From structure topology to chemical composition. XII. Titanium silicates: the crystal chemistry of rinkite, Na₂Ca₄REETi(Si₂O₇)₂OF₃

Abstract

Rinkite, ideally Na2Ca4REETi(Si2O7)2OF3, is a common mineral in alkaline and peralkaline rocks. The crystal structures of five rinkite crystals from three alkaline massifs: Ilimaussaq, Greenland; Khibiny, Kola Peninsula, Russia and Mont Saint-Hilaire, Canada, have been refined as two components related by the TWIN matrix (–1 0 0, 0 –1 0, 1 0 1) (Mo-αa radiation). The crystals, a = 7.4132-7.4414, b = 5.6595-5.6816, c = 18.8181-18.9431 Å, β = 101.353-101.424(2)°, V= 776.1-786.7 Å3, space group P21lc, Z = 2, Dcalc = 3.376—3.502 g cm–3, were analysed using an electron microprobe subsequent to collection of the X-ray intensity data. Transmission electron microscopy confirmed the presence of pseudomerohedral twinning in rinkite crystals. The crystal structure of rinkite is a framework of TS (titanium silicate) blocks. The TS block consists of HOH sheets (H-heteropolyhedral, O-octahedral). The TS block in rinkite exhibits linkage and stereochemistry typical for Group I (Ti = 1 a.p.f.u.) of Ti disilicate minerals: two H sheets connect to the O sheet such that two (Si2O7) groups link to the trans edges of a Na polyhedron of the O sheet. The crystal chemistry of rinkite and nacareniobsite-(Ce) is discussed.

NACARENIOBSITE-(Ce) AND BRITHOLITE-(Ce) IN PERALKALINE GRANITES FROM THE MORRO REDONDO COMPLEX, GRACIOSA PROVINCE, SOUTHERN BRAZIL: OCCURRENCE AND COMPOSITIONAL DATA

“…Following Cámara et al (2011), we have fitted the data reported here along with those presented by them REE + Y contents in our nacareniobsite-(Ce) vary between 21.1 and 24.9 wt.% (1.08-1.28 apfu) and are, in general, higher than literature data (cf. Table 3).…”

“…Structural formulae were computed on the basis of 18 (O + F) for nacareniobsite-(Ce) and 26 (O,F) for britholite-(Ce), following Cámara et al (2011) and Oberti et al (2001), respectively.…”

“…Thus, Cámara et al (2011) concluded that mosandrite requires redefinition. Accordingly, the mineral name rinkite will be adopted in this work.…”

“…The authors also indicate that the observed compositional variations among the rinkite-related minerals, such as rinkolite and götzenite, follow the [Ca 2+ ] -2 [REE 3+ Na + ] substitution. Recently,Cámara et al (2011) showed that the available rinkite and nacareniobsite-(Ce) compositions lie on a plane within (Nb,Ta-REE,Y,Th-Na) cationic space, defined by the equation Na = 1.34 + 0.81(Nb + Ta) + 0.67(REE + Y + Th).The main compositional variations for representative rinkite and nacareniobsite-(Ce) as well as the ideal rinkite and nacareniobsite-(Ce) endmembers, are shown inFigures 5A and 5B. Below, the main exchange reac-TABLE 1.…”

Paragenesis and decomposition assemblage of a Mn-rich eudialyte from the Sushina peralkaline nepheline syenite gneiss, Paschim Banga, India

“…Rinkite [ideally Na 2 Ca 4 REETi(Si 2 O 7 ) 2 OF 3 ; Camara et al (2011) ], occurs as long (~5-10 × 50-200 μm) needle-like prismatic crystals ( Fig. 6) with the altered eudialyte aggregates.…”