Enrico Caprilli

Monazite-huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy

Abstract The crystal-chemical relationships occurring within a single grain of monazite-(Ce) from Vetralla, Vico Volcanic Complex, north of Rome, are outlined. The sample is from a miarolitic cavity in a holocrystalline ejectum consisting of K-feldspar plus minor plagioclase, mica and Fe-oxides, collected from a pyroclastic explosive level. The Gandolfi film (Cu-Kα radiation) can be indexed in space group P21 /n with a = 6.816(4); b = 6.976(4); c = 6.471(3) Å ,β = 103.63(3)° :V = 299.0(6) Å3. Electron-probe microanalyses plot within the field of monazite along the huttonite-monazite edge of the huttonite-monazite-brabantite triangle. Despite patchy and irregular zoning, the grain shows a clear enrichment towards pure monazite at the outer rim. A constant Th:Si ratio of 1:1 indicates the existence of a simple solid-solution between huttonite and monazite. The substitution can be written as Th4+ + Si4+ → REE3+ + P5+ without requiring any electrostatic compensation by divalent cations, or by anionic groups. The REE distribution pattern is compatible with that of monazites from syenitic rocks.

Crystal chemistry of ferroan phlogopites from the Albano maar lake (Colli Albani volcano, central Italy)

This work considers micas present both as phenocrysts in granular xenoliths and as large xenocrysts in the phreatomag-matic deposits, which characterize the final volcanic activity of Colli Albani volcanic complex, Roman Comagmatic Region (central Italy). In these micas the Fe/(Fe+Mg) ratio spans from 0.06 to 0.55, thus the micas range from phlogopite to magnesian annite through ferroan phlogopite. Heterovalent octahedral substitutions involve Ti, Al and also Cr but in very limited amounts. In the interlayer, Ba for K substitution occurs up to 0.06 apfu. Crystal structure refinements were determined in space groups C 2/m and C 2/c for the 1M and 2M 1 polytypes, respectively (agreement factor, R obs : 0.021 ≤ R obs ≤ 0.035). The crystal structure of these micas is greatly affected by the annitic substitution. An increase in the annitic component is reflected by a decrease in distortion affecting both individual tetrahedra (tetrahedral angle variance, TAV: 0.49 ≤ TAV ≤ 1.13° 2 ) and the entire tetrahedral ring [6.6 ≤ α ≤ 10.34°]. The interlayer topology is affected as well. A strong correlation exists between Fe and Ti; thus, the same correlations found for Fe apply to Ti also. Another strong correlation exists between heterovalent octahedral substitutions and the (OH)- site deprotonation. In contrast, heterovalent interlayer substitutions are accompanied by substitution of F- for (OH)-. Despite their scattered occurrence in pyroclastics, these micas nicely represent the overall crystal chemical trends of micas, which re-crystallized undisturbed in magma before this reached explosive conditions. One crystal of the dataset considered may represent a phenocryst in a hypabissal body disrupted by one of the many explosive phases of the Alban Hills volcanism.

Crystal chemistry of trioctahedral micas in alkaline and subalkaline volcanic rocks: A case study from Mt. Sassetto (Tolfa district, Latium, central Italy)

Abstract This work provides a crystal-chemical description of trioctahedral micas from volcanic rocks (lavas, tuffs, ignimbrites, and xenoliths) outcropping at Mt. Sassetto (Tolfa district, Tuscan Province, central Italy). Mica crystals vary in composition from ferroan phlogopite to magnesian annite. Heterovalent octahedral substitutions are mainly related to Al3+, Ti4+, and, only in a few samples, to Fe3+. The two main mechanisms regulating Ti inlet into the mica structure are the Ti-oxy [VITi4+VI(Mg,Fe)2+-1(OH)--2O2-2] and Ti-vacancy [VITi4+VI □VI(Mg,Fe)2+-2] substitutions. In these micas, Ti content is the predominant crystal-chemical parameter and significantly affects octahedral and interlayer topology as well. Micas with the highest Ti contents deviate from the expected fractional crystallization trend in the Ti vs. Mg/(Mg + Fetot) diagram, possibly as a consequence of a variation in intensive parameters (T, P, fH₂, fO₂, fH₂O) during crystallization in the magmatic chamber. In micas with significant Fe3+ contents, the layer charge balance is accomplished by the following mechanisms: VIFe2+-3VIFe23+VI□, VIFe2+-1VIFe3+(OH)--1O2-, and VIFe2+-1VIFe3+IVSi-14+IVAl3+. These micas show mean electron counts greater for the octahedral M2 site than for M1, and unit-cell parameters signifcantly smaller than usual for other micas. Atmospheric weathering is unlikely to produce the observed Fe oxidation. The Fe3+-vacancy and Fe-oxy substitutions could represent secondary, re-equilibration effects related to post-eruptive water-rock hydrothermal processes (acid-sulfate alteration).

