Ti-rich Garnets Research Articles

Not Only Garnets…

Abstract Garnets have been known to man since time immemorial and are used in a wide variety of applications as well as being prototypes of useful synthetic materials. Our investigations show that in nature, garnets and minerals with a langasite-type structure can be very close in composition. Examples are cubic Ti-rich garnets with the common formula Ca3(Ti4+,Fe3+,Al)2(Si,Fe3+,Al)3O12 and the new trigonal mineral qeltite, Ca3Ti(Fe3+2Si)Si2O14, which occur in paralavas of the pyrometamorphic Hatrurim Complex, Israel. Synthetic compounds of the langasite family are important because of their functional properties, such as unique piezoelectricity, high thermal stability, and low acoustic losses, as well as optical nonlinearity and multiferroicity. Qeltite is the first high-temperature terrestrial mineral with a langasite-type structure, the description of which was a catalyst for the discovery in pyrometamorphic rocks of the Hatrurim Complex of a whole series of new natural phases with langasite-type structure and varied composition (A3BC3D2O14, where A = Ca and Ba; B = Ti, Nb, Sb, and Zr; C = Ti, Al, Fe, and Si; and D = Si). We think that qeltite and other minerals with langasite-type structure may be relatively widely distributed in terrestrial rocks that form under similar conditions to those of Ti-rich garnet but are missed by researchers.

Thaumasite in calcic skarns from the Zvezdel-Pcheloyad Pb-Zn deposit, Eastern Rhodopes, Bulgaria

Thaumasite was found in zoned calcic skarns hosted by the monzonitic rocks of the Zvezdel pluton, Eastern Rhodopes. Associated minerals are wollastonite, clinopyroxene, grossular-andradite garnets, Ti-rich garnets, plagioclase, calcite, quartz, epidote, prehnite, and chlorite. Titanite, apatite and magnetite are present as accessories. The mineral has been characterized using optical microscopy, PXRD, SEM/EDS, EPMA, and FTIR. Thaumasite forms sheaf-like and fan-shaped aggregates, which consist of acicular crystals elongated along the c axis. The IR spectrum of thaumasite is in accordance with the chemical composition and contains absorption bands in the range 3500–3291 cm–1 (O–H stretching vibrations); 1683, 1650 cm–1 (bending vibrations of H2O); 1389 cm–1 (stretching vibrations of CO32–); 1100, 1069 cm–1 (asymmetric stretching vibrations of SO42–); 882 cm–1 (symmetric bending vibration of CO32–); 768, 742, 680 cm–1 (Si–O stretching vibrations of Si(OH)6 octahedra); 670 cm–1 (bending vibrations of SO42–).

Major, Trace, and Rare-Earth Element Geochemistry of Nb-V Rich Andradite-Schorlomite-Morimotoite Garnet from Ambadungar-Saidivasan Alkaline Carbonatite Complex, India: Implication for the Role of Hydrothermal Fluid-Induced Metasomatism

In situ major, trace and rare-earth element composition of Ti-rich garnets from Ambadungar-Saidivasan alkaline carbonatite complex (ASACC) are presented to constrain its likely genesis. The garnets are characterized by high andradite (42.7–57.3), schorolomite (22.0–31.0), and morimotoite (15.6–26.5) end members. No distinct chemical zonation is noticed except for minor variations in Ti content. The garnets are enriched in LREE (average 731 ppm) and relatively depleted in HREE (average 186 ppm) and show an M-type first tetrad that leads to a convex upward pattern between Ce and Gd. Mildly positive to no Eu anomalies are observed (Eu/Eu* = 1.06–1.17). The REE patterns (LaN/YbN = 1.11–2.11) are similar to those of garnets from skarn deposits. The presence of tetrad effect in the LREE pattern suggests an active role of metasomatic processes involving hydrothermal fluids during the growth of the garnets. These garnets also contain high Nb (282–2283 ppm) and V (1083–2155 ppm) concentrations, which stand out against the composition of the host rock. Therefore, late-stage metasomatic reactions of earlier formed minerals with hydrothermal fluid enriched in Fe, Si, LREE, Nb, V, and Ti led to the formation of garnet. The primary source for these elements could be magnetite, ilmenite, and pyrochlore present in different varieties of carbonatites in the ASACC, with the required elements being released during their interaction with the hydrothermal fluid. The hydrothermal fluid was likely to be moderately acidic, and having fluoride and sulfate as the primary ligands.

