Pyrope Crystals Research Articles

Synthesis of Pyrope Garnet with High Samarium Content at 5 GPa and 1300 °C

In natural garnets associated with diamonds, an increased content of "light" rare-earth elements, especially samarium, is often noticed. There are various estimates of the composition of the crystallization medium. Experimental data on the synthesis of Cr-rich garnets containing rare-earth elements are still scarce. The experiments were carried out on a multi-anvil apparatus of the “split-sphere” type (BARS) at a pressure of 5 GPa and a temperature of 1300 °С. The high pressure cell was made of Zr02 and CaO. A tubular graphite element was used as a heater. The measurement error of pressure and temperature was ± 0.2 GPa and ± 25 °С. The starting materials for the experiments were natural serpentine-antigorite, chromium spinel, and corundum. As a result of the experiments, pyrope crystals with a high content of samarium were synthesized in the range of 0.77 to 2.34 wt% Sm2O3. The study demonstrates that the interaction of components in the serpentine-chromite-corundum-Sm system in the presence of fluid leads to crystallization of pyrope garnet with samarium content exceeding the values known for natural garnets associated with diamond.

Micro- and nano-size hydrogrossular-like clusters in pyrope crystals from ultra-high-pressure rocks of the Dora-Maira Massif, western Alps

The supracrustal metamorphic rocks of the Dora-Maira Massif, western Alps, have been intensively studied. Certain ultra-high-pressure lithologies contain coesite and nearly end-member composition pyrope, Mg3Al2Si3O12, making this locality petrologically and mineralogically unique. Structural OH-, loosely termed “water”, in pyrope crystals of different composition has been investigated numerous times, using different experimental techniques, by various researchers. However, it is not clear where the minor OH- is located in them. IR single-crystal spectra of two pyropes of composition {Mg2.79,Fe2+0.15,Ca0.04}Σ2.98[Al]2.02(Si)2.99O12 and {Mg2.90,Fe2+0.04,Ca0.02}Σ2.96[Al]2.03(Si)3O12 were recorded at room temperature (RT) and 80 K. The spectra show five distinct OH- bands located above 3600 cm-1 at RT and seven narrow bands at 80 K and additional fine structure. The spectra were curve fit and the OH- stretching modes analyzed and assigned. It is argued that OH- is located in microscopic- and nano-size Ca3Al2H12O12-like clusters. The basic substitution mechanism is the hydrogarnet one, where (H4O4)4- ⇔ (SiO4)4-, and various local configurations containing different numbers of (H4O4)4- groups define the cluster type. The amounts of H2O range between 5 and 100 ppm by weight, depending on the IR calibration adopted, and are variable among crystals. Hydrogrossular-like clusters are also present in a synthetic pyrope with a minor Ca content grown hydrothermally at 900 °C and 20 kbar, as based on its IR spectra at RT and 80 K. Experiment and nature are in agreement, and OH- groups are partitioned into various barely nano-size hydrogrossular-like clusters. This proposal is new and significant mineralogical, petrological, and geochemical implications result. Ca and proton ordering occur. Hypothetical “defect” and/or coupled-substitution mechanisms to account for structural OH- are not needed to interpret experimental results. OH- incorporation in pyrope of different generations at Dora-Maira is discussed and OH- could potentially be used as an indicator of changing P_{{{text{H}}_{{text{2}}} {text{O}}}} {text{(}}a_{{{text{H}}_{{text{2}}} {text{O}}}} {text{) - }}T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira pyropes can now be interpreted atomistically.

Hydrous Na-garnet from Garnet Ridge; products of mantle metasomatism underneath the Colorado Plateau

