# Dollaseite-(Ce)

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, CaREE$^{3+}$ Mg $_2$AlSi $_3$O $_{11}$,(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 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). 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 $_2$Si $_3$O $_{12}$(OH).

## Composition

With a general formula for the epidote group of A$_2$M$_3$Si$_3$O$_{13}$H dollaseite-(Ce) received its formula of CaREE$^{3+}$ Mg $_2$AlSi $_3$O $_{11}$,(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$_{.06}$)(Mg$_{1.81}$ Fe$_{.25}$)Al$_{.97}$ Si$_{3.0}$(OH)$_{1.25}$ F$_{.88}$ O$_{10.99}$ from which the standard formula is then derived after applying Levinson’s rules for renaming REE minerals.

## 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)$^{3+}$ + O$^{2−}$ = Mg$^{2+}$ + F$^−$ . The average epidote-group mineral’s structure has chains of edge-sharing octahedral such as Al$^{3+}$, Fe$^{3+}$, Mn$^{3+}$. The cavities that are formed by the octahedral chains that are occupied by A(1) and A(2) cations are occupied by Ca$^{2+}$ and its REE, Ce$^{3+}$. Like the rest of the members of the epidote group, the aluminium octahedra in dollaseite-(Ce) share edges, forming endless chains. Similar to the other epidote-group minerals, dollaseite-(Ce) is monoclinic and thus is part of the space group P2$_1$/m. 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. 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). Another dollaseite-(Ce) specimen with composition slightly similar to that of dissakisite-(Ce) can be found in rock composed of fluorite and fluorian phlogopite.