Titanium-rich phases in the Earth's transition zone and lower mantle: Evidence from experiments in the system MgO–SiO2–TiO2(±Al2O3) at 10–24 GPa and 1600 °C

Phase relations in the MgSiO3–MgTiO3 and Mg3Al2Si3O12–MgTiO3 systems were studied at 10–24 GPa and 1600 °C using a high-pressure Kawai-type multianvil apparatus. We investigated the full range of starting compositions for the enstatite-geikielite system to derive a P–X phase diagram and synthesize titanium-bearing phases, such as olivine/wadsleyite, rutile, pyroxene, MgTiSi2O7 weberite, bridgmanite and MST-bridgmanite in a wide pressure range. Olivine and pyroxene in run products are characterized by a low titanium content (<0.6 and <0.3 wt% TiO2, respectively) whereas the content of TiO2 in wadsleyite reaches 2 wt% at 12 GPa. The concentration of Ti in MgTiSi2O7 weberite decreases with pressure from 52 wt% TiO2 at 14 GPa to 43 wt% TiO2 at 18 GPa. Two perovskite-type structures (MgSiO3 bridgmanite and Mg(Si,Ti)O3 bridgmanite) were detected in the studied system. MgSiO3 bridgmanite (Brd) is formed at a pressure of >20 GPa and characterized by significant titanium solubility (up to 13 wt% TiO2 at 24 GPa). Mg(Si,Ti)O3 perovskite is formed at a pressure of >17 GPa. The concentration of TiO2 in this phase varies from 29 wt% to 49 wt%. It was found that addition of Ti to the system moves the boundaries of Ol/Wad phase transformations to lower pressures. Addition of Al to the starting material allows us to simulate the composition of natural Ti-rich garnets and bridgmanites. It is important to note that garnet in the Prp-Gkl system is stable throughout a wide pressure range (10–24 GPa). Al incorporation does not affect the distribution of titanium between two types of bridgmanite. It is shown that high contents of Ti stabilize bridgmanite-like compounds at considerably lower pressure than that at the lower mantle/transition zone boundary. Our experiments simulate the composition of natural Ti-rich primary garnet found in eclogite from the Sulu ultrahigh-pressure (UHP) terrane.

In-situ major and trace element chemistry of melanite from Tieshan Fe–Cu skarn deposit, Hubei Province, Eastern China: Implications for hydrothermal fluid evolution

The Tieshan Fe–Cu skarn deposit is located in the Edong ore district, the largest ore district of the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in Eastern China. The Fe–Cu mineralization is spatially associated with the Tieshan pluton, which intruded the carbonates of the Lower Triassic Daye Formation. The orebodies are predominantly hosted along the intrusive contact zone between the (quartz)-diorite and (dolomitic)-marble. EMPA analysis shows that the Tieshan garnets belong to high-Ti andradite, with TiO2 contents ranging from 0.03 to 2.59 wt%, and is designated as melanite. The melanite contains low Na content, suggesting the absence of Na+-REE3+ coupled substitution. LA–ICP–MS analysis shows high total LREE (∑LREE), low ∑HREE and positive Eu anomalies, suggesting high fO2 and rapid growth of the melanite. Compared with other Ti-rich garnets worldwide, the Tieshan melanite grains are more Al-rich and Si-rich, and shows negative SiO2 vs. TiO2 correlation, which suggests predominant Ti-substitution in the tetrahedral site. The more positive Eu anomalies and higher percentage of andradite are present in the rim (than in the core), indicating lower salinities and higher fO2 of the late-stage ore-forming fluid. The magmatic-hydrothermal fluid source of these garnets supports the metasomatic ore genesis at Tieshan.

Origin of Ti-rich garnets in the groundmass of Wajrakarur field kimberlites, southern India: insights from EPMA and Raman spectroscopy

Although Ti-rich garnets are commonly encountered in the groundmass of many alkaline igneous rocks, they are comparatively rare in kimberlites. Here we report on the occurrence of Ti-rich garnets in the groundmass of the P-15 and KL-3 kimberlites from the diamondiferous Wajrakarur field in the Eastern Dharwar craton of southern India. These garnets contain considerable Ti (11.7–23.9 wt.% TiO2), Ca (31.3–35.8 wt.% CaO), Fe (6.8–15.5 wt.% FeOT) and Cr (0.04–9.7 wt.% Cr2O3), but have low Al (0.2–5.7 wt.% Al2O3). In the case of the P-15 kimberlite they display a range in compositions from andradite to schorlomite, with a low proportion of grossular (andradite(17.7–49.9)schorlomite(34.6–49.5)-grossular(3.7–22.8)-pyrope(1.9–10.4)). A few grains also contain significant chromium and represent a solid solution between schorlomite and uvarovite. The Ti-rich garnets in the KL-3 kimberlite, in contrast, are mostly schorlomitic (54.9─90.9 mol %) in composition. The Ti-rich garnets in the groundmass of these two kimberlites are intimately associated with chromian spinels, perhaps suggesting that the garnet formed through the replacement of spinel. From the textural evidence, it appears unlikely that the garnets could have originated through secondary alteration, but rather seem to have formed through a process in which early magmatic spinels have reacted with late circulating, residual fluids in the final stages of crystallization of the kimberlite magma. Raman spectroscopy provides evidence for low crystallinity in the spinels which is likely to be a result of their partial transformation into andradite during their reaction with a late-stage magmatic (kimberlitic) fluid. The close chemical association of these Ti-rich garnets in TiO2-FeO-CaO space with those reported from ultramafic lamprophyres (UML) is also consistent with results predicted by experimental studies, and possibly implies a genetic link between kimberlite and UML magmas. The occurrence of Ti-rich garnets of similar composition in the Swartruggens orangeite on the Kaapvaal craton in South Africa, as well as in other kimberlites with an orangeitic affinity (e.g. the P-15 kimberlite on the Eastern Dharwar craton in southern India), is inferred to be a reflection of the high Ca- and high Ti-, and the low Al-nature, of the parent magma (i.e. Group II kimberlites).