This is the first report on amphibole exsolution in pyrope from the Colorado Plateau. Pyrope crystals delivered from mantle depths underneath the Colorado Plateau by kimberlitic volcanism at 30 Ma were collected at Garnet Ridge, northern Arizona. The garnet grains analyzed in this study occur as discrete crystals (without adjacent rock matrix) and are classified into two major groups, Cr-rich pyrope and Cr-poor pyrope. The Cr-poor pyrope group is divided into four subgroups based on exsolved phases: amphibole lamella type, ilmenite lamella type, dense lamellae type, and clinopyroxene/amphibole lamellae type. Exsolved amphibole occurs in amphibole lamella type, dense lamellae type, and clinopyroxene/amphibole lamellae type of Cr-poor pyrope. The amphibole crystals tend to have preferred orientations in their garnet hosts and occur as monomineralic hexagonal or rhombic prisms and tablets, and as multimineralic needles or blades with other exsolved phases. Exsolved amphibole has pargasitic compositions (Na2O up to 1.6 apfu based on 23 oxygen). Garnet host crystals that have undergone amphibole exsolution have low OH contents (2–42 ppmw H2O) compared to garnets that do not have amphibole lamellae (up to 115 ppmw H2O). The low OH contents of garnets hosting amphibole lamellae suggest loss of OH from garnet during amphibole exsolution. Amphibole exsolution from pyrope resulted from breakdown of a precursor “hydrous Na-garnet” composition (Mg,Na+ x)3(Al2 − x, Mgx)2Si3O12 − 2x(OH)2x. Exsolution of amphibole and other phases probably occurred during exhumation to depths shallower than 100 km prior to volcanic eruption. Based on the abundance and composition of exsolved clinopyroxene and amphibole lamellae in one garnet, hydrous Na-garnet had excess silicon (Si3.017 apfu, 12 oxygen normalization, vs. X3Y2Si3O12 for typical garnet). Comparison with experimental data suggests crystallization at pressures near 6–8 GPa. Garnet crystals that host exsolved amphibole have compositions (Pyp49-76, 3–10 wt% CaO, and up to 0.6 wt% Cr2O3) similar to garnets reported from pyroxenites, and have pyrope-almandine-grossular compositional ranges that overlap with the Cr-rich pyrope (typical lherzolitic garnet). Hydrous Na-garnet was likely formed by metasomatic reactions between Cr-rich pyrope and Na-rich aqueous fluid in the deep upper mantle. The most likely source of metasomatic Na-rich fluid is ancient oceanic crust that was subducted before subduction of the Farallon Plate beneath the Colorado Plateau.

Kinetics of deuteration in pyrope

Hydrogen-deuterium exchange experiments were performed at temperatures between 973 and 1223 K on a Dora Maira pyrope and on two pyrope samples from mantle xenoliths. The Fourier transform infrared (FTIR) spectra of hydrogen in the natural Dora Maira and xenolith pyropes are different, with several OH bands between 3600 and 3650 cm-1 and between 3510 and 3575 cm -1 , respectively. The self-diffusion of deuterium in Dora Maira pyrope is given by D = D 0 exp [-140 ± 38 kJmol -1 /RT] with log D 0 (m 2 /s) = -5.8 ± 1.9. This activation energy for diffusion is identical to those measured in diopside and olivine, but the pre-exponential term D 0 is one to two orders of magnitude smaller. This suggests that the mechanism of hydrogen self-diffusion in the pyrope structure is similar to that for other upper mantle mineral structures, such as olivine and pyroxene. In contrast to the Dora Maira pyrope, for the two pyrope crystals extracted from xenoliths, the total concentration of hydrous species is not preserved during H-D exchange experiments; dehydration occurs concurrently with deuteration. This renders the analysis of the kinetics of H-D exchange difficult. In the pyropes from mantle xenoliths, the kinetics of dehydration under reducing conditions is close to that for hydrogen dehydrogenation from experiments performed in air by Wang et al. (1996). This suggests that hydrogen exchange in mantle pyropes may be independent of the oxygen fugacity.

