Dollaseite-(Ce) | |
---|---|
General | |
Category | Silicate mineral |
Formula (repeating unit) | CaCeMg2AlSi3O11F(OH) |
IMA symbol | Dls-Ce[1] |
Crystal system | Monoclinic |
Crystal class | Prismatic (2/m) (same H-M symbol) |
Space group | P21/m |
Identification | |
Formula mass | 551.04 g/mol |
Color | Brown |
Crystal habit | Massive, somewhat prismatic, equant when subhedral |
Twinning | On [100] |
Cleavage | Indistinct, epidote has one good cleavage on one side |
Fracture | Flat regular to uneven |
Mohs scale hardness | 6.5–7 |
Luster | Vitreous |
Streak | light brown |
Diaphaneity | Translucent |
Specific gravity | 3.9 |
Optical properties | Biaxial (+) |
Refractive index | nα = 1.715 nβ = 1.718 nγ = 1.733 |
Birefringence | δ = 0.018 |
Pleochroism | Strong |
References | [2][3][4][5] |
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+
Mg
2AlSi
3O
11,(OH)F, dollaseite-(Ce) can be partially identified by its content of the rare earth element cerium.
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)MgAl
2Si
3O
12(OH).[8]
With a general formula for the epidote group of A2M3Si3O13H[9] 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.91Ce.45La.20Nd.20Pr.09Sm.08Gd.06)(Mg1.81Fe.25)Al.97Si3.0(OH)1.25F.88O10.99 from which the standard formula is then derived after applying Levinson’s rules for renaming REE minerals.[10]
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.
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]