Cristiano Ferraris

Nomenclature of the apatite supergroup minerals

The apatite supergroup includes minerals with a generic chemical formula IX M12 VII M23( IV TO4)3 X( Z ¼ 2); chemically they can be phosphates, arsenates, vanadates, silicates, and sulphates. The maximum space group symmetry is P63/m, but several members of the supergroup have a lower symmetry due to cation ordering and deviations from the ideal topology, which may result in an increase of the number of the independent sites. The apatite supergroup can be formally divided into five groups, based on crystal-chemical arguments: apatite group, hedyphane group, belovite group, britholite group, and ellestadite group. The abundance of distinct ions which may be hosted at the key-sites (M ¼ Ca 2þ , Pb 2þ , Ba 2þ , Sr 2þ , Mn 2þ , Na þ , Ce 3þ , La 3þ ,Y 3þ , Bi 3þ ;T ¼ P 5þ , As 5þ ,V 5þ , Si 4þ ,S 6þ ,B 3þ ;X ¼ F � , (OH) � , Cl � ) result in a large number of compositions which may have the status of distinct mineral species. Naming of apatite supergroup minerals in the past has resulted in nomenclature inconsistencies and problems. Therefore, an ad hoc IMA-CNMNC Subcommittee was established with the aim of rationalizing the nomenclature within the apatite supergroup and making some order among existing and potentially new mineral species. In addition to general recommendations for the handling of chemical (EPMA) data and for the allocation of ions within the various sites, the main recommendations of this subcommittee are the following: 1. Nomenclature changes to existing minerals. The use of adjectival prefixes for anions is to be preferred instead of modified Levinson suffixes; accordingly, six minerals should be renamed as follows: apatite-(CaF) to fluorapatite, apatite-(CaOH) to hydroxylapatite, apatite-(CaCl) to chlorapatite, ellestadite-(F) to fluorellestadite, ellestadite-(OH) to hydroxylellestadite, phospho- hedyphane-(F) to fluorphosphohedyphane. For the apatite group species these changes return the names that have been used in thousands of scientific paper, treatises and museum catalogues over the last 150 years. The new mineral IMA 2008-009, approved without a name, is here named stronadelphite. Apatite-(SrOH) is renamed fluorstrophite. Deloneite-(Ce) is renamed deloneite. The new mineral IMA 2009-005 is approved with the name fluorbritholite-(Y).

Nano- to micro-scale decompression products in ultrahigh-pressure phengite: HRTEM and AEM study, and some petrological implications

Abstract Samples of phengite-3T, (K0.93Na0.01)(Al1.44Mg0.56Ti0.02)(Si3.54Al0.46)O10(OH1.93F0.07), which formed at about 3 GPa and 1000 K in ultrahigh-pressure metamorphic rocks of the Dora-Maira Massif (Western Alps, Italy), were investigated by HRTEM, AEM, EMPA, and SEM. The matrix of phengite- 3T, which is almost free of stacking faults, contains single-crystal α-quartz platelets 100-700 Å thick that are confined by (001) planes of phengite. In the vicinity of quartz, the 30 Å period of the phengite-3T matrix is faulted by short sequences with about 19 Å periodicity, interpreted as talc-2M. The occurrence of these phases is not connected with any defects in the host phengite nor is their spatial distribution homogeneous on the TEM scale. The shape control exerted by phengite on the quartz crystals, the absence of deformation around them, and the nearby presence of an interlayered 19 Å phase, suggest that quartz and talc may have been produced within mica by a reaction of the type: 3 alumino-celadonite + 2 H+ = 1 muscovite + 1 talc + 5 quartz + 2 H2O + 2 K+ which leads to a less celadonite-rich phengite during decompression, after the rock had left the coesite stability field. In addition, examination by optical microscopy along [001] of “thick” sections of the Dora-Maira phengite and of phengite samples from other high-pressure terranes (Monte Mucrone and Sesia Zone in the Italian Western Alps, Dabie Mountains in central China), revealed the presence of micrometer-wide, amoeboid platelets of quartz interlayered at various depths parallel to (001) of micas. In spite of the different observation scale, these are interpreted as the same reaction product as identified by HRTEM. These new observations show that high-pressure white micas may not be homogeneous and should be examined more carefully. Some consequences for thermobarometry of such heterogeneity and intracrystalline re-organization during decompression are considered; they depend on the resolution of the analytical method employed. Implications for thermochronometry still have to be evaluated.