A multi-analytical study of the crystal structure of unusual Ti–Zr–Cr-rich Andradite from the Maronia skarn, Rhodope massif, western Thrace, Greece

Unusual Ti–Cr–Zr-rich garnet crystals from high-temperature melilitic skarn of the Maronia area, western Thrace, Greece, were investigated by electron-microprobe analysis, powder and single-crystal X-ray diffraction, IR, Raman and Mossbauer spectroscopy. Chemical data showed that the garnets contain up to 8 wt.% TiO2, 8 wt.% Cr2O3 and 4 wt.% ZrO2, representing a solid solution of andradite (Ca3Fe3+ 2Si3O12 ≈46 mol%), uvarovite (Ca3Cr2Si3O12 ≈23 mol%), grossular (Ca3Al2Si3O12 ≈10 mol%), schorlomite (Ca3Ti2[Si,(Fe3+,Al3+)2]O12 ≈15 mol%), and kimzeyite (Ca3Zr2[Si,Al2]3O12 ≈6 mol%). The Mossbauer analysis showed that the total Fe is ferric, preferentially located at the octahedral site and to a smaller extent at the tetrahedral site. Single-crystal XRD analysis, Raman and IR spectroscopy verified substitution of Si mainly by Al3+, Fe3+ and Ti4+. Cr3+ and Zr4+ are found at the octahedral site along with Fe3+, Al3+ and Ti4+. The measured H2O content is 0.20 wt.%. The analytical data suggest that the structural formula of the Maronia garnet can be given as: (Ca2.99Mg0.03)Σ=3.02(Fe3+ 0.67Cr0.54Al0.33Ti0.29Zr0.15)Σ=1.98(Si2.42Ti0.24Fe0.18Al0.14)Σ=2.98O12OH0.11. Ti-rich garnets are not common and their crystal chemistry is still under investigation. The present work presents new evidence that will enable the elucidation of the structural chemistry of Ti- and Cr-rich garnets.

Schorlomite: a discussion of the crystal chemistry, formula, and inter-species boundaries

Examination of schorlomite from ijolite at Magnet Cove (USA) and silicocarbonatite at Afrikanda (Russia), using electron-microprobe and hydrogen analyses, X-ray diffraction and Mossbauer spectroscopy, shows the complexity of substitution mechanisms operating in Ti-rich garnets. These substitutions involve incorporation of Na in the eightfold-coordinated X site, Fe2+ and Mg in the octahedrally coordinated Y site, and Fe3+, Al and Fe2+ in the tetrahedrally coordinated Z site. Substitutions Ti4+Fe3+Fe3+−1Si−1 and Ti4+Al3+Fe3+−1Si−1 are of major significance to the crystal chemistry of schorlomite, whereas Fe2+ enters the Z site in relatively minor quantities (<3% Fe∑). There is no evidence (either structural or indirect, such as discrepancies between the measured and calculated Fe2+ contents) for the presence of [6]Ti3+ or [4]Ti4+ in schorlomite. The simplified general formula of schorlomite can be written as Ca3Ti4+2[Si3-x(Fe3+,Al,Fe2+) x O12], keeping in mind that the notion of end-member composition is inapplicable to this mineral. In the published analyses of schorlomite with low to moderate Zr contents, x ranges from 0.6 to 1.0, i.e. Ti4+ in the Y site is 15 mol.%) proportions of andradite (Ca3Fe3+2Si3O12), morimotoite (Ca3Fe2+TiSi3O12), and Ca3MgTiSi3O12. The importance of accurate quantitative determination and assignment of Fe, Ti and other cations to the crystallographic sites for petrogenetic studies is discussed.

