Crystal-chemical complexity in natural garnets: structural constraints on chemical variability

Merli, Marcello; Callegari, Athos Maria; Cannillo, Elio; Caucia, Franca; Leona, Marco; Oberti, Roberta; Ungaretti, L.

doi:10.1127/ejm/7/6/1239

Cited by 46 publications

(32 citation statements)

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“…The experimental procedure was previously applied in high-pressure crystallographic studies using diamond anvil cells and on a single olivine inclusion in diamond (Nestola et al, 2011). The structure data, together with the formulation of an improved structure-composition model for garnet (according to Merli et al, 1995) and the very reasonable assumption that the garnet is under relatively low internal pressure (below 1 GPa), allowed us to estimate the chemical composition as, expressed as end-member fractions, Pyr 0.41(5) Alm 0.36(7) Gro 0.22(1) Uva 0.01(1) . Uncertainties are higher than for a conventional electron microprobe analysis, but sufficiently small for a precise classification of the garnet and thus broad petrological purposes, without having to destroy the inclusion as would be required by conventional chemical analysis.…”

Section: Discussion and Concluding Remarksmentioning

confidence: 99%

“…Based on the work by Merli et al (1995), we present an updated automatic procedure for the determination of reliable garnet compositions starting from crystal-structure refinement data. The procedure is based on multiple regression equations for the independent estimates of the unit-cell edges and variable structural parameters (i.e., the fractional coordinates of the oxygen atom) as a function of the cation content expressed in terms of garnet end-members.…”

Section: X-ray Micro-tomographymentioning

confidence: 99%

“…The structure refinement was performed in the Ia3d space group using the SHELXL software (Sheldrick, 2008). Ionic and neutral scattering factors were used for cations (Mg and Mg 2þ , Fe and Fe 2þ , Ca and Ca 2þ , Al and Al 3þ , Cr and Cr 3þ , Si and Si 4þ ) and for oxygen (O and O 2-) as suggested in Merli et al (1995).…”

Section: Single-crystal X-ray Diffractionmentioning

confidence: 99%

“…De Stefano et al, 2009) Relevant geometrical parameters (i.e., the unit-cell edge a and fractional atomic coordinates of oxygen x(O), y(O), z(O)) were fitted using a model based on the classical Margules formalism, which takes into account the interaction between paired end-members. The following refined parameters were obtained by fitting up to 281 selected experimental data reported in Merli et al (1995):…”

Section: X-ray Micro-tomographymentioning

confidence: 99%

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In situ analysis of garnet inclusion in diamond using single-crystal X-ray diffraction and X-ray micro-tomography

Nestola¹,

Merli²,

Nimis³

et al. 2012

ejm

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A single crystal of garnet enclosed in a diamond from the Jericho kimberlite (Slave Craton, Canada) has been investigated using X-ray diffraction and X-ray micro-tomography. The novel experimental approach allowed us to determine the crystal structure of the garnet. The unit-cell edge a and fractional atomic coordinates of oxygen were used to determine the composition via an updated Margules model for garnets. The composition is Pyr 0.41(5) Alm 0.36(7) Gro 0.22(1) Uva 0.01(1) , which is indistinguishable from the eclogitic garnets found in other Jericho diamonds. We also demonstrated that residual pressures on the inclusion of up to 1 GPa do not affect significantly the determination of the garnet composition by structure refinement.

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Section: Discussion and Concluding Remarksmentioning

confidence: 99%

Section: X-ray Micro-tomographymentioning

confidence: 99%

Section: Single-crystal X-ray Diffractionmentioning

confidence: 99%

Section: X-ray Micro-tomographymentioning

confidence: 99%

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In situ analysis of garnet inclusion in diamond using single-crystal X-ray diffraction and X-ray micro-tomography

Nestola¹,

Merli²,

Nimis³

et al. 2012

ejm

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“…The ferric ions reduce to ferrous ions, but, apparently, they do not stay in the same octahedral site as there was no evidence in the Mössbauer spectra, where it is not possible to ascribe any doublet to Fe 2+ in the octahedral site. The other factor controlling the instability of Prp 80 Alm 20 garnet involves the different ion size of Ca 2+ (1.12 Å), Fe 2+ (0.92 Å), and Mg 2+ (0.89 Å), which could increase the strain and distortion of the garnet lattice, affecting not only the dodecahedral sites, but also the octahedral and even tetrahedral sites [26]. …”