Crystal-chemistry of zirconolite and calzirtite from Jacupiranga, Sao Paulo (Brazil)

Abstract A specimen of zirconolite, collected from the type locality of the mineral originally described as zirkelite at Jacupiranga, São Paulo, Brazil has been re-examined and its mineral chemistry more completely characterized. All crystals studied are metamict and display very fine lamellar oscillatory zoning (1-3 μm in width) superimposed on a sector zonation. Such zoning, observed in backscattered electron images, is primarily related to differences in the concentration of Th. In comparison with other reported zirconolite samples from a variety of geological occurrences, Jacupiranga zirconolite has higher Ca, Th, (Nb + Ta) and lower Ti and REE, which is consistent with its occurrence in carbonatitic rocks. The compositional variation with respect to an ideal zirconolite is described by two main coupled substitutions: (Th,U)4+ + (Fe,Mg)2+ ⇌ Ca2+ + Ti4+, and (Th,U)4+ + (REE)3+ + (Nb,Ta)5+ + (Fe,Mg)2+ ⇌ (Ca)2+ + 3(Ti)4+. Calzirtite, Ca2Zr5Ti2O16, although intergrown with zirconolite and with identical major components, shows much less compositional variability with only minor amounts of Nb and Ta substituting for Ti. Unlike zirconolite, the REE and actinide elements are not easily accommodated in the calzirtite structure.

Non-metamict betafite from Le Carcarelle (Vico volcanic complex, Italy): occurrence and crystal-structure

Abstract Non-metamict betafite, a pyrochlore-group mineral with general formula A2-mB2XO6Y(O,OH,F)1-n·pH2O and 2 Ti > Nb+Ta and U > 20% at the A site, has been found at Le Carcarelle, (Latium, Italy). It occurs within miarolitic cavities of a foid-bearing syenitic ejectum enclosed within the pyroclastic formation known as “ignimbrite C″, which belongs to the main effusive phase of the Vico volcanic complex. The host rock is composed of K-feldspar, biotite, augitic clinopyroxene, magnetite and minor sodalite. Electron microprobe analyses gave the following crystal-chemical formula: (Ca1.24Na0.17U0.49REE0.03)∑=1.93 (Ti1.05Nb0.76Zr0.14Fe0.04Ta0.01)∑=2.00O6(O,OH). Compared with other occurrences reported in the mineralogical literature, betafite from Le Carcarelle is extremely enriched in U and depleted in Th. Due to its young age of formation (~150 k.y.), this betafite sample is highly crystalline, thus allowing structure refinement of unheated material. Betafite from Le Carcarelle is cubic Fd3̄m, with a = 10.2637(13) Å, and V = 1081.21(35) Å3, and has a smaller A site (consistent with the higher U content), and a larger and more distorted B site (consistent with the higher Ti content) than calciobetafite from Campi Flegrei, Italy (Mazzi and Munno, 1983). Analysis of the atomic displacement parameters provides evidence for static disorder at the X site.

Poppiite, the V3+ end-member of the pumpellyite group: Description and crystal structure

Abstract Poppiite, a new mineral from Gambatesa mine (Val Graveglia, Genova, Northern Italy), is the V3+ end-member of the pumpellyite group [[VII](Ca7.68 Na0.27 K0.03 Rb0.02)Σ8.00[VI](V3+1.26 Fe3+1.02 Mg0.78 Mn2+0.59 Al0.31 Cu2+0.04)Σ4.00[VI](V3+6.89 Al1.07 Ti0.04)Σ8.00[IV](Si11.69Al0.31)Σ12.00 O42 (OH)14; C2/m, a = 19.2889(6), b = 6.0444(2), c = 8.8783(3) Å, β = 97.328(2)°, V = 1026.66(6) Å3, Dmeas = 3.36(2) g/cm3, and Dcalc = 3.44 g/cm3]. Poppiite crystals, with size varying from 0.1 to 0.6 mm, are minute, greenish-brown, and prismatic, and are associated with roscoelite, ganophyllite, manganaxinite, goldmanite, and calcite. The strongest lines in the X-ray powder diffraction pattern [dobs (Å), Irel, (hkl)] are: 2.930, 100, (511); 3.817, 70, (202); 2.548, 65, (3.13); 2.551, 62, (420); 1.612, 57, (7.31,424); and 2.367, 51, (222, 403). Poppiite is optically negative, with 2Vcalc = 44°, nα = 1.768(9), nβ 1.804(8), nγ 1.810(9). The pleochroic scheme is α = light yellowish brown, β = deep greenish brown, and γ = brown to reddish brown. The crystal structure was refined using 1918 unique reflections to R = 0.0307. Like the other pumpellyite-group minerals the crystal structure of poppiite consists of chains of edge-sharing octahedra linked by SiO4, Si2O7, and CaO7 polyhedra

