David K. Teertstra

Substitutional mechanisms, compositional trends and the end-member formulae of scapolite

Compositional trends of natural samples of scapolite extend toward the ideal (anhydrous) end-members marialite (Ma) Na4Al3Si9O24Cl and meionite (Me) Ca4Al6Si6O24CO3. Intermediate members do not follow the plagioclase substitution, although Si contents closely correlate with Me% [calculated as 100{Σ(divalent cations)}/4]. The solid solution may be explained by substitutional interactions among the M, T and A sites, as constrained by net charge-balance and maximum site-occupancy requirements of the structural formula M4T12O24A. Two changes in compositional trends located at [(Na3.4Ca0.6)(Al3.6Si8.4)O24]+1 and [(Na1.4Ca2.6)(Al4.7Si7.3)O24]+1.9 divide the series into three portions. Nomenclature consistent with these divisions is marialite (Me 0–15), calcian marialite (15 < Me ≤ 50), sodian meionite (50 < Me < 65) and meionite (Me 65–100). A large number of scapolite occurrences have Si values near 7.3 apfu, corresponding to the 14/m-P42/n symmetry transition and to the stabilization of scapolite in a Cl-poor environment. Trends of Cl with Si or (Na + K) apfu indicate complex substitutional mechanisms, and should not be used to suggest that “mizzonite” or “dipyre” are end-members. Calculation of excess positive charge from measurement of M and T cations indicates that scapolite typically has the maximum content of Cl expected by charge-balance. Anion substitution is complicated by substition of H as hydroxyl or bicarbonate-bisulfate anions. The calculation CO2 = 1−ClFS may apply only to anhydrous scapolites which are rare, otherwise it over-estimates CO2 because it ignores the role of H. CO2 content calculated by charge-balance equations gives better agreement with constraints of the known substitutional mechanism. More H is present than is required by stoichiometry, and molecular H2O may be present in channels along the c-axis.

Silvialite, a new sulfate-dominant member of the scapolite group with an Al-Si composition near the 14/m-P4 2 /n phase transition

Abstract Silvialite, ideally Ca4Al6Si6O24SO4, is tetragonal, I4/m, Z = 2, with a = 12.160(3), c = 7.560(1) Å, V = 1117.9(8) Å3, c:a = 0.6217:1, ω= 1.583, Ɛ= 1.558 (uniaxial negative), Dm = 2.75 g/cm3, Dcalc = 2.769 g/cm3 and H (Mohs) = 5.5. It is transparent and slightly yellow, has a good {100} cleavage, chonchoidal fracture, white streak and a vitreous lustre. It occurs in upper-mantle garnet-granulite xenoliths hosted by olivine nephelinite, from McBride Province, North Queensland, Australia. The empirical formula, derived from electron-microprobe analysis, is (Na1.06Ca2.86)(Al4.87Si7.13)O24 [(SO4)0.57(CO3)0.41]. Crystal-structure refinement shows disordered carbonate and sulfate groups along the fourfold axis. Silvialite is a primary cumulate phase precipitated from alkali basalt at 900−1000°C and 8−12 kbar under high fSO₂ and fO₂ . The name silvialite, currently used in literature to describe the sulfate analogue of meionite, was suggested by Brauns (1914).

Rubidium- and cesium-dominant micas in granitic pegmatites

Abstract The mode of occurrence and chemical composition of five types of micas with Rb- or Cs-dominant populations of interlayer cations, collected from the Red Cross Lake rare-element pegmatites in north-central Manitoba, are described here. All five micas are candidates for new mineral species but crystal-structural data and Li contents could not be determined to date because of extremely small particle size, restricted to the margins of strongly zoned microcrystals. Based on electron-microprobe analyses, on Li contents estimated from Li/F (at.) = 1.0, and on bulk analysis of ferromagnesian micas for FeO and Fe2O3, the micas correspond to Rb- and Cs-dominant polylithionite (with representative interlayer populations of Rb0.82K0.12Cs0.07 and Cs0.74Rb0.12K0.08 apfu, respectively), Rb and Cs-dominant magnesian annite (Rb45K0.37Cs0.20 and Cs0.67Rb0.20K0.12 apfu, respectively), and Cs-dominant ferroan phlogopite (Cs0.92Rb0.04K0.02 apfu).

