Crystal-chemical and structural characterization of fluorapatites in ejecta from Somma-Vesuvius volcanic complex

Abstract

The mineralogy and crystal chemistry of apatites occurring in 14 ejecta of historical eruptions (1631 and 1872 A.D.) of the Somma-Vesuvius volcanic complex were investigated by a multi-methodological approach including polarized optical microscopy, scanning electron microscope, electron microprobe analysis in wavelength-dispersive mode, laser ablation inductively coupled plasma mass spectroscopy, and single-crystal X‑ray diffraction. Five different groups of apatite, with different mineralogical and crystal-chemical features, were identified. Apatite crystals occur with well-developed hexagonal prismatic habit and, more rarely, with skeletal acicular forms. The crystals are yellow (type 1), transparent and colorless (type 2 and type 3), green (type 4) and aquamarine colored (type 5), with different paragenesis: macro-crystalline aggregates of clinopyroxenes, phlogopite, and apatite (type 1; type 4); clusters of apatite with micro-crystalline clinopyroxene, minerals of the cancrinite group, feldspars, and opaque minerals (type 2); aggregates of apatite, sellaite, wagnerite, gypsum, and phlogopite (type 3); aggregates of davyne, nepheline, mica group minerals, and apatite (type 5). Chemical analyses of apatites show variable amounts of Na, REE, Mg, Sr, and Fe replacing Ca, not negligible amounts of Si and S substituting P, and a significant substitution of Cl and OH instead of F. Five crystals representative of each apatite-type were studied by single-crystal X‑ray diffraction. Their crystal structure was refined in the hexagonal P63/m space group. A significant variation of the unit-cell parameters with the composition was observed. A comparative crystal-chemical analysis between apatites from Somma-Vesuvius and those from other localities is carried out

Crystal Chemistry and Luminescence Properties of Eu-Doped Polycrystalline Hydroxyapatite Synthesized by Chemical Precipitation at Room Temperature

“…Results are reported in Table 3. The space group determination step, executed by using the powder pattern of Eu-HAp900 gave the most common hexagonal P6 3 /m space group of apatite [27][28][29]. The possibility of the monoclinic (pseudo-hexagonal) P2 1 /b space group, also reported in literature [30], was not considered for two reasons: (1) it is much less frequent; (2) XRPD is not able to distinguish between the two space groups P6 3 /m and P2 1 /b of apatite.…”

“…Considering the larger size of Cl and OH groups with respect to the F anion, Structural sites of HAp are those known for hexagonal P6 3 /m HAp, with Ca1, Ca2 and P1 occupying special positions, as well as O1, O2; O3 is the only atom in a general position, while an additional discussion is worthwhile for the OH group, which is on the 4e special position. The apatite group comprises three main species, fluoro-, chloro-and hydroxyapatite [28]; in fluoro-apatite the F atom is at special position 2a, corresponding to the intersection of 6 axis and mirror planes at z = 1 4 and z = 3 4 . Each F anion is bound with three symmetry-related Ca2 cations, lying at the same z level for symmetry reasons.…”

“…. O3, slightly outside the bonding sphere of Ca, with distances >2.80 Å; actually, in many apatites the Ca1 polyhedron is described as nine-coordinated [28,36,37]. Ca2 shows a CaO 6 (OH) geometry (Figure 2b), resembling a distorted CaO 7 pentagonal bipyramid with an equatorial plane occupied by O1 atom and two couples of symmetry-related O3 atoms, and vertices occupied by O2 atom and OH group (corresponding to O4 atom).…”

Neutron and XRD Single-Crystal Diffraction Study and Vibrational Properties of Whitlockite, the Natural Counterpart of Synthetic Tricalcium Phosphate

“…These applications are mostly based on the similarity of the density of these compounds with that of human hard tissues [2]. The most abundant natural CaP belongs to the apatite series Ca 10 (PO 4 ) 6 (F,Cl,OH) 2 [3][4][5]; however, there is currently an arising interest for the mineral whitlockite, ideally Ca 9 Mg(PO 4 ) 6 [PO 3 (OH)], a rare species found as a secondary phase in some granite pegmatites [6]. The interest for whitlockite, is largely due to its relation with merrillite Ca 9 NaMg(PO 4 ) 7 and keplerite Ca 9 (Ca 0.5 0.5 )Mg(PO 4 ) 7 , two phosphate minerals found in meteorites, which represent the main reservoir of phosphate phosphorus in the Solar System [7].…”

Compositional variation of turquoise-group minerals from the historical collection of the Real Museo Mineralogico of the University of Naples

“…The present research takes place within the context of a general survey on natural phosphate which some of the authors have been undertaking (Rossi et al, 2011;Capitelli et al, 2011;2014a,b;Ventruti et al, 2016). In particular, the objectives of this investigation is to present a detailed characterization of minerals from the turquoise group, evaluating their provenance, various mineralogical arrangements, and different compositions.…”