REE fluorcarbonates group are also known as the bastnäsite series (Donnay & Donnay, 1953). The group consists of four minerals: bastnäsite (REEFCO3), synchysite (REEFCO3 · CaCO3), parisite (2REEFCO3 · CaCO3), and röntgenite (3REEFCO3 · 2CaCO3).
Bastnäsite accounts for approximately 90% of the world’s REE production; synchysite occurs subordinately and is associated with bastnäsite. In almost all the cases where bastnäsite is exploited, parisite and röntgenite are comparatively rare.
The general mineral formula for the group is nXYCO3 · mCaCO3, where:
X = LREE,
Y = (F, OH)
m = 0 (bastnäsite) or 1 (synchysite, parisite, röntgenite),
n = 1 (bastnäsite, synchysite), 2 (parisite) or 3 (röntgenite)
Synchysite displays monoclinic symmetry, whereas other species of the bastnäsite group show trigonal or hexagonal symmetry.
The bastnäsite minerals group present two common features:
the structure can be broadly described as the stacking of three types of layers (CeF, CO3 and Ca) along the c axis;
the Ce/F ratios in all the phases are 1, indicating that this feature is common in all minerals of the group.
According to Donnay & Donnay (1953), in most cases, fluorcarbonates are polycrystals with syntaxial intergrowth of two species in contact along an irregular surface or along repeated parallel planes (0001). All pairs have been observed, except the bastnäsite-synchysite pair.
Van Landuyt & Amelinckx (1975) claim, that the syntactic intergrowths can be described as mixtures of bastnäsite and synchysite. The authors considered bastnäsite-(Ce) and synchysite-(Ce) as two end-members and that parisite and röntgenite are ordered mixtures of bastnäsite (B) and synchysite (S) in single layers stacked along the c crystallographic axis direction. Parisite can be considered a BS stacking and röntgenite a BS2 stacking (Manfredi et al. 2013).
Atomic arrangement of bastnäsite-(Ce), synchysite-(Ce) and parisite-(Ce) projected on (010). Triangles represent (CO3) groups, and O atoms lie at the apices of the triangles. Circles from the largest to the smallest represent F, Ce, and Ca atoms, respectively. The unit cells are outlined.
Additional REE minerals that commonly occur in fluorocarbonate-bearing REE deposits include monazite {REEPO4}, allanite {(Ca,REE)2(Al,Fe,Mg)3Si3012(OH)}, ancylite {REESr(CO3(OH)·H2O}, burbankite {(REE,Na,Ca,Sr,Ba)6(CO3)5}, calkinsite {REE2(CO3)3·4H2O} , lanthanite {REE2(CO3)3·8H2O}, and fluocerite {REEF3}.
Bastnäsite Group minerals
The name came from the type locality at the Bastnäs mines, Riddarhyttan, Skinnskatteberg, Västmanland, Sweden. The most common member of this group is bastnäsite-(Ce). F-enriched species in this group can form in an environment relatively low in F content, whereas OH-species are rare and occur only in low-temperature environments essentially devoid of F (Hsu, 1992).
Pseudomorph of yellowish bastnäsite-(Y) YCO3F after gagarinite-(Y) NaCaYF6 crystals in quartz SiO2 core of alkaline granitic pegmatite from type locality; Verkhnee Espe Massif, Akzhaylyautas Mts, Tarbagatai Range, Eastern Kazakhstan Province, Kazakhstan; 25 x 13 x 8 mm
Honey-orange hexagonal crystal of bastnäsite-(Ce) CeCO3F with sharp faces and good luster associated with a white orthoclase var. adularia KAlSi3O8 crystals; Zagi Mountain, Hameed Abad Kafoor Dheri, Peshawar, Khyber Pakhtunkhwa, Pakistan; crystal: 11 x 8 x 6 mm
Bastnäsite-(Ce) Ce(CO3)F in kaolinite Al2(Si2O5)(OH)4 (darkest) and muscovite (var. sericite) KAl2(AlSi3O10)(OH)2; Miłków, Jelenia Góra District, Lower Silesia (Dolnośląskie), Poland; SEM-BSE image
Pinkish to orange balls of Bastnäsite-(Ce) CeCO3F; La Flèche quarry, Bertrix, Luxembourg Province, Belgium; FOV 12 mm
Yellowish pseudomorph of bastnäsite-(Y) YCO3F after gagarinite-(Y) NaCaYF6 in quartz core of alkaline granitic pegmatite from type locality; Verkhnee Espe Massif, Akzhaylyautas Mts, Tarbagatai Range, Eastern Kazakhstan Province, Kazakhstan; 25 x 13 x 8 mm
Complete hexagonal crystal of bastnäsite-(Ce) CeCO3F with inclusions of black aegirine NaFeSi2O6 and yellowish rutile TiO2; Zagi Mountain, Hameed Abad Kafoor Dheri, Peshawar, Khyber Pakhtunkhwa, Pakistan; crystal: 11 x 8 x 6 mm
Synchysite Group minerals
Named in 1901 by Gustav Flink from the Greek σύγχΰσις “synchys” for “confounding” in allusion to its initially being mistaken for parisite.
