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The Angolan alkaline-carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals; whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, wöhlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected. [ABSTRACT FROM AUTHOR]

Here we report a nomenclature and classification for the seidozerite-supergroup minerals. The TS (Titanium-Silicate) block is the main structural unit in all seidozerite-supergroup structures; it consists of a central O (O = Octahedral) sheet and two adjacent H (H = Heteropolyhedral) sheets where Si2O7 groups occur in the H sheets. The TS block is characterized by a planar minimal cell based on translation vectors, t1 and t2, the lengths of these vectors are t1 ≈ 5.5 and t2 ≈ 7 Å, and t1 ^ t2 is close to 90°. The forty-five minerals of the sedozerite supergroup are divided into four groups based on the content of Ti and topology and stereochemistry of the TS block: in rinkite, bafertisite, lamprophyllite and murmanite groups, Ti (+ Nb + Zr + Fe3+ +Mg + Mn) = 1, 2, 3 and 4 apfu (atoms per formula unit), respectively. All TS-block structures consist either solely of TS blocks or of two types of block: the TS block and an I (Intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, H2O groups and oxyanions (PO4)3-, (SO4)2- and (CO3)2-.The general formula of the TS block is as follows A2P B2P M2H M4O (Si2O7)2X4+n, where M2H and M4O = cations of the H and O sheets; MH = Ti, Nb, Zr, Y, Mn, Ca + REE, Ca;MO = Ti, Zr, Nb, Fe3+, Fe2+, Mg, Mn, Zn, Ca, Na; AP and BP = cations at the peripheral (P) sites = Na, Ca + REE, Ca, Zn, Ba, Sr, K; X = anions = O, OH, F, H2O; X4+n=X4O +XnP, n = 0, 1, 1.5, 2, 4; XP = XMP and XAP = apical anions of MH and AP cations at the periphery of the TS block. [ABSTRACT FROM AUTHOR]

In the Ilímaussaq alkaline complex, minerals from the rinkite–nacareniobsite-(Ce) solid solution series have been found in pulaskite pegmatite, sodalite foyaite, naujaite and naujaite pegmatite from the roof sequence, and in marginal pegmatite, kakortokite and lujavrite from the floor sequence. The electron microprobe analyses embrace almost the full extension of the solid solution series and confirm its continuity. The solid solution series shows similar compositional variations in the roof and floor sequences: Rinkite members of the series are found in the less evolved rocks in the two sequences, whereas nacareniobsite-Ce members occur in the most evolved rocks and pegmatites in the two sequences. The REE (+Y) content varies from 0.83 atoms per formula unit (apfu) in rinkite from pulaskite pegmatite to 1.31 apfu in nacareniobsite-(Ce) from naujaite pegmatite. The main substitution mechanisms in the solid solution series investigated in this work are 2Ca2+ = Na+ + REE3+ and Ti4+ + Ca2+ = Nb5+ + Na+. The increased contents of Nb5+ and REE3+ are only to a minor degree compensated through the F1– = O2– substitution. The chondrite normalised REE patterns of the minerals develop in a similar way in the two sequences, showing relative La-enrichment and Y-depletion from the less to the most evolved rocks. Hainite has not previously been found in the Ilímaussaq complex. It was here identified in a pulaskite pegmatite sample by a combination of X-ray diffraction giving the unit cell dimensions a = 9.5923(7) Å, b = 7.3505(5) Å, c = 5.7023(4) Å, α = 89.958(2)°, β = 100.260(1)°, γ = 101.100(2)°, and X-ray powder pattern and electron microprobe data giving the empirical formula (Ca1.62 Zr0.16Y 0.22) (Na0.87Ca1.11) (Ca 1.65 REE0.35)Na(Ti0.81Nb0.09Fe0.08 Zr0.02)(Si2O7)2O0.99F2.96. Based on published and the present data it is documented that minerals from the hainite-götzenite solid solution series show a compositional variation between the ideal end members (Y,REE,Zr)Na2Ca4Ti(Si2O7)2OF3 and NaCa6Ti(Si2O7)2OF3. [ABSTRACT FROM AUTHOR]

Abstract: Proterozoic peralkaline nepheline syenite gneiss occurring at Sushina Hill (Paschim Banga, India) consists of minor eudialyte together with potassium feldspar, albite, aegirine, and nepheline. Perthites are absent and the precursor syenite is considered to have been a two feldspar sub-solidus eudialyte aegirine nepheline syenite . The major minerals have near-endmember compositions and are not altered. In contrast, eudialyte is partially-to-completely altered to a decomposition assemblage consisting principally of hilairite and members of the pectolite-serandite solid solution series together with minor ceriothorianite, pyrochlore, and perraultite. Rinkite occurs as early-forming prismatic inclusions in eudialyte. The eudialyte occurs as an interstitial late-forming magmatic phase with hydrothermal eudialyte overgrowths. Eudialytes from Sushina are characterized by low-to-negligible Fe and high Mn and Nb contents relative to most eudialyte from other localities. Late magmatic eudialyte has relatively high CaO (12–14wt.%) and low MnO (5.5–7wt.%) relative to the hydrothermal variety (7–8.5wt.% CaO; 9.5–10.5wt.% MnO). The late magmatic Ca–Mn–Nb-rich eudialytes are principally solid solutions between Ce-zirsilite, Mn-taseqite, kentbrooksite, and possibly hydrated eudialyte or aqualite. The Mn–Sr-rich hydrothermal eudialyte is also a solid solution between these end members, although the presence of significant concentrations of Mn at the M(1) site and Sr at the N-site suggests that the mineral could be a new member of the eudialyte group; possibly a Mn analog of taseqite or a Sr analog of manganoeudialyte. The mineralogy of the decomposition assemblage of Sushina eudialyte is unlike those found for other eudialytes and in particular lacks zircon, fergusonite, fluorite and bastnaesite. The deuteric/hydrothermal fluids responsible for the alteration are considered as not buffered by the coexisting silicate assemblage and to be water-rich with very high Na/Cl and Na/F ratios. [Copyright &y& Elsevier]

The average structure, space group Pnnm [subcell: A = 5.666(16), B = 18.844(5), C = 3.728(11) Å, V = 398.0(2) ų, Z = 1], of the new mineral dovyrenite Ca6Zr[Si2O7]2(OH)4 has been refined from single-crystal X-ray data to R = 7.97%. The modular structure of dovyrenite is build by alternate stacking of Ca-polyhedral layers characteristic of the tobermorite structure and octahedral layers with attached disilicate groups known from the rosenbuschite group of minerals. No indications of ordered polytypes were detected for the potential OD-structure. Either the small crystal size producing only weak diffraction intensities did not allow detecting diffuse diffraction features (or "super-structure" reflections) or the structure is build by disordered stacks of OD layers. Nevertheless, the resolved average structure allowed unraveling the possible order patterns within the rosenbuschite-like octahedral layers. The key for understanding the polytypic character of this structure is the short periodicity of the tobermorite-like Ca polyhedral layer of only 3.73 Å along c, whereas the periodicity of the attached rosenbuschite-like octahedral layer is doubled. In dovyrenite Ca occurs in sixfold-, sevenfold-, and eightfold-coordination. The octahedral Ca site is only half occupied and may reveal additional vacancies, which must be charge balanced by disordered OH-groups replacing O. A corresponding modular structure with the same subunits but different composition and without octahedral vacancies exists for rinkite (Ti,Nb,Al,Zr)(Na,Ca)3(Ca,Ce)4[Si2O7]2(O,F)4, which has hitherto been considered as heterophyllosilicate. [ABSTRACT FROM AUTHOR]