Garnet pyroxenite xenoliths and pyropic megacrysts in Scottish alkali basalts

Synopsis Pyropic garnet occurs within garnet pyroxenite xenoliths and as discrete crystals in Carboniferous–Permian alkali basaltic tuffs and hypabyssal intrusions. Eight localities are now known from the Midland Valley together with another in the NW Highlands. The xenoliths are inferred to have been derived from minor pyroxenitic intercalations in spinel lherzolite within the lithospheric mantle. Although pyrope crystals at Elie Ness are regarded as high-pressure phenocrysts cognate with the basanitic host, it is indeterminable whether discrete garnets at the other localities are phenocrysts or xenocrystal fragments derived from garnet pyroxenites. All, however, whether phenocrystic, xenocrystic or integral components of pyroxenitic xenoliths, are regarded as cognate in the sense of being products of high-pressure crystallization of late Palaeozoic basanite magmas. Kaersutite, phlogopite, hercynitic spinel, ilmenite and pseudomorphs after orthopyroxene occur as minor components in some of the garnet pyroxenite xenoliths. Mineral compositional data (electron- and ion-microprobe) are presented, with comparisons to xenolith-megacryst suites elsewhere. The garnets have strong similarities with each other and with megacryst populations from alkali basalts outside the U.K. It is concluded that the pyroxenites originally crystallized from basaltic magmas as high-pressure cumulates from depths of >50 km and represent some of the deepest samples from the Scottish lithosphere.

Carmichaelite, a new hydroxyl-bearing titanate from Garnet Ridge, Arizona

Carmichaelite [MO2‐x(OH)x, where M includes Ti, Cr, Fe, Mg, and Al] is a new hydrous titanate from Garnet Ridge, an ultramafic diatreme on the Colorado Plateau in Arizona. It is named in honor of Ian S.E. Carmichael, Professor of Geology at the University of California, Berkeley, in recognition of his many contributions to petrology, especially his studies of Fe-Ti oxides in volcanic rocks. The new mineral occurs as platy, cinnamon-brown inclusions in mantle-derived pyrope crystals, which also contain inclusions of rutile, srilankite, ilmenite, minerals of the crichtonite group, spinel, and olivine. Carmichaelite is commonly in contact with rutile and occasionally with srilankite. The average composition is TiO2 = 62.16, Cr2O3 = 18.43, Al2O3 = 1.88, V2O3 = 0.87, Nb2O5 = 0.37, FeO = 7.61, MgO = 2.80, H2O (calc) = 5.76, total 99.89 wt%, giving an empirical formula close to Ti0.62Cr0.19Fe0.09Mg0.06Al0.03V0.01O1.5(OH)0.5. A cation (non-H) to (O+OH) ratio of 1:2 was confirmed by microprobe analysis of oxygen, and the presence of the hydroxyl component was supported by IR data. Carmichaelite is monoclinic, space group P21/c, with a = 7.706(1), b = 4.5583(6), c = 20.187(3) A, β = 92.334(2)°, V = 708.5(3) A 3 , and Z = 22. The calculated density is 4.13 g/cm 3 . The strongest diffraction lines [ d(I, h k l)] from a calculated powder pattern are 2.842 (100, ‐3.773 (94, 013), 2.664 (70, 213), 1.688 (54, ‐1.679 (44, 226), 1.661 (44, ‐and 1.648 (34, 1.1.11). The crystal structure, which has been determined from single-crystal X-ray diffraction data, consists of stacked chains of cation octahedra within layers of hexagonal closest-packed anions. The arrangement of cation octahedra within a single chain has the same basic structural unit of five edge-sharing octahedra found in olivine, the humite group, and leucophoenicite. Carmichaelite is inferred to have co-precipitated with its pyrope host and other titanates in the presence of a fluid or melt phase (i.e., mantle metasomatism) at temperatures of 650‐750 °C and pressures of 20‐25 kbar. It contains the highest OH content reported for titanates of mantle origin and offers a potential storage site for water and high field strength elements in the upper mantle. The new mineral is probably stabilized by high pressure and the high concentration of Cr. Certain features of the carmichaelite structure raise the possibility of a similar high-pressure phase in the Si-AlO-H system.

Mineral inclusions in pyrope crystals from Garnet Ridge, Arizona, USA: implications for processes in the upper mantle

