Dollaseite-(Ce)

Dollaseite-(Ce) is a sorosilicate end-member epidote mineral and was discovered by Gajer (1927) in the Ostanmossa mine, Norberg district, Sweden, the mineral wrongly named "magnesium orthite" after Gajer himself was renamed after the structure refinement made to the mineral by Peacor and Dunn (1988) led to the proper classification. The name dollaseite-(Ce) was chosen in honor of Wayne Dollase, who performed broad research on the epidote-group 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). Dollaseite-(Ce), although not very well known is part of a broad group of minerals, the epidote group 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+
Mg
2AlSi
3O
11,(OH)F, dollaseite-(Ce) can be partially identified by its content of the rare earth element cerium.

Dollaseite-(Ce)
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
LusterVitreous
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]

Composition

With a general formula for the epidote group of A2M3Si3O13H[5] dollaseite-(Ce) received its formula of CaREE3+
Mg
2AlSi
3O
11,(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.[6] Before any of these technological analyses were performed on dollaseite-(Ce) specimens, mistakes were made on the composition of the mineral. After further analysis were made it was determined that dollaseite-(Ce) was not the magnesium analogue of allanite as originally believed by Gaijer and was renamed. Geijer however, was not the only one to have had reported minerals with compositions that would be similar to a magnesium analogue of allanite. 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)MgAl
2Si
3O
12(OH).[7]

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−
.[8] The original confusion of the composition of this mineral when it was first discovered can be due to this exchange of cations and anions, an exchange which is not always complete and thus can resemble dissakisite-(Ce), the Mg dominant allanite.[8] The average epidote-group mineral’s structure has chains of edge-sharing octahedral such as Al, Fe3+, Mn3+ which are represented in Figure 1.[9] The cavities that are formed by the octahedral chains that are occupied by A(1) and A(2) cations[9] are occupied by Ca and its REE, Ce3+.[5] Like the rest of the members of the epidote group, the aluminum octahedral in dollaseite-(Ce) share edges, forming endless chains.[10]

Physical properties

Similar to the other epidote-group minerals, dollaseite-(Ce) is monoclinic and thus is part of the space group P 2 1/M.[11] 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. Dollaseite-(Ce) can generally be found in mineralized dolomite-tremolite rocks in the form of dark brown and subhedral crystals

Geologic occurrence

Dollaseite-(Ce) can occur in a number of locations, but the most prominent is at Ostanmossa, Sweden. The type of dollaseite-(Ce) that can generally be found at this location tends to be iron-poor and occurs in tremolite skarn.[7] 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).[7] Another dollaseite-(Ce) specimen with composition slightly similar to that of dissakisite-(Ce) can be found in rock composed of fluorite and fluorian phlogopite.[8]

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. Dollase, W.A. (1971) Refinement of the crystal structures of Epidote, Allanite and Hancockite. American Mineralogist, 56, 447-464
  6. 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
  7. 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
  8. Holtstam, D. and Andersson, U.B. (2007) The REE minerals of the Bastanas-type deposits, South-Central Sweden, The Canadian Mineralogist, 45, 1073-1114
  9. Dollase, W.A. (1969) Refinement and comparison of the structures of Zoisite and Clinozoisite. American Mineralogist, 53, 1882-1898
  10. Dollase, W.A. (1968) Crystal structure and cation ordering of piemontite. American Mineralogist, 54, 710-717
  11. Armbruster, T. (2006) Recommended nomenclature of epidote-group minerals. European Mineralogical Journals, 18 551-567
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