Structural mechanisms of solid solution and cation ordering in augite-jadeite pyroxenes; I, A macroscopic perspective

Ballaran, Tiziana Boffa; Carpenter, Michael A.; Domeneghetti, M. Chiara; Tazzoli, V.

doi:10.2138/am-1998-5-602

Cited by 55 publications

(30 citation statements)

References 29 publications

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“…At all temperatures where the P2/n phase is 56 stable, the M11 and M22 sub-sites are more ordered than the M21 and M22 sub-sites. It has been 57 observed in both experimental (Boffa Ballaran et al 1998) and computational (Vinograd et al 58 2007) studies that, at moderate temperatures, the relationship between cation order on the two sites 59 closely follows Q M1 = 2 Q M2 , where Q M1 and Q M2 are the long range order parameters of the M1 60 and M2 sites, respectively. 61…”

Section: Introduction 29mentioning

confidence: 95%

“…However, the slow 64 kinetics of cation diffusion in omphacite at low temperature inhibits the ordering process, and 65 natural omphacite always possess some cation disorder (Carpenter 1978). Vinograd et al (2007) 66 compared order parameters calculated using Monte Carlo simulations with values obtained for a 67 natural omphacite (Boffa Ballaran et al 1998), and concluded that cation ordering is kinetically 68 ineffective below 600 C. The order parameters decrease systematically to ~850 °C, where both 69 order parameters are effectively zero. 70…”

Section: Introduction 29mentioning

confidence: 99%

“…As both experiments 140 (eg. Rossi et al 1983;Boffa Ballaran et al 1998) and simulations (Vinograd et al 2007) have 141…”

Section: Introduction 29mentioning

confidence: 99%

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The effect of cation order on the elasticity of omphacite from atomistic calculations

Skelton

Walker

2015

Phys Chem Minerals

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9Omphacite, a clinopyroxene mineral with two distinct crystallographic sites, M1 and M2, and 10 composition intermediate between diopside and jadeite, is abundant throughout the Earth's upper 11 mantle, and is the dominant mineral in subducted oceanic crust. Unlike the end-members, 12 omphacite exists in two distinct phases, a P2/n ordered phase at low temperature and a high-13 temperature C2/c disordered phase. The crystal structure and full elastic constants tensor of ordered 14 P2/n omphacite have been calculated to 15 GPa using plane-wave density functional theory. Our 15 results show that several of the elastic constants, notably C 11 , C 12 , and C 13 deviate from linear-16 mixing between diopside and jadeite. The anisotropy of omphacite decreases with increasing 17 pressure and, at 10 GPa, is lower than that of either diopside or jadeite. The effect of cation disorder 18 is investigated through force-field calculations of the elastic constants of Special Quasirandom 19 2 Structures supercells with simulated disorder over the M2 sites only, and over both cation sites. 20These show that cation order influences the elasticity, with some components displaying particular 21 sensitivity to order on a specific cation site. C 11 , C 12 , and C 66 are sensitive to disorder on M1, while 22 C 22 is softened substantially by disorder on M2, but insensitive to disorder on M1. This shows that 23 the elasticity of omphacite is sensitive to the degree of disorder, and hence the temperature. We 24 expect these results to be relevant to other minerals with order-disorder phase transitions, implying 25 that care must be taken when considering the effects of composition on seismic anisotropy. 26

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Section: Introduction 29mentioning

confidence: 95%

Section: Introduction 29mentioning

confidence: 99%

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The effect of cation order on the elasticity of omphacite from atomistic calculations

Skelton

Walker

2015

Phys Chem Minerals

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“…Single-crystal studies were preferred, as they provide accurate cell parameters and quasi-homogeneous chemical compositions. More than one hundred Na-clinopyroxene data from single-crystal structural determination (Edgar et al, 1969;Griffin & Mottana, 1982;Mottana, 1983;Davoli, 1986;Pasqual et al, 1995;Oberti, personal communication;Boffa Ballaran et al, 1998;Nestola et al, 2006Nestola et al, , 2007 were projected on the triangular diagram according to their chemical compositions; no literature data were located in the aegirine-augite compositional field, suggesting the probable presence of a miscibility gap. Three different sets of isoparametric lines -for the reflections " XRPD pattern of a mixed Na-pyroxenite between 28 and 38 2 , the angle interval which contains the three most significant reflections for Na-pyroxenes.…”

Section: Development Of a New Analytical Methodsmentioning

confidence: 99%

An easy non-invasive X-ray diffraction method to determine the composition of Na-pyroxenes from high-density `greenstone' implements

