Chemistry:Dollaseite-(Ce)

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Short description: Epidote supergroup, sorosilicate mineral
Dollaseite-(Ce)
Dollaseite.JPG
General
CategorySilicate mineral
Formula
(repeating unit)		CaCeMg2AlSi3O11F(OH)
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/m
Identification
Formula mass551.04 g/mol
ColorBrown
Crystal habitMassive, somewhat prismatic, equant when subhedral
TwinningOn [100]
CleavageIndistinct, epidote has one good cleavage on one side
FractureFlat regular to uneven
Mohs scale hardness6.5–7
|re|er}}Vitreous
Streaklight brown
DiaphaneityTranslucent
Specific gravity3.9
Optical propertiesBiaxial (+)
Refractive indexnα = 1.715 nβ = 1.718 nγ = 1.733
Birefringenceδ = 0.018
PleochroismStrong
References[1][2][3][4]

Dollaseite-(Ce) is a sorosilicate end-member epidote rare-earth mineral which was discovered by Per Geijer (1927) in the Ostanmossa mine (Östanmossa gruva), Norberg district, Sweden. Dollaseite-(Ce), although not very well known, is part of a broad epidote group of minerals which are primarily silicates, the most abundant type of minerals on earth. Dollaseite-(Ce) forms as dark-brown subhedral crystals primarily in Swedish mines. With the ideal chemical formula, CaREE3+Mg2AlSi3O11,(OH)F, dollaseite-(Ce) can be partially identified by its content of the rare earth element cerium.

History

The mineral provisionally named "magnesium orthite" by Geijer himself[6] was renamed after structural refinement by Peacor and Dunn in 1988 led to its proper classification. The name dollaseite-(Ce) was chosen in honor of Wayne Dollase, who performed broad research on epidote minerals. The original confusion of the mineral's composition was because of a complex atomic composition where an exchange of cations leads to a form of dollaseite-(Ce) that at first glimpse resembles an Mg analogue of allanite, now known as dissakisite-(Ce).[7] Enami and Zang, Meyer, Hanson and Pearce also reported minerals that resembled the composition of a Mg analogue of allanite but none had sufficient data, or exact composition to be declared as a Mg analogue of allanite. It was not until 1991 that Edward S. Grew established dissakisite as the actual Mg-dominant allanite mineral with formula Ca(Ce,La)MgAl2Si3O12(OH).[8]

Composition

With a general formula for the epidote group of A2M3Si3O13H[9] dollaseite-(Ce) received its formula of CaREE3+Mg2AlSi3O11,(OH)F based on data from electron microprobe analytical procedures. The results give the empirical formula (Ca.91 Ce.45 La.20 Nd.20 Pr.09 Sm.08 Gd.06)(Mg1.81 Fe.25)Al.97 Si3.0(OH)1.25 F.88 O10.99 from which the standard formula is then derived after applying Levinson’s rules for renaming REE minerals.[10]

Structure

The atomic structure of dollaseite-(Ce) can be somewhat complex at times due to the charge-coupled substitution involving both cations and anions: (Fe,Al)3+ + O2− = Mg2+ + F−.[7] The average epidote-group mineral’s structure has chains of edge-sharing octahedral such as Al3+, Fe3+, Mn3+.[11] The cavities that are formed by the octahedral chains that are occupied by A(1) and A(2) cations[12] are occupied by Ca2+ and its REE, Ce3+.[9] Like the rest of the members of the epidote group, the aluminium octahedra in dollaseite-(Ce) share edges, forming endless chains.[13] Similar to the other epidote-group minerals, dollaseite-(Ce) is monoclinic and thus is part of the space group P21/m.[14] Peacor and Dunn (1988) refined the lattice parameters of dollaseite-(Ce) and concluded that the parameters were a=8.934(18) Å, b=5.721(7) Å, c=10.176(22) Å.

Geologic occurrence

Dollaseite-(Ce) can generally be found in mineralized dolomite-tremolite rocks in the form of dark brown and subhedral crystals. Dollaseite-(Ce) can occur in a number of locations, but the most prominent is at Östanmossa, Sweden. The type of dollaseite-(Ce) that can generally be found at this location tends to be iron-poor and occurs in tremolite skarn.[8] The fact that dollaseite-(Ce) can be prominently found in Sweden comes as no surprise since Sweden has been known to host many REE epidote-group minerals such as dissakisite-(Ce) and allanite-(Ce).[8] Another dollaseite-(Ce) specimen with composition slightly similar to that of dissakisite-(Ce) can be found in rock composed of fluorite and fluorian phlogopite.[7]

See also

References

  1. ↑ http://www.minsocam.org/ammin/AM73/AM73_838.pdf, Peacor, D.R. and Dunn, P.J., 1988, Dollaseite-(Ce)(magnesium orthite redefined): structure refinement and implications for F + M 2+ substitution in epidote-group minerals. American Mineralogist: 73: 838–842
  2. ↑ http://rruff.geo.arizona.edu/doclib/hom/dollaseitece.pdf Handbook of Mineralogy
  3. ↑ http://www.mindat.org/min-1303.html Mindat
  4. ↑ http://www.webmineral.com/data/Dollaseite-(Ce).shtml Webmineral
  5. ↑ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine 85 (3): 291–320. doi:10.1180/mgm.2021.43. Bibcode: 2021MinM...85..291W. 
  6. ↑ Geijer, P.; Bygdén, A.O.B. (1927). "Some mineral associations from the Norberg district". Avhandlingar och Uppsatser (Sveriges Geologiska Undersökning) C 343. 
  7. ↑ 7.0 7.1 7.2 Holtstam, D. and Andersson, U.B. (2007) The REE minerals of the Bastanas-type deposits, South-Central Sweden, The Canadian Mineralogist, 45, 1073–1114
  8. ↑ 8.0 8.1 8.2 Grew, E.S., Essene, E.J., Peacor D.R., Su, S. and Asami, M. (1991) Dissakisite-(Ce), a new member of the epidote group and the Mg analogue of allanite-(Ce), from Antarctica, American Mineralogist, 76, 1990–1997
  9. ↑ 9.0 9.1 Dollase, W.A. (1971) Refinement of the crystal structures of Epidote, Allanite and Hancockite. American Mineralogist, 56, 447–464
  10. ↑ Peacor, D.R. and Dunn P.J. (1988) Dollaseite-(Ce)(magnesium orthite redefined): Structure refinement and implications for F + M2+ substitutions in epidote-group minerals, American Mineralogist, 73, 838–842
  11. ↑ Dollase, W. A. (1968). "Refinement and comparison of the structures of zoisite and clinozoisite" (in en). American Mineralogist 53 (11–12): 1882–1898. ISSN 0003-004X. https://pubs.geoscienceworld.org/msa/ammin/article-abstract/53/11-12/1882/542541/Refinement-and-comparison-of-the-structures-of. 
  12. ↑ Dollase, W.A. (1969) Refinement and comparison of the structures of Zoisite and Clinozoisite. American Mineralogist, 53, 1882–1898
  13. ↑ Dollase, W.A. (1968) Crystal structure and cation ordering of piemontite. American Mineralogist, 54, 710–717
  14. ↑ Armbruster, T. (2006) Recommended nomenclature of epidote-group minerals. European Mineralogical Journals, 18 551–567



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