Mineral inclusions in pyrope crystals from Garnet Ridge in the Navajo Volcanic Field on the Colorado Plateau are investigated in this study with emphasis on the oxide minerals. Each pyrope crystal is roughly uniform in composition except for diffusion halos surrounding some inclusions. The pyrope crystals have near constant Ca:Fe:Mg ratios, 0.3 to 5.7 wt% Cr2O3, and 20 to 220 ppm H2O. Thermobarometric calculations show that pyrope crystals with different Cr contents formed at different depths ranging from 50 km (where T ≈ 600 °C and P = 15 kbar) to 95 km (where T ≈ 800 °C and P = 30 kbar) along the local geotherm. In addition to previously reported inclusions of rutile, spinel and ilmenite, we discovered crichtonite series minerals (AM21O38, where A = Sr, Ca, Ba and LREE, and M mainly includes Ti, Cr, Fe and Zr), srilankite (ZrTi2O6), and a new oxide mineral, carmichaelite (MO2−x(OH)x, where M = Ti, Cr, Fe, Al and Mg). Relatively large rutile inclusions contain a significant Nb (up to 2.7 wt% Nb2O5), Cr (up to ∼6 wt% Cr2O3), and OH (up to ∼0.9 wt% H2O). The Cr and OH contents of rutile inclusions are positively related to those of pyrope hosts, respectively. Needle- and blade-like oxide inclusions are commonly preferentially oriented. Composite inclusions consisting mainly of carbonate, amphibole, phlogopite, chlorapatite, spinel and rutile are interpreted to have crystallized from trapped fluid/melt. These minerals in composite inclusions commonly occur at the boundaries between garnet host and large silicate inclusions of peridotitic origin, such as olivine, enstatite and diopside. The Ti-rich oxide minerals may constitute a potential repository for high field strength elements (HFSE), large ion lithophile elements and light rare earth elements (LREE) in the upper mantle. The composite and exotic oxide inclusions strongly suggest an episode of metasomatism in the depleted upper mantle beneath the Colorado Plateau, contemporaneous with the formation of pyrope crystals. Our observations show that mantle metasomatism may deplete HFSE in metasomatic fluids/melts. Such fluids/melts may subsequently contribute substantial trace elements to island arc basalts, providing a possible mechanism for HFSE depletion in these rocks.

Diffusion of the hydrous component in pyrope

Other| June 01, 1996 Diffusion of the hydrous component in pyrope Liping Wang; Liping Wang University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States Search for other works by this author on: GSW Google Scholar Youxue Zhang; Youxue Zhang Search for other works by this author on: GSW Google Scholar Eric J. Essene Eric J. Essene Search for other works by this author on: GSW Google Scholar Author and Article Information Liping Wang University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States Youxue Zhang Eric J. Essene Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online Issn: 1945-3027 Print Issn: 0003-004X Copyright © 1996 by the Mineralogical Society of America American Mineralogist (1996) 81 (5-6): 706–718. https://doi.org/10.2138/am-1996-5-618 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Liping Wang, Youxue Zhang, Eric J. Essene; Diffusion of the hydrous component in pyrope. American Mineralogist 1996;; 81 (5-6): 706–718. doi: https://doi.org/10.2138/am-1996-5-618 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Dehydrogenation experiments have been performed on natural pyrope megacrysts (Py70Alm16Gr14) containing 22-112 ppm total H20 by weight (H/Si is from 0.00035 to 0.0018). The concentrations of OH in pyrope crystals were determined by Fourier-trans-form infrared spectroscopy. Two methods were used to obtain diffusivities. One method involved measurement of OH concentration profiles after a heating experiment. The dif-fusivity of the hydrous component was found to be proportional to the OH concentration along each profile, indicating that the diffusing species is not OH and is probably a minor or trace species with a concentration proportional to the square of the OH concentration (such as H2). Hence, the diffusivities are referred to as the apparent diffusivities (D*, which equals D*0C/C0, where C0 is the initial OH concentration and D*0 is the D* when C = C0). The other method involved measurement of the overall decrease of OH content across a wafer as a function of cumulative heating duration. From the overall decrease of OH content, the apparent bulk diffusion-out diffusivities (D*out) were obtained. (D*out,D*in, and D*0 are different if D* depends on concentration.) The diffusivities from the two methods are consistent for a given pyrope crystal. However, the D*out values in different pyrope crystals are roughly inversely proportional to the initial OH content in the crystal, opposite the concentration dependence of D* along a diffusion profile. This apparent paradox places strong constraints on the diffusion and incorporation mechanism of the hydrous component in pyrope and indicates a control on the concentration of the diffusing species by factors other than OH content (such as Fe3+/Fe2+). Any proposed diffusion mechanism must be able to explain this apparent paradox (both the proportionality and the inverse proportionality).The D*out values (in squared micrometers per second) in a crystal with 82–90 ppm initial total H2O can be described by In D*out = [(28.20 ± 1.30) − (30580 ± 1450)]/T (where Tis temperature in kelvins and errors are at the 2σ level). The D*out values are greater by a factor of ∼ 3 in a crystal with 22–23 ppm initial total H2O. The D*0 values can be determined by dividing the D*out values by 0.347, and D* at each concentration can be determined from D* = D*0C/Co. The average activation energy for hydrous component diffusion in the two crystals is 253 ± 13 (2σ) kJ/mol. The diffusion-out diffusivities of the hydrous component are seven to eight orders of magnitude greater than the Fe-Mg interdiffusivities. They are so large that the OH content of a pyrope crystal can adjust to changing environmental conditions on a time scale of hours at temperatures as low as 800 °C. Pyrope crystals from the mantle may dehydrogenate during ascent. Caution should be exercised in using OH content in natural pyrope crystals to infer conditions of the source region. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Single crystal absorption spectra of synthetic Ti, Fe-substituted pyropes