Complexly zoned Ti-rich melanite-schorlomite garnets from Ambadungar carbonatite-alkalic complex, Deccan Igneous Province, Gujarat State, Western India

Ti-rich garnet phenocrysts from a tephrite (‘nephelinite’) plug in the Ambadungar complex situated in the Chhota Udaipur alkalic subprovince show concentric zoning. Based on paired orthogonal traverses across three selected crystals (total 81 step-scan point analyses), andradite content ranges from 55 to 86 mol% (the remainder being almost entirely schorlomite), corresponding to the following wt% oxide variations: TiO 2 5.5–15.8, CaO 29.6–32.5, MgO 0.3–1.6, Fe 2O 3 20–26, Al 2O 3 0.7–3.5%; MnO, V 2O 3, Na 2O and ZrO 2 each rarely exceeds 0.5%. Zoning patterns in individual grains from this one rock differ considerably in several ways: (1) there may be 2–5 alternating pale and dark zones, the pale generally being more andradite-rich enriched in Ti, Mg and usually Zr but impoverished in Al; (2) the two orthogonal traverse may or may not be mirror images; (3) monotonic trends (decreasing Ca, Al, increasing Mg, Zr from core to rim) may or may not be present; (4) oscillatory zoning varies in amplitude; and (5) apparent substitutions differ, although all crystals clearly show the Si–Ti substitution inferred for Ti-rich garnets elsewhere. An idealized case is developed from an observed complexly zoned phenocryst population to piece together a history of the alkaline host magma that experienced several events such as polybaric differentiation, magma-mixing, and kinetic effects.

Hydroxyl defects in garnets from mantle xenoliths in kimberlites of the Siberian platform

A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v OH vibrational range in search of the occurrence, energy and intensity of the v OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the Siberian platform. The v OH single-crystal spectra show either one or a combination of two or more of the following major v OH bands, I 3645–3662 cm−1, II 3561–3583 cm−1, III 3515–3527 cm−1, and minor bands, Ia 3623–3631 cm−1, IIa 3593–3607 cm−1. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO2 > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v OH spectra do not correspond to those of OH− defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications of v OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems I to III. Instead, they exhibit one weak, broad band (Δv OH 200–460 cm−1) near 3570 cm−1, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of all v OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between values below the detection limit of our single-crystal IR method, near 2 × 10−4 wt%, up to 163 × 10−4 wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role. Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of the graphite-pyrope facies. This latter result (2) may point to significantly lower f H2O and f O2 in the former as compared to the latter facies.

Iron site populations from Mössbauer spectroscopy in Ti-bearing garnets from Mt Vulture (Italy)

The present work concerns the determination of the cation site populations and crystal chemical formulae of some Ti-rich silicate garnets. The samples come from Mt. Vulture, a volcanic complex located on the east side of the Lucania Apennines (south Italy). Mossbauer spectroscopy (MS) has been used in order to obtain the iron site occupancies. Two different models for interpreting the Fe2+ peak positions in the Mossbauer spectra are compared, and some methodological aspects are discussed. The derived iron distributions have then been combined with the data obtained by electron probe microanalysis (EPMA) and single-crystal X-ray diffraction analysis (XRD) to obtain consistent crystal chemical formulae for these compounds.

The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa

166 garnets of dominantly mantle origin were analyzed for OH content by infrared (IR) spectroscopy. IR spectra in the 3400–3700 cm^(-1) region display consistent absorption patterns attributable to OH structurally bound within the garnet crystal, occasionally contaminated by low intensity OH absorptions from microscopic inclusions. The principal structural OH absorption occurs near 3570 cm^(-1), with the appearance of additional absorptions near 3512 cm^(-1) and 3650 cm^(-1) dependent on garnet composition or paragenesis. Samples derive from a wide variety of rock types occurring as xenoliths in kimberlites of southern Africa. OH abundances, using the best currently available calibration, range from less than 1 up to 135 ppm H_2O, and increase in the general order as follows: on-craton eclogites Group 1 (basaltic) kimberlite>alnoite>alkali basalt. The OH contents of common lithospheric granets from coarse peridotites, including several phlogopite-bearing samples are typically less than 20 ppm H_2O, for tectonic settings of kimberlites both on and off the Archaean Kaapvaal craton. Ti-rich garnets from deformed peridotites are richer in OH, supporting previous suggestions of association of these xenoliths with putative megacryst magmas. Subcalcic Cr-rich xenocrysts, diamond inclusion garnets and garnets from diamondiferous eclogites have very low OH contents, similar to eclogites and depleted peridotites without macroscopic diamonds. The OH content of southern African peridotite and eclogite garnets are significantly lower on average than those previously examined from the Colorado Plateau diatremes. While details of emplacement-related H mobility in garnets remain to be established, our results suggest that garnets record useful information on the role of water or other hydrous volatile species in petrological processes at their source regions in the mantle. Although garnets do not appear to constitute a large reservoir of mantle hydrogen, the large stability range of OH-bearing garnet in the crust and mantle implies wide applicability as a qualitative hydrobarometer.