Compositional and textural evolution of pollucite in pegmatites of the moldanubicum

SummaryPollucite is found in five pegmatites in the Moldanubicum, all of them within the Czech Republic: Susice III (elbaite subtype) and Nová Ves near Český Krumlov (petalite subtype) in southern Bohemia, Jeclov I and Puklice II (lepidolite subtypes) and Věžná I (beryl-columbite subtype) in western Moravia. Pollucite displays a broad range of primary compositions at these localities, with CRK = 100(Cs + Rb + K)/Σ cations variable from 72 to 85, and Si/Al from 2.43 to 2.60. Exsolution to Cs,Al-rich and Na,Si-rich components is not widespread: the maximum CRK is 92 at Si/Al of 2.30. However, Na-exchanged compositions and local veinlets of analcime are much more common; they reach CRK of 5 and 7, respectively. Three occurrences are comparable to the usual pollucite-bearing assemblages, except for the geochemically rather primitive pegmatites at Susice and Věžná. At the latter locality, an extremely steep fractionation gradient generates minor amounts of a highly evolved Li-,Cs-,Be-, Ca-,Sr-,Ba-,Mn-,Bi-, REE-,Zr-,Sn-,Th-,W-,B-,P-,F-rich assemblage, with pollucite closely associated with chabazite and harmotome.ZusammenfassungPollucit ist nur in fünf Pegmatiten des Moldanubikums bekannt, die alle auf dem Gebiet der Tschechischen Republik liegen: Sušice III (Elbait-Subtyp), Novä Ves in der Nähe von Český Krumlov in Süd-Böhmen (Petalit-Subtyp), Jeclov 1 und Puklice II (Lepidolith-Subtyp) und Věžná I (Beryll-Columbit Subtyp) in West-Mähren. Pollucit zeigt ein breites Spektrum an primärer Zusammensetzung mit CRK = 100 (Cs + Rb + K)/Σ Kationen von 85–72, und Si/Al 2.43 bis 2.60. Entmischung von Cs,Al-reichen und Na,Si-reichen Komponenten ist nicht verbreitet; maximal erreicht CRK = 92 bei Si/Al = 2.30. Weitver-breiteter Na-Austausch in Mineralen und lokale Gängchen von Analcim erreichen CRK von 5 bzw. 7. Alle Vorkommen sind vergleichbar mit den üblichen Pollucit-führenden Vergesellschaftungen ausser dem geochemisch seltenen primitiven Pegmatit von Věžná. Ein extrem steiler Fraktionierungsgradient erzeugt hier eine hochentwickelte Li-,Cs-, Be-,Ca-,Sr-,Ba-,Mn-,Bi-,REE-,Zr-,Sn-,Th-,W-,B-,P-,F-reiche Mineralisation mit Pollucit in enger Vergesellschaftung mit Chabasit und Harmotom.

Scapolite cell-parameter trends along the solid-solution series

Abstract Scapolites are tetragonal framework aluminosilicates with the general formula M4T12O24A,the major constituents being M = Na + Ca, T = Si + Al, and A = Cl + CO3 + S-bearing anions, along with minor Fe, Sr, Ba, K, and H. Substitutions among the M, T, and A sites are independent and constrained only by net charge balance and maximum site occupancy. The cell parameters vary primarily with framework chemistry and Al-Si order and are not a strong function of the M- or A-site substitutions. End-member marialite has a = 12.06(1) and c = 7.551(5) Å, and meionite has a = 12.20(1) and c = 7.556(5) Å. Changes in cell- parameter trends in the series occur at Al3.6Si8.4O24 and Al4.7Si7.3O24 and are correlated with the locations of P42/n-I4/m phase transitions. The scapolite group can be divided into three isomorphous series: 9.0 > Si > 8.4, 8.4 > Si > 7.3, and 7.3 > Si > 6.0, each with its own patterns of Al-Si order. Because scapolites with the same Si content have variable cell parameters, there might be variable degrees of Al-Si order or a minor, secondary influence of the M- or A-site substituents.

