Synchysite-(Ce) from Cinquevalli (Trento, Italy): Stacking Disorder and the Polytypism of (Ca,REE)-Fluorcarbonates

Giancarlo Capitani

doi:10.3390/min10010077

Abstract

Synchysite-(Ce) at Cinquevalli occurs as fine needles intergrown with quartz in quartz-dikes and in association with altered K-feldspar and oxidized chalcopyrite as major constituents. Synchysite-(Ce) [Ca1.00(Ce0.43La0.26Nd0.17Y0.07Pr0.04Sm0.02Gd0.01)Σ=1.00(CO3)2(F0.58(OH)0.42)], shows an overgrowth rim of bastnäsite-(Ce) [(Ce0.34La0.25Nd0.17Pb0.07C a0.06Y0.06Pr0.04S m0.02Gd0.01)Σ=1.00C O3(F0.75(OH)0.25)]. Unit cell refinement of synchysite (C2/c) and bastnäsite (P62c) led to a = 12.272(4), b = 7.100(2), c = 18.640(5) Å, β = 102.71(5)°, and a = 7.085(1), c = 9.746(2) Å, respectively. Polysomatic faults are sporadic, but polytypic disorder is widespread. High resolution transmission electron microscopy images taken along [100] or ⟨130⟩ show an apparent order and the related diffraction patterns are streak-free. Conversely, along [010] or ⟨110⟩, a high density of stacking faults is observed and the related diffraction patterns show hhl rows with h ≠ 3n affected by streaks. No ordered domain larger than a few unit cells was detected. The stacking sequence of (Ca,REE)-fluorcarbonates can be compared with subfamily-B mica polytypes (2M2, 2O and 6H), which are characterized by n·60° (n = odd) rotations. Subfamily-A polytypes (1M, 2M1 and 3T), characterized by n·60° (n = even) rotations, should not be possible. Synchysite, characterized by ±60° rotations, can be likened to the 2M2 polytype.

Highlights

Synchysite [CaREE(CO3 )2 F] represents the Ca-rich end member of the (Ca,rare earth elements (REEs))-fluorcarbonates’polysomatic series
A number of additional polysomes/polytypes have been observed with transmission electron microscopy (TEM) by several authors [1,2,3,4,5,6,7]
The mining located on the southern slope of the Mount Fravort, along the valley carved by the Argenta stream, area is located on the southern slope of the Mount Fravort, along the valley carved by the Argenta at altitude around 1500 m

Summary

Introduction

Synchysite [CaREE(CO3 ) F] represents the Ca-rich end member of the (Ca,REE)-fluorcarbonates’. The REE-rich end member is bastnäsite [REE(CO3 )F]. Both are important ore minerals for rare earth elements (REEs). The chemical formula of individual, intermediate polysomes can be expressed as [REE(CO3 )F]m [CaREE(CO3 ) F]n , where m and n refer to the number of bastnäsite (B). Synchisite (S) basic units of the Bm Sn series. Notable intermediate members of the series are parisite (BS) and röntgenite (BS2 ). A number of additional polysomes/polytypes have been observed with transmission electron microscopy (TEM) by several authors [1,2,3,4,5,6,7].

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Conclusion