Section: Variation Of Onset Temperature With Respect To the Fe Contenmentioning

confidence: 99%

The influence of the iron content on the reductive decomposition of A3−xFexAl2Si3O12 garnets (A = Mg, Mn; 0.47 ≤ x ≤ 2.85)

Aparicio¹,

Filip²,

Mashlan³

et al. 2014

AIP Conference Proceedings

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Abstract. Thermally-induced reductive decomposition of natural iron-bearing garnets of the almandine-pyrope and almandine-spessartine series were studied at temperatures up to 1200 °C (heating rate of 10 °C/min) under atmosphere of forming gas (10% of H 2 in N 2 ). Crystallochemical formula of the studied garnet was calculated as VIII (A 3-x Fe 2+x ) VI (Al, Fe 3+ ) 2 Si 3 O 12 , where the amount of Fe 3+ in the octahedral sites is negligible with the exception of pyrope, A = Mg, Mn, and 0.47 x 2.85. The observed decomposition temperature, determined from differential scanning calorimetry and thermogravimetry, is greater than 1000 °C in all cases and showed almost linear dependence on the iron content in the dodecahedral sites of the studied garnets, with the exception of garnet with a near-pyrope composition (Prp 80 Alm 20 ). The initial garnet samples and decomposition products were characterized in details by means of X-ray powder diffraction and 57 Fe Mössbauer spectroscopy. We found that all studied garnets have common decomposition products such as metallic iron (in general, rounded particles below 4 μm) and Fe-spinel; the other identified decomposition products depend on starting chemical composition of the garnet: Fe-cordierite, olivine (fayalite or tephroite), cristobalite, pyroxene (enstatite or pigeonite), and anorthite. Anorthite and pigeonite were only present in garnets with Ca in the dodecahedral site. All the identified phases were usually well crystallized.

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Determination of skarn garnet compositions using Raman spectroscopy

Zhu

Cook

Xie

et al. 2022

J Raman Spectroscopy

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Garnet is a key and defining mineral of skarns and associated metalliferous deposits. Variation in garnet composition, commonly expressed by the proportions of different end‐members, is widely used to determine the physical–chemical features of hydrothermal fluids. Skarn garnets from the Fujiashan W‐Cu‐Mo deposit, eastern China, were investigated by Raman spectroscopy and electron probe microanalysis to assess the quantitative correlation between Raman band positions and proportions of garnet end‐members. Compositions and Raman band positions of so‐called “grandite” garnet (Adr18–98Grs0–79), where Adr and Gr are the end‐members andradite and grossular, respectively, display a spatial zonation at Fujiashan that correlates with the distance from the contact between skarn and causative intrusion. Raman band positions determined in the ranges 234–246, 351–368, 369–375, 515–544, 814–826, and 873–882 cm−1 demonstrate moderate to very strong (R2 up to 0.99) linear correlations with mol% andradite and grossular components. This is attributed to homovalent substitution between Fe3+ and Al3+ in the octahedral site, which has an indirect effect on bond lengths and angles of Si‐O vibration, resulting in linear variation of Raman band positions. The band between 515 and 544 cm−1 is the most sensitive to compositional variation, and its position enables robust estimation of end‐member proportions within 10% of results calculated from electron probe microanalysis. This research highlights the potential of Raman spectroscopy as a rapid, powerful method to assess the composition of skarn garnet, enabling accelerated construction of spatial zonation models for skarns during skarn deposit exploration.

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Crystal-chemical complexity in natural garnets: structural constraints on chemical variability

Cited by 46 publications

References 0 publications

In situ analysis of garnet inclusion in diamond using single-crystal X-ray diffraction and X-ray micro-tomography

In situ analysis of garnet inclusion in diamond using single-crystal X-ray diffraction and X-ray micro-tomography

The influence of the iron content on the reductive decomposition of A3−xFexAl2Si3O12 garnets (A = Mg, Mn; 0.47 ≤ x ≤ 2.85)

Determination of skarn garnet compositions using Raman spectroscopy

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