Subsolidus rubidium-dominant feldspar from the Morrua Pegmatite, Mozambique; paragenesis and composition

Abstract At the Morrua pegmatite, Mozambique, alkali feldspar has replaced pollucite under low-temperature (250−150°C) hydrothermal conditions. Fluids invading a fracture system in pollucite formed round granular aggregates of (K-Rb)-feldspar in three stages: (1) a compositionally heterogeneous core of the feldspar cluster (+cookeite±apatite) with 7−20 mol.% RbAlSi3O8, grading outward into a Rb- dominant feldspar with 66 mol.% RbAlSi3O8 (20 wt.% Rb2O); (2) an intermediate layer of non-porous, inclusion-free, end-member K-feldspar; (3) an outer layer of porous end-member K-feldspar. Feldspars of all three stages seem to be monoclinic and disordered, with metastable sanidine structure. Zoning in K/Rb, preserved on a fine scale, was formed during growth at a temperature too low for subsequent alkali-cation diffusion or (Al,Si)-ordering.

Pollucite in Sweden: Occurrences, crystal chemistry, petrology and subsolidus history

Abstract Pollucite is known from three highly fractionated peraluminous pegmatites in the Swedish Svecokarelides at Varutrask, Uto, and Akerberg. At all three localities, it was penetrated by several generations of feldspar‐, mica‐ and quartz‐bearing veins, replaced by (K, Rb)‐feldspar, and subsequently argillized. The primary homogeneous pollucite has a uniform composition close to Poll80Anal20 with Si/Al from 2.4 to 2.5. The pollucite underwent breakdown into Cs.Al‐ and Na,Si‐en‐riched blebs, crosscut by several generations of Cs,Al‐rich pollucite veinlets (± quartz), locally flanked by diffuse Na,Si‐enrichment. Late‐stage hydrothermal fluids generated (K,Rb)‐feldspar and forced analcimization by cation exchange along fractures and vein surfaces. Teertstra, D.K., Cerny, P., Langhof, J., Smeds, S.‐A. & Grensman, F., 1996: Pollucite in Sweden: occurrences, crystal chemistry, petrology and subsolidus history. GFF, Vol. 118 (Pt. 3, September), pp. 141–149. Stockholm. ISSN 1103–5897.

The compositional evolution of pollucite from African granitic pegmatites

Occurrences of pollucite are known from nine highly fractionated granitic pegmatites in Zimbabwe, Namibia and Mozambique. The pollucite is typically associated with spodumene, petalite, amblygonite, quartz and feldspar. The primary pollucite has SiAl (at.) ratios of 2.47 to 2.57 and CRK = 100(Cs + Rb + K)/(Cs +, Rb + K + Na + Ca + Li) of 73.5 to 87.7. Extensive re-equilibration generated blebs and veins of (Cs, Al) rich pollucite with compositions trending towards end-member pollucite CsAIS2O6 with SiAl = 2.00 and CRK = 100. Characteristic sequences of alteration by coarse veins of lepidolite, quartz and feldspars, fine veins of muscovite and spodumene, and replacement by feldspar and clay minerals are well-developed in African pollucite. The crystal chemistry, compositional ranges, character of subsolidus re-equilibration, and veining, alteration plus metasomatic features correspond to those observed at other pollucite localities, documenting general uniformity of pollucite crystallisation and subsolidus reactions in granitic pegmatites. Veining, replacement and cryptic processes such as analcimisation may significantly reduce Cs grades in apparently monomineralic pollucite ore, but late cation exchange to analcime is limited at the localities axamined. This suggests that leaching of Cs from undisturbed capsules of ceramic radwaste, which utilise a pollucite phase for containment of radioactive Cs isotopes, will be negligible in a Na poor environment.