2007

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A large number of polished stone implements from Palaeolithic to Bronze Age sites of Northern Italy and Southern France are made of high-pressure (HP) metamorphic rocks (eclogite and related rocks), mainly consisting of Napyroxene (jadeite to omphacite) from the metamorphic belt of the Western Alps. The standard archaeometric study of prehistoric stone implements follows a procedure that is invasive, expensive and time-consuming. Since Na-pyroxenes may show a large compositional range, a thorough study of the variations affecting the d hkl values, obtained by X-ray diffraction, of three selected reflections as a function of different chemical composition was carried out, in order to determine the chemistry of Na-pyroxene isomorphic mixtures and roughly evaluate their relative amounts. These reflections ( " 2 221, 310, 002) are sharp, intense and sensitive to the variation of pyroxene chemical composition. Using such d hkl values measured on pyroxenes of known chemistry, a Ca-pyroxene(Di)-jadeite(Jd)-aegirine(Ae) compositional diagram was constructed, from which the composition of an unknown pyroxene can be estimated within an error of about 5%. When the size of the object is relatively small and a flat polished surface is present, the proposed analytical procedure becomes totally non-invasive. The data obtained shed light on the provenance sources of such implements and the prehistoric trade routes.

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“…Zone in Figure 8 (Oberha nsli et al, 1985;Castelli, 1991;Boffa Ballaran et al, 1998). Oberha nsli et al (1985) reported jadeite in fine grained pelitic gneiss (Sample SE14A) within a metagranitoid body and omphacite from both metagranitoid (Sample SE14) and mylonite (Sample SE18) deformed from granitoid from the Monte Mucrone area, central part of the EMC of the Sesia Zone (Fig.…”

Section: Clinopyroxene Data Equilibrated At T 500 C and P = 15 Kbarmentioning

confidence: 99%

The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, Western Alps, Italy

Matsumoto

Hirajima

2005

Journal of Mineralogical and Petrological Sciences

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An association of jadeite and omphacite has been newly found in an eclogite facies metaquartz diorite collected from the Orco Valley area, southern part of the Eclogitic Micaschist Complex of the Sesia Zone, Western Alps, Italy. Both the jadeite and omphacite occur as idiomorphic to subidiomorphic grains in the matrix, and some of them are in contact with each other with a sharp grain boundary. Most of the jadeite shows faint and irregular zoning, with a chemical variation of X jd (Al VI jd /(Na + Ca)) = 0.75 0.90. Half of the omphacite grains is homogeneous, and the remainder shows various zoning patterns. Some omphacite grains exhibit prograde zoning with an increase in X jd from 0.31 to 0.55 and a decrease in X aeg (= Fe 3+ /(Na + Ca)) from 0.14 to 0.04 from the core to the rim. The rim composition of the zoned omphacite is similar to that of homogeneous omphacite (X jd = 0.40 0.56). The average rim composition of the jadeite omphacite pairs in direct contact shows an apparent miscibility gap between X jd = 0.50 0.06, X aeg = 0.09 0.03, and X aug (= Ca/(Na + Ca)) = 0.41 0.05 in omphacite, and X jd = 0.79 0.04, Xaeg = 0.08 0.03, and Xaug = 0.13 0.03 in jadeite. Application of Powell (1985) garnet clinopyroxene geothermometer gives T = 470 30 C at P = 12 kbar and application of Waters and Martin (1993) garnet omphacite phengite geobarometer gives P = 12.4 kbar at T = 440 C and P = 12.0 kbar at T = 500 C as peak metamorphic conditions. Composition data of the pyroxene P2/n and C2/c compositional fields obtained from both this study and the literature suggest that: (1) the shape of two miscibility gaps between jadeite and omphacite and between omphacite and augite in the jadeite augite aegirine phase diagram of Carpenter (1983) is more concordant with the natural data than that of Holland (1990), (2) the miscibility gap between omphacite and augite closes at T 500 C and P = 15 kbar, but the miscibility gap between jadeite and omphacite may still exist, and (3) the miscibility gap between jadeite and omphacite closes at T = 700 850 C and P =15 45 kbar.

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Structural mechanisms of solid solution and cation ordering in augite-jadeite pyroxenes; I, A macroscopic perspective

Cited by 55 publications

References 29 publications

The effect of cation order on the elasticity of omphacite from atomistic calculations

The effect of cation order on the elasticity of omphacite from atomistic calculations

An easy non-invasive X-ray diffraction method to determine the composition of Na-pyroxenes from high-density `greenstone' implements

The coexistence of jadeite and omphacite in an eclogite-facies metaquartz diorite from the southern Sesia Zone, Western Alps, Italy

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