A prismatic crystal of synchisite-(Ce) CaCe(CO3)2F, orange; Wanni glacier – Scherbadung area, Kriegalp Valley, Binn Valley, Wallis, Switzerland; crystal 4 mm long
Prismatic elongated orange crystal of synchysite-(Ce) CaCe(CO3)2F on gneiss; Wanni glacier – Scherbadung area, Kriegalp Valley, Binn Valley, Wallis, Switzerland; crystal 4 mm long
Synchisite-(Ce) CaCe(CO3)2F crystal with honey color and evident cleavage on gneiss; Mt. Cervandone area (Scherbadung; Cherbadung), Devero Alp, Baceno, Devero Valley, Antigorio Valley, Ossola Valley, Verbano-Cusio-Ossola Province, Piedmont, Italy; crystal about 3 mm
Short prismatic and striated cristal of synchysite-(Ce) CaCe(CO3)2F with smoky quartz SiO2 on gneiss; Mt. Cervandone area (Scherbadung; Cherbadung), Devero Alp, Baceno, Devero Valley, Antigorio Valley, Ossola Valley, Verbano-Cusio-Ossola Province, Piedmont, Italy; quartz crystal: 7 mm
Yellowish crystal of synchysite-(Ce) CaCe(CO3)2F on gneiss; Wanni glacier – Scherbadung area, Kriegalp Valley, Binn Valley, Wallis, Switzerland; FOV: 15 mm
Orange-brown crystal of prismatic synchisite-(Ce) CaCe(CO3)2F on matrix; Wanni glacier – Scherbadung area, Kriegalp Valley, Binn Valley, Wallis, Switzerland; crystal 4 mm long
Named after J.J. Paris, former Manager of the Muzo emerald mine, Muzo, Columbia (leasee of mine from 1828-1848).
Very rare; can be distinguished from Synchysite-(Ce) only by analytical methods.
Quite common on the market are pseudomorphs composed of earthy microporous muscovite aggregate with very minor admixture of earthy anatase from Mount Malosa, Zomba District, Malawi, sell as parisite (or pseudomoprhs after parisite). Recent investigation shows, that it is probably pseudomorphose after some silicate minerals (beryl, mylarite, cancrinite or something else).
Prismatic, striated pseudomorph after parisite-(Ce) CaCe2(CO3)3F2 with yellow-orange zircons ZrSiO4 and colorless quartz SiO2; Mount Malosa, Zomba District, Malawi; 26 x 21 x 13 mm; (two types of visually similar pseudomorphs are known from this locality a) type formed after REE-fluorcarbonates and always contain residual REEs and b) type formed after unknown hexagonal REE-free silicate and replaced by hydromuscovite with rutile admixture; it is rather imposible to recognize which one is this)
Cluster of lustrous, brown, elongated parisite-(Ce) CaCe2(CO3)3F2 crystals; Snowbird mine, Fish Creek, Alberton, Mineral Co., Montana, USA; 18 x 10 x 6 mm
Nice quality group of slender, deep reddish-brown prismatic parisite-(Ce) CaCe2(CO3)3F2 crystals; Snowbird mine, Fish Creek, Alberton, Mineral Co., Montana, USA; 18 x 10 x 6 mm
Prismatic and striated pale brown pseudomorph after crystal of parisite-(Ce) CaCe2(CO3)3F2 with yellow-orange zircon ZrSiO4 and colorless quartz SiO2; Mount Malosa, Zomba District, Malawi; 26 x 21 x 13 mm; (two types of visually similar pseudomorphs are known from this locality a) type formed after REE-fluorcarbonates and always contain residual REEs and b) type formed after unknown hexagonal REE-free silicate and replaced by hydromuscovite with rutile admixture; it is rather imposible to recognize which one is this)
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