Synthetic pyrope crystals up to 0.5 mm in diameter, substituted by titanium or by titanium plus iron, were grown under defined conditions of P, T, $$f_{O_2 }$$ in the presence of water using a piston-cylinder device. The crystals were characterized by X-ray and microprobe techniques. Their single-crystal optical absorption spectra were measured by means of a microscope-spectrometer. Two absorption bands at 16100 and 22300 cm{cm-1} in the spectra of pale-blue Fe-free Ti-bearing pyropes, grown under reduced conditions, were identified as originating from spin-allowed transitions, derived from 2 T 2g → 2 E g of octahedral Ti3+ ions. The splitting value of the excited 2E g state, 6200 cm-1, and the crystal field parameter of Ti3+ in pyrope Δ 0 = 19 200 cm-1 are both in agreement with literature data. In spectra of brown Fe, Ti-bearing garnets, a broad band at 23000 cm-1 was interpreted as a Fe2+[8] → Ti4+[6] charge-transfer band. The spectral position and width of this band agree with those observed for a FeTi charge transfer band in natural garnets. Fe, Ti-containing garnets synthesized at relatively high oxygen fugacity (10-11,0 atm), which permits a fraction of Fe3+ to enter the garnet, show an additional Fe2+[8] → Fe3+[6] charge transfer band at 19800 cm-1.

Titanium substitution and OH-bearing defects in hydrothermally grown pyrope crystals

A series of Ti-substituted pyrope crystals was synthesized in the system MgO-(Na2O)-Al2O3-TiO2-SiO2-H2O at PH20 = Ptot between 25 and 30 kbars and 975 and 1000° C, using graphite heated piston-cylinder devices. The crystals, ranging up to 500 μm in diameter, were studied by X-ray, electron-microprobe and FTIR-microscope spectrometric techniques. The pyrope crystals were colourless when hem/mt or mt/wu buffers were used during the synthesis, and pale blue with the wu/iron buffer and in unbuffered runs. Sodium was not found in the synthetic crystals, titanium was always near 0.06 Ti atoms pfu, independent on the Ti-excess used in the starting material. A substitution Al2+[6]+Si4+[4]+4O2-= Ti4+[6]+□[4]+[(OH)3O^5-, providing charge balance for octahedral Ti4+-substitution is found to be compatible with all properties (number, widths, position, integrated intensity) of the stretching vibrations of defect hydroxyls, which have energies 3684, 3568, 3525 cm-1.

The structure analyses of pyrope (Mg3Al2Si3Ol2) and grossular (Ca3Al2Si3O12) glasses by X-ray diffraction method.