Geochemistry and petrology of late K-and Rb-feldspars in the Rubellite pegmatite, Lilypad Lakes, NW Ontario

SummaryThree types of late, subsolidus (K-Rb)-feldspars occur in the spodumene-subtype, complex-type, pollucite-bearing Rubellite rare-element pegmatite at Lilypad Lakes, near Fort Hope in northwestern Ontario: (1) Exsolution followed by hydrothermal reconstitution generated diffuse bands, patches and veinlets of Rb-enriched (< 43 mol. % Rbf) and Rb-depleted triclinic K-feldspar in black blocky microcline; (2) metasomatism of pollucite produced aggregates of adularia which range from 14 to 73 mol.% Rbf, rimmed by pure 100 mol.% Kfs feldspar, and are most likely highly disordered; (3) blocky K-feldspar was replaced by salmon-coloured, fine-grained, Na-poor and Rb-free maximum microcline with a composition close to 100 mol.% Kfs, associated with calcite and fluorite. After initial solid-state unmixing in blocky microcline, the reconstituted and metasomatic (K-Rb)-feldspars of the first two types could have formed simultaneously, under the influence of the same aqueous fluid. The exsolved and reconstituted Rb-rich feldspar inherited the (Al,Si)-ordered framework of the blocky precursor, whereas metasomatic neocrystallization generated metastable disordered (K-Rb)-adularia. In contrast, a different, F- and CO3-rich fluid produced the metasomatic salmon-coloured K-feldspar, and promoted its perfect (Al,Si)-order.ZusammenfassungIn dem Pollucit and seltene Elemente führenden Rubellit-Pegmatit (Spodumen-Subtyp, komplexer Typ) bei Lilypad Lakes, nahe Fort Hope in Nordwest-Ontario kommen drei Typen von späten (K,Rb)-Feldspäten vor: (1) Entmischung gefolgt von hydrothermaler Rekonstitution schuf diffuse Bänder, Flecken and Gdngchen von an Rb angereicherten (< 43 Mol-% Rbf) and an Rb verarmtem triklinem K-Feldspat in schwarzem, blockigem Mikroklin; (2) Pollucit-Metasomatismus führte zur Bildung von Adular-Aggregaten mit von 14 bis 73 Mol-% Rbf, der von reinem 100 Mol-% Kfs gesäumt and höchst-wahrscheinlich stark ungeordnet ist; (3) blockiger K-Feldspat wurde durch lachsfarbigen, feinkömigen, Na-armen and Rb-freien maximalen Mikroklin mit einer Zusammensetzung nahe 100 Mol-% Kfs ersetzt, der mit Calcit and Fluorit vergesellschaftet ist. Nach der ursprünglichen Festkörperentmischung in blockigen Mikroklin konnten sich die neugebildeten und metasomatischen (K,Rb)-Feldspäte der beiden ersten Typen gleichzeitig and unter dem Einfluß des gleichen wäßrigen Fluides gebildet haben. Die entmischten and neugebildeten Rb-reichen Feläspdte erbten die (Al,Si)-Ordnung des Gerüstes des blockigen Vorläufers, während die metasomatische Neokristallisation metastabil ungeordnete (K,Rb)-Adulare bildete. Im Gegensatz dazu bildete ein F- und CO3-reiches Fluid den metasomatischen, lachsfarbigen K-Feldspat und verursachte seine vollständige (Al,Si)-Ordnung.

The informal discreditation of “doranite” as the magnesium analog of analcime

Abstract Zeolites that fill vugs in basalt from the type locality of “doranite” near Carrickfergus, UK, were reexamined with the hope of establishing the magnesium analog of analcime as a valid species. However, contents of Mg in the zeolites as measured by electron microprobe analysis were not above the limits of detection (0.01 wt% MgO). X-ray powder diffraction and optical microscopy show a vug assemblage of thomsonite + natrolite + analcime + Mg-rich clay minerals.