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This paper describes a new occurrence of the rare minerals perrierite-(Ce) and zirconolite in the tephra of the Roccamonfina volcano. The host rock is a foid-bearing alkali-syenite and is composed predominantly of sanidine with minor clinopyroxene, magnetite and sodalite. Accessory phases include titanite, zircon and baddeleyite. Intense phyllic and argillic alteration is observed at alkali-feldspars grain boundaries or along fractures within crystals. Perrierite-(Ce) occurs as brownish-red, transparent, well-developed, euhedral, prismatic crystals inside the miarolitic cavities of the ejectum. Its powder pattern can be indexed in the monoclinic P2(1)/m space group, with (in Angstrom) a = 13.676(11), b = 5.655(2), c = 11.763(8), beta = 113.75 (4). It is Zr-rich and the entry of Zr in the C site of the structure can be described by the charge balancing substitution: Ca-[A]([C])(Ti4+,Zr4+) = (REE3+[C])-R-[A](M-3+,M2+). Zirconolite occurs as very small acicular or stubby prismatic crystals typically associated with baddeleyite in the altered parts of the rock. In the ternary REE-Act-Me5+ its composition plots well within the field of zirconolites from alkali syenitic environments. The genesis of this unusual mineral assemblage is related to late metasomatic stages at hypabyssal conditions of the regional volcanic activity.

Amphiboles were synthesized at 750 8C, 1 kbar (H2O) on the binary joins (nickel, magnesium)-richterite and (magnesium, cobalt)-richterite. Structural variations and site occupancies were characterized by Rietveld structure refinement, with final RBragg indices in the range 4–9%, and by powder infrared spectroscopy in the principal OH-stretching region. Site-occupancy refinement of Ni-Mg and Mg-Co distributions give the partition coefficients over M1,3 and M2 where KM21 5 (M21/Mg)M1,3/(M21/Mg)M2, and M21 5 Ni21 or Co21, K 5 2.98 6 0.37 and K 5 1.34 6 0.31. Both Kd Ni Co values are greater than 1.0, d d whereas [6]r(Ni21 ) , [6]r(Mg) , [6]r(Co21 ); this indicates that cation size is not the primary factor affecting the ordering of Ni-Mg and Mg-Co over the octahedral sites. The infrared spectra of intermediate binary compositions show fine structure caused by ordering of Ni- Mg or Mg-Co over the M1,3 sites and by ordering of Na and M (vacancy) at the A site; thus intermediate compositions show an eight-band spectrum in the principal OHstretching region. Precise band intensities were derived by nonlinear least-squares fitting of Gaussian band shapes to the observed spectra. The relative observed intensities of the combinations of bands 3IA1 2IB1 IC and IB 1 2IC 1 3ID are in accord with the equations o o o o o o of Burns and Strens (1966), indicating that there is no significant variation in molar absorptivity with frequency (energy) for individual bands within a single sample (spectrum). Combined with the results of Skogby and Rossman (1991) on polarized single-crystal infrared spectra of amphiboles, this result suggests that different local configurations of M1,3 cations in amphiboles couple such that the transition probabilities of the associated OH groups are equal.

Detailed X-ray single-crystal structure refinements and complete (SIMS + EMPA) microprobe chemical analyses of a series of non-metamict samples of hellandite recently found in Latium (Italy), together with a critical re-evaluation of the existing data on hellandite, allow a better understanding of the crystal-chemistry of the hellandite group. Relative to the crystal structure determined by Mellini and Merlino (1977) for a Y-rich sample from Predazzo (Italy), a new tetrahedrally coordinated site has been detected; this may be fully, or in part, occupied by Li and Be. These cations occur at the center of the tetrahedral cavity where the H atom, which is bonded to the O5 O atom, protrudes; thus the H content in hellandite is constrained to values less than or equal to 2 - (Li + Be + F). A new general formula for hellandite-group minerals is proposed: X(4)Y(2)ZT(2)[B4Si4O22]W-2, where X = Na, Ca, Y, LREE3+ at the eightfold-coordinated M3 and M4 sites; Y = Ca, Y, HREE3+, Th4+, U4+ at the eightfold-coordinated M2 site; Z = Al, Mn3+, Fe3+, Ti4+ at the octahedral M1 site; T = square (vacancy), Li, Be at the new tetrahedrally coordinated site; and W = OH, F, O-2 at the O5 site. Eight root end-member compositions were identified; four of these correspond to known compositions: hellandite-(REE), tadzhikite-(REE), and two new minerals, mottanaite-(Ce) and ciprianiite, which are described in a companion paper (Della Ventura et al. 2002). The root-name tadzhikite must be used for samples with Ti4+ > 0.5 apfu, with no reference to the OH content. Hellandite is an REE mineral. Rare-earth elements occur at two distinct sites (M2 and M4), with a strong preference for M2 (particularly HREE). Thus, one or two Levinson modifiers can be added to the root name to correctly describe the species. A sequence of incorporation based on crystal-chemical arguments is provided and allows evaluation of the site populations for nomenclature purposes. This redefinition of the hellandite group and the new nomenclature rules have been approved by the IMA Commission on New Minerals and Mineral names (code 00-F).

The structure of aluminosilicate melts and/or glasses plays a key role in the earth sciences for the understanding of rock-forming igneous processes, as well as in the materials sciences for their technical applications. In particular, the alkaline-earth aluminosilicate glasses are an extremely important group of materials, with a wide range of commercial application, as well as serving as an analog for natural basaltic melts. However, definition of their structure and properties is still controversial, and in particular the role and effect of Al has long been a subject of debate. Here we report a series of experimental x-ray absorption near-edge structure spectra at the Al K edge on a series of synthetic glasses of peralkaline composition in the $\mathrm{CaO}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}\ensuremath{-}{\mathrm{SiO}}_{2}$ system, together with a general theoretical framework for data analysis based on an ab initio full multiple-scattering theory. We propose an Al/Si tetrahedral network model for aluminosilicate glasses based on distorted polyhedra, with varying both the $T\ensuremath{-}\mathrm{O} (T=\mathrm{Al}$ or Si) bond lengths and the $T\ensuremath{-}\mathrm{O}\ensuremath{-}T$ angles, and with different Al/Si composition. This model achieves a significant agreement between experiments and simulations. In these glasses, experimental data and theoretical results concur to support a model in which Al is network former with a comparatively well ordered local medium-range order (up to 5 \AA{}).

This paper describes the occurrence of Th-rich hellandite-(Ce) and britholite-(Ce) within a volcanic ejectum from a pyroclastic formation of the Vico volcanic complex, north of Rome, Italy. The host rock is a saturated alkali-syenite which consists mainly of potassium feldspar and plagioclase, together with fluor-biotite, plagioclase, titanite, clinopyroxene and magnetite. Hellandite-(Ce) and britholite-(Ce) are associated with this mineral assemblage which contains also several accessory phases including zircon, fluorapatite, allanite, and silico-phosphates of Th, U, REE close in composition to cheralite-brabantite. Hellandite-(Ce) and britholite-(Ce) are typically intergrown with hellandite in all case observed rimming britholite. These unusual REE, Th, U minerals are secondary to the formation of the host rock and the compositions of the associated minerals suggest that they originated from fluids rich in F, P, and possibly S and C. The presence of fractures filled by clays containing dispersed REE-minerals, probably fluorocarbonates, suggests a very late-stage hydrothermal remobilization, transportation and redeposition of the REE. An important implication of such widespread secondary mineralization is that great care should be exerted when using bulk-rock trace-element data from these volcanic samples for geochemical modelling.