SEM/TEM-AEM characterization of micro- and nano-scale zonation in phengite from a UHP Dora-Maira marble petrologic significance of armoured Si-rich domains

Chemically zoned phengite flakes from an impure marble of the ultra-high pressure Brossasco-Isasca Unit (Dora-Maira massif, Western Alps - Italy) were investigated by scanning, transmission and analytical electron microscopies (SEM-TEM-AEM), revealing a complex nano-structure within the crystal cores. Diffraction patterns show that this phengite is a highly ordered 3 T polytype. A close inspection of the diffracted spots reveals that, for some classes of crystallographic indexes, weak satellite spots surround a central stronger spot. In bright-field TEM images, 60–120 A wide areas of mottled contrast cross, at an angle of about 40°, the host phengite \[Phe (a)] that represents the matrix. The satellite spots, the spatially ordered mottled contrast and the AEM analyses are all indicative of the presence of high-Si phengite relics [Phe (b)\] (Si = 3.61 apfu) within the ordered matrix (Si = 3.38 apfu). The higher-Si phengite has the c cell parameter [29.68 A; Phe (b)] different from that of the lower-Si matrix [29.75 A; Phe (a)]. A zoned 3 T phengite mantle (3.41-3.31 Si apfu) surrounds the phengite cores showing decreasing c values from the inner towards the outer part ( i.e. , 29.78-29.85 A). A later 2 M1 polytype with Si = 3.22 apfu has been locally detected at the edge of the phengite crystals. Both the crystallographic and crystal-chemical data of the higher-Si phengite relics, as well as petrologic information have been used to reconstruct prograde, peak and decompressional growth stages of the phengite flakes. Differences in both the octahedral and tetrahedral contents between the two types of phengite nano-domains suggest that the formation of the high-Si Phe (b) is due to an incomplete ionic reaction affecting the prograde Phe (a) at the metamorphic peak. The proposed ionic reaction is: 1 Phe (a) + 0.20 Mg2+ + 0.23 Si4+ → 1 Phe (b) + 0.44 Al3+.

Intergrowth of graphite within phlogopite from Finero ultramafic complex (Italian Western Alps): implications for mantle crystallization of primary-texture mica

High Resolution Transmission and Analytical Electron Microscopy (HRTEM-AEM) of phlogopite crystals from the Finero lherzolite (Italian Western Alps) revealed nanometric intergrowths of graphite-like layers. The intergrowths consist of 4 to 6 graphite-like layers (c ≈ 3.3 A) with the basal plane parallel to (001) of the phlogopite host. The phlogopite crystals contain between 0.024 and 0.048 wt % of carbon. The carbon isotope composition varies from δ 13 C graphite = −16.1 %° to −10.4 %° (VPDB). These values are compatible with carbon phases found in other mantle-derived ultramafic rocks. A Raman analysis confirms the presence of a graphite-like phase intergrown within phlogopite. Geochemical and microtextural data point to a primary origin of the phlogopite-graphite intergrowths and suggest simultaneous crystallization of both phases from a trapped partial melt.

Phlogopite exsolutions within muscovitea first evidence for a higher-temperature re-equilibration, studied by HRTEM and AEM techniques

Nano- to micro-scale lamellae of ferro-aluminian phlogopite (Phl) within centimetre-size crystals of muscovite (Ms-matrix) from a pegmatite near Gorduno (Lepontine Domain, Central Alps, Switzerland) were discovered using high-resolution transmission (HRTEM) and analytical electron microscopy (AEM). The Msmatrix is a 2 M 1 polytype with occasional disordered sequences. The Phl lamellae, covering a large range of sizes, are parallel to the basal plane of the host. The interfaces parallel to (001) are straight, whereas the interfaces perpendicular to the basal plane are irregular. The lamellae are concentrated on levels of the Ms-matrix with disordered stacking. The Ms-matrix has close to end-member composition with 0.15 tri-octahedral cations p.f.u. The Phl-lamellae show considerable di-octahedral and eastonite substitution and are zoned, with rims slightly richer in di-octahedral component. The Ms-matrix itself is homogeneously exsolved into two sets of nanometre-sized lamellae at an angle of about 40° to the basal plane. The Phl-lamellae and the fine, homogeneously nucleated lamellae in the Ms-matrix are exsolutions of a primary pegmatitic muscovite, richer in tri-octahedral component. Micas along the di-tri-octahedral join exsolve with increasing temperature and the composition of the exsolved matrix points to an equilibration temperature of 600°C. The recalculated primary composition points to a crystallisation temperature around 500°C. The nearby, younger Novate intrusion body probably acted as heat source for the post-magmatic increase of temperature.

Crystal chemistry of mimetite, Pb10(AsO4)6Cl1.48O0.26, and finnemanite, Pb10(AsO3)6Cl2

The crystal chemistries of synthetic mimetite, Pb(10)(As(5+)O(4))(6)(Cl(2 - x)O(x/2)), a neutral apatite, and finnemanite, Pb(10)(As(3+)O(3))(6)Cl(2), a reduced apatite, were characterized using a combination of X-ray powder diffraction, neutron diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. Both phases conform to hexagonal P6(3)/m symmetry; however, the temperature-driven transformation of clinomimetite to mimetite described earlier was not confirmed. The average mimetite structure is best described through the introduction of partially occupied oxygen sites. A better understanding of the mixed arsenic speciation in apatites can guide the formulation of waste form ceramics and improve models of long-term durability after landfill disposal.