The structures of Mg3Al2Si3O12 and Ca3Al2Si3O12 glasses were investigated by means of the radial distribution function (RDF) analysis based on X-ray diffraction data. The Al and Si atoms in these glasses are essentially in four-fold coordination. The T–O (T=A1 and Si) distance of Mg3Al2Si3O12 glass (1.68A) estimated from the peak position of radial distribution function curve is almost equal to that of Ca3Al2Si3O12 glass (1.70A). These T–O distances are consistent with the common average T–O distance in AlO4 and SiO4 tetrahedra. Mg–O distance in Mg3Al2Si3O12 glass is 2.1A and coordination number of Mg atom is 4.8. These values are smaller than those in pyroxene and pyrope crystals. Ca–O distances in Ca3Al2Si3O12 glass is 2.4A and coordination number of Ca atom is 7.1–7.4. These values are in conformity with those obtained in grossular and diopside crystals. The short range structures of Mg3Al2Si3O12 and Ca3Al2Si3O12 glasses, the oxygen coordination around T, Mg, and Ca atoms resemble those of MgSiO3 and CaSiO3 glasses, respectively. Especially, the G(r) curve of Mg3Al2Si3O12 glass is very similar to that of MgSiO3 glass.

The pyrope-coesite rocks and their country rocks at Parigi, Dora Maira Massif, Western Alps: detailed petrography, mineral chemistry and PT-path

Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits.

Genesis of carbonate in pyrope from ultramafic diatremes on the Colorado Plateau, southwestern United States

Mineral associations and compositions of carbonates within pyrope crystals are clues to the genesis of mantle carbonate and to the character of metasomatic melts in depleted peridotite. The pyrope crystals are in ultramafic diatremes of the Navajo field on the Colorado Plateau. Although inclusions of olivine and pyroxene are typically monomineralic, 4 of 6 inclusions of carbonates and hydrates are polymineralic. Polymineralic assemblages include: pargasite-magnesite-dolomite-apatite-spinel; pargasite-dolomite-Ba phlogopite (with 10% BaO); olivine-dolomite-spinel; edenite-chlorite; and olivine-ilmenite-spinel. Magnesite and chlorite are present also as monomineralic inclusions. The two inclusions with pargasite plus carbonate are in the same garnet; the association of carbonates plus hydrates and the enrichment in Ba are evidence that the included minerals originated from melt trapped in pyrope. The pargasite and mica are F-poor and contain about 0.4 and 1.1 wt% Cl, respectively, more than any other analyzed mantle amphibole or mica. If the parent melts of such inclusions are similar to those responsible for trace-element metasomatism of continental lithosphere, then these melts have higher Cl/F ratios than those inferred from typical xenolith minerals. Amphibole-garnet and olivine-spinel equilibration temperatures are in the range 500–700° C, so the garnets cooled to low temperatures within the mantle following inclusion of melt. All the hydrates and carbonates may have formed from trapped melt, but evidence is strong only for the complex pargasite-carbonate-mica inclusions. Two garnets containing chlorite are more Cr-rich and Fe-poor than most other inclusion-bearing pyropes, and the chlorite may have been included during prograde metamorphism of subducted lithosphere.

Ellenbergerite, a new high-pressure Mg-Al-(Ti,Zr)-silicate with a novel structure based on face-sharing octahedra

Ellenbergerite occurs as purple millimetre-size grains associated with talc, kyanite, clinochlore, rutile, and zircon in composite inclusions within decimetre-large pyrope crystals (90–98 mole percent end-member) in the quartzite layer of the Dora Maira massif, Western Alps, from which coesite has been recently reported (Chopin 1984). It is hexagonal, a=12.255(8), c=4.932(4) A, Z=1, space group P63. Mohs hardness 6.5; Dmes 3.15, Dcal 3.10; no cleavage. Uniaxial negative and vividly pleochroic,ω colourless,e colourless to deep lilac with colour zoning. The intensely coloured variety hasω 1.6789(5),e 1.670(1); microprobe analysis yields SiO2 39.1, P2O5 0.45, Al2O3 25.1, TiO2 4.0, MgO 22.2, FeO 0.20, sum 99.05 wt.% including H2O 8.0 (coulometrically). The formula calculated on a O28(OH)10 basis (implying 7.5 wt.% H2O) is Mg6.71 Fe0.03 Ti0.61 Al6.00 Si7.92 P0.08 O28(OH)10 The colour zoning is due to nearly complete Ti⇌Zr substitution. In addition ellenbergerite may contain more than 8 wt.% P2O5 with strictly correlated changes of Si, Mg, Al and Ti+Zr contents, over 80% of which represent the SiAl⇌PMg substitution.