Crystal structure and chemistry of trilithionite-2M2 and polylithionite-2M2

The crystal chemistry of three Li-bearing mica-2 M 2 crystals from pegmatites has been studied by chemical analyses and single crystal X-ray diffraction; their belonging to the trilithionite-polylithionite join is highlighted by the following compositional ranges in atoms per formula unit [based on O 12-(x+y) (OH) x F y ]: 3.198 ≤ Si ≤ 3.538, 0.462 ≤ [IV] Al ≤ 0.811, 1.195 ≤ [VI] Al ≤ 1.390, 0.031 ≤ (Fe+Mn) ≤ 0.072, 1.522 ≤ Li ≤ 1.757, 0.872 ≤ K ≤ 0.906, 0.030 ≤ Na ≤ 0.073, 0.000 ≤ (Cs+Rb) ≤ 0.099, 1.541 ≤ F ≤ 1.722. The correlation between F and Li content is confirmed, as observed in Li-rich micas. Crystal structure refinements were carried out in space group C 2/ c ( R values vary between 0.030 and 0.031). The crystal chemistry is mostly influenced by tetrahedral chemical composition. Increasing [IV] Al content, α and ψ M1 parameters increase; Si content involves a lowering of the interlayer separation and tetrahedral thickness. Li content affects octahedral thickness. The stability of 2 M 2 polytype seems to be induced by a relative increase of Δz tetrahedral parameter, which reduces the repulsion between basal tetrahedral oxygen atoms. Unlike Li-bearing muscovite, trioctahedral Li-bearing mica crystals show an octahedral occupancy not related to octahedral charge.

Crystal structure and crystal chemistry of fluorannite and its relationships to annite

Abstract This study focuses on the crystal-chemical characterization of fluorannite from the Katugin Ta-Nb deposit, Chitinskaya Oblast’, Kalar Range, Transbaikalia, eastern Siberia, Russia. The chemical formula of this mineral is (K0.960Na0.020Ba0.001)(Fe2.1022+Fe0.4253+Cr0.0023+Mg0.039Li0.085Ti0.210Mn0.057)(Al0.674Si3.326) O10(F1.060OH0.028O0.912). This mica belongs to the 1M polytype (space group C2/m) with layer parameters a = 5.3454(2) Å, b = 9.2607(4) Å, c = 10.2040(5) Å, β = 100.169(3)º. Structure refinement,using anisotropic displacement parameters, converged at R = 0.0384. When compared to annite, fluorannite shows a smaller cell volume (Vfluorannite = 497.19 Å3; Vannite = 505.71 Å3), because of its smaller lateral dimensions and c parameter. Flattening in the plane of the tetrahedral basal oxygen atoms decreases with F content, together with the A-O4 distance (i.e. the distance between interlayer cation A and the octahedral anionic position) due to the reduced repulsion between the interlayer cation and the anion sited in O4.

THE CRYSTAL STRUCTURE OF ROSCOELITE-1M

Single-crystal X-ray diffraction experiments were carried out on roscoelite crystals from Reppia, Val Graveglia, Italy. Roscoelite [structural formula: XII(Ba0.006K0.994)IV(Si3.150Al0.850) VI(Al0.040Fe0.150Mg0.100Mn0.062V1.696Ti0.003)O10(OH)2] shows a near-perfect three-dimensional stacking order with cell parameters a = 5.292(1), b = 9.131(2), c = 10.206(3) Å, β = 100.98(2)° and space group C2/m, which indicate a 1M polytype. The crystal structure was refined on the basis of Fo2 for 846 unique reflections to R1 = 3.29% calculated using 746 unique observed reflections [|Fo| ⩾ 4σ(Fo)]. The mean tetrahedral cation–oxygen atom distance, = 1.641 Å, is close to the mean value obtained for dioctahedral true micas from the literature, whereas the octahedral sheet is characterized by a larger cis-octahedral cation–oxygen atom bond distance = 2.020 Å which, together with the mean electron count, is consistent with V occupancy. The presence of V within the octahedral sheet produces the smallest tetrahedral rotation (α = 2.3°), the lowest flattening of the basal oxygen surface (Δz = 0.118 Å) and the narrowest interlayer separation (3.030 Å) in dioctahedral micas.

Nb-containing titanite: new data and crystal structure refinement.

Nb-containing titanite, [ 4 ] Si 1 . 0 0 0 [ 6 ] (Ti 0 . 7 4 8 Al 0 . 0 9 5 Zr 0 . 0 4 5 Nb 0 . 0 4 1 Ta 0 . 0 0 1 Fe 3 + 0.067 Sn 0 . 0 0 2 Sb 0 . 0 0 1 ) 1 . 0 0 0 [ 7 ] (Ca 0 . 9 8 6 Ce 0 . 0 0 4 La 0 . 0 0 1 Mn 0 . 0 0 4 Na 0 . 0 0 5 K 0 . 0 0 1 ) 1 . 0 0 1 F 0 . 0 5 5 (OH) 0 . 0 6 5 O 4 . 8 8 0 , from San Vito quarry, Mt. Somma, Naples, Italy, is characterised by extensive substitutions in the six- and seven-fold coordination sites. Single-crystal X-ray diffraction study indicates this niobian titanite to be monoclinic (a = 6.557(1), b = 8.701(1), c = 7.077(1) A, β = 113.86°) with C2/c symmetry. Crystal structure refinement (agreement factor, R = 0.027) suggests that the substitution mechanism (Al, Fe) 3 + + (Nb, Ta) 5 + = 2Ti 4 + is primarily responsible for the incorporation of pentavalent metal ions in the octahedral site.