Petrogenesis of mineral micro-inclusions in an uncommon carbonado

A unique Brazilian sample of a carbonado, displaying unusually large amount of diamond clasts merged within a fine-grained diamond matrix, has been studied by Secondary Ion Mass Spectrometry (SIMS), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) on Focused Ion Beam (FIB)-extracted foils. We found for the first time in the diamond clasts (stage-1) micrometer to nanometer-sized inclusions of augite, ilmenite and phlogopite (all Fe-rich). Inclusions of metallic phases (Fe, Ti, Cr, Al and alloys of Fe-Cr, Al-Cr, Al-Fe Cr) described in worldwide carbonado occur in the studied sample exclusively within the fine-grained matrix (stage-2). The carbon isotopic composition of the diamond clasts and the fine-grained matrix falls within the range −27 ‰ to −32 ‰, like worldwide carbonado. The iron-rich silicate-oxide assemblage isolated inside clasts points to an initial growth of that diamond from mafic-rock minerals under oxidizing conditions ( f O 2 > IW). On the other hand metallic phases within the fine-grained matrix indicate an oxygen fugacity drop of at least 15 log units. This change in redox conditions is coeval with a deformation event under shearing stress at upper-mantle depth. During this metamorphic event, stage-1 diamonds were broken giving rise to the stage-2 fine-grained matrix, and syngenetic oxide inclusions were reduced to their metallic elements. This unique sample sheds new light on early 1970s hypotheses that interpreted carbonado as a high-pressure product from prograde metamorphism of crustal mafic rocks.

Thermal stability of a reverse-graded SiGe buffer layer for growth of relaxed SiGe epitaxy

We have recently developed a novel reverse-graded (RG) buffer system, in which the Ge content decreases with distance from the Si interface. These thin (90 nm) RG layers are capable of supporting the growth of relaxed SiGe layers (85% relaxed) with defect densities as low as 10 5 /cm 2 . Good quality strained Si has also been successfully grown on these substrates. However, the thermal stability of this novel heterostructure has not been explored. In this paper, we establish, by high-resolution X-ray diffraction, Raman spectroscopy, atomic force microscopy, and transmission electron microscopy, that the heterostructure is stable up to 1000°C with no further strain relaxation in both the RG layer and strained Si layer. Hence, it is clear that this thin RG heterostructure is highly suitable as a buffer system for the growth of high-mobility strained Si or Ge devices.

Low-dislocation-density strain relaxation of SiGe on a SiGe∕SiGeC buffer layer

We report an observation of strain relaxation in lattice-mismatched heteroepitaxial Si1−xGex layers, accompanied by a reduction in threading dislocation density (TDD). This occurs on a Si0.77Ge0.23 layer grown on top of alternating layers of Si0.77Ge0.23∕Si0.76Ge0.23C0.01. The present scheme allows us to grow a high-quality 85% relaxed Si0.77Ge0.23 layer with a TDD of ∼104∕cm2. The high-resolution transmission electron microscope results showed the presence of Si1−x−yGexCy domains (with x⩽0.23 and y⩽0.01) after annealing at 1000°C. We infer that the formation of these domains assist the low TDD relaxation by releasing the epitaxial misfit strain as localized discrete strain and by blocking the propagation of misfit dislocations.

A multi-domain gem-grade Brazilian apatite

Abstract A gem-grade apatite from Brazil of general composition (Ca,Na)10[(P,Si,S)O4]6(F,Cl,OH)2 has been studied using single-crystal X-ray and neutron diffraction together with synchrotron powder X-ray diffraction. Earlier electron microscopy studies had shown the nominally single-phase apatite contains an abundant fluorapatite (F-Ap) host, together with chloro-hydroxylapatites (Cl/OH-Ap) guest phases that encapsulate hydroxylellestadite (OH-El) nanocrystals. While the latter features appear as small (200-400 nm) chemically distinct regions by transmission electron microscopy, and can be identified as separate phases by synchrotron powder X-ray diffraction, these could not be detected by singlecrystal X-ray and neutron analysis. The observations using neutron, X-ray and electron probes are however consistent and complementary. After refinement in the space group P63/m the tunnel anions F− are fixed at z = ¼ along <001>, while the anions Cl− and OH− are disordered, with the suggestion that O-H···O-H··· hydrogen-bonded chains form in localized regions, such that no net poling results. The major cations are located in the 4f AFO6 metaprism (Ca+Na), 6h ATO6X tunnel site (Ca only), and 6h BO4 tetrahedron (P+Si+S). The structural intricacy of this gem stone provides further evidence that apatite microstructures display a nano-phase separation that is generally unrecognized, with the implication that such complexity may impact upon the functionality of technological analogues.