ↑ a b c Sytle M. Antao: Crystal structure of morimotoite from Ice River, Canada . In: Powder Diffraction . tape 29 , no. 4 , 2014, p. 325-330 ( cambridge.org [PDF; 287 kB ; accessed on January 14, 2018]).
  4. ↑ a b List of localities for morimotoite in the Mineralienatlas and Mindat
  5. ↑ KT Fehr, & G. Amthauer: Comment on 'Morimotoite, Ca3TiFe2 + Si3O12, new titanian garnet from Fuka, Okayama Prefecture, Japan' by Henmi et al. (1995). In: Mineralogical Magazine . tape 60 , no. 5 , 1996, pp. 842–845 ( minersoc.org [PDF; 287 kB ; accessed on January 15, 2018]).
  6. ↑ Irene T. Rass: Morimotoite, a new titanian garnet? - discussion . In: Mineralogical Magazine . tape 61 , no. 5 , 1997, pp. 728-730 ( rruff.info [PDF; 287 kB ; accessed on January 15, 2018]).
  7. ↑ Tatsuya Kageyama, Chiyoko Henmi: Synthesis of morimotoite . In: 日本 鉱 物 学会 年 会 講演 要旨 集 . tape 2002 , 2002, pp. 122-122 , doi : 10.14824 / kobutsu.2002.0.122.0 .
  8. ↑ a b c d e Edward S. Grew, Andrew J. Locock, Stuart J. Mills, Irina O. Galuskina, Evgeny V. Galuskin and Ulf Hålenius: IMA Report - Nomenclature of the garnet supergroup . In: American Mineralogist . tape 98 , 2013, p. 785–811 ( main.jp [PDF; 2,3 MB ; accessed on July 8, 2017]).
  9. ^ A b c Andrew Locock, Robert W. Luth, Ronald G. Cavell, Dorian GW Smith, M. John, M. Duke: Spectroscopy of the cation distribution in the schorlomite species of garnet . In: American Mineralogist . tape 80 , 1995, pp. 27–38 ( minsocam.org [PDF; 1.7 MB ; accessed on December 11, 2017]).
  10. ↑ AR Chakhmouradian, CA McCammon: Schorlomite: a discussion of the crystal chemistry, formula, and inter-species boundaries . In: Physics and Chemistry of Minerals . tape 32 , 2005, pp. 277–289 ( researchgate.net [PDF; 478 kB ; accessed on January 22, 2018]).
  11. ↑ Leone Melluso, Rajesh K. Srivastava, Vincenza Guarino, Alberto Zanetti, Anup K. Sinha: MINERAL COMPOSITIONS AND PETROGENETIC EVOLUTION OF THE ULTRAMAFIC-ALKALINE - CARBONATITIC COMPLEX OF SUNG VALLEY, NORTHEASTERN INDIA . In: The Canadian Mineralogist . tape 48 , 2010, p. 205–229 ( res.in [PDF; 3.1 MB ; accessed on January 21, 2018]).
  12. ↑ Howie, RA, AR Wooley, JH Scoon, RC Tyler & JN Walsh: The role of titanium and the effect of TiO2 on the cell size, refractive index, and specific gravity in the andradite-melanite-schorlomite series . In: Mineralogical Magazine . tape 36 , 1968, pp. 775–790 ( rruff.info [PDF; 2,3 MB ; accessed on January 21, 2018]).
  13. ^ Georg Amthauer and George R. Rossman: The hydrous component in andradite garnet . In: American Mineralogist . tape 83 , 1998, pp. 835–840 ( minsocam.org [PDF; 78 kB ; accessed on January 21, 2018]).