Magnesiodumortierite, a new mineral from very-high-pressure rocks (western Alps). II. Crystal chemistry and petrological significance

Chopin, Christian; Ferraris, Giovanni; Ivaldi, Gabriella; Schertl, Hans‐Peter; Schreyer, Werner; Compagnoni, Roberto; Davidson, Cameron; Davis, Andrew M.

doi:10.1127/ejm/7/3/0525

Cited by 23 publications

(23 citation statements)

References 12 publications

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“…Magnesiodumortierite is the Mg-analogue of dumortierite. Here, Mg 2+ is dominant at the X site (over vacancies, Ti 4+ , Fe 2+ , and Fe 3+ ); up to 0.40 Mg has been observed at the M H (Al4) site (Chopin et al 1995), although Al 3+ is still dominant at the latter. The Mg 2+ replaces Al 3+ in the dumortierite structure via the mechanisms 2Al 3+ → Mg 2+ + Ti 4+ and Al 3+ + O 2-→ Mg 2+ + OHwith the latter mechanism favored at high pressures (Chopin et al 1995).…”

Section: Review Of Dumortierite-like Materialsmentioning

confidence: 94%

“…Holtite is known from complex granitic pegmatites at four localities thus far: Greenbushes, Western Australia (Pryce 1971, Hoskins et al 1989; Voron'i Tundry, Kola Peninsula, Russia (Voloshin et al 1977); Szklary, Lower Silesia, Poland (Pieczka & Marszałek 1996, Pieczka et al 2011; and Virorco, San Luis, Argentina (Galliski et al 2012). Magnesiodumortierite has been found only in ultrahigh-pressure rocks of the western Alps (Chopin et al 1995. Together, these three minerals comprise the dumortierite group of orthorhombic aluminum borosilicates.…”

Section: Introductionmentioning

confidence: 99%

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Structure and Topology of Dumortierite and Dumortierite-Like Materials

Evans

Groat

2012

The Canadian Mineralogist

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Dumortierite, ca. (Al,)Al 6 (BO 3)Si 3 O 13 (O,OH) 2 , has a complex and unusual crystal structure that is shared by several minerals and an increasing number of synthetic materials, which we group together as dumortierite-like materials (DLMs). Dumortierite has a strongly pseudo-hexagonal orthorhombic structure based on rod-like double-chains of Al octahedra in a framework based on the {6•4•3•4} semi-regular planar tiling. The large hexagonal channels contain chains of face-sharing Al octahedra attached to the framework by rings of SiO 4 tetrahedra. Smaller triangular channels contain planar BO 3 groups, while in other materials the triangular channels are occupied by tetrahedral or pyramidal groups. Holtite and magnesiodumortierite are isostructural with dumortierite, while ellenbergerite, phosphoellenbergerite, ekatite, and the synthetic materials have very similar structures to dumortierite, but with hexagonal symmetry. These minerals and materials differ from one another in the identity of the metal cations occupying the framework and face-sharing chains, and in the tetrahedral, pyramidal, or triangular groups within the hexagonal and triangular channels. We write the following structure-generating function, which describes both topological and stoichiometric properties of DLMs: X(M 2 Φ) 3 (T h Q h 3 q h) 3 (T t Q t 3 q t) where X is the face-sharing hexagonal channel octahedrally coordinated site, M is the framework octahedrally coordinated site, T h and T t are three-to four-coordinate cation sites in the hexagonal and triangular channels, Φ is a four-coordinate anion site, Q h and Q t are three-coordinate basal anion sites, and q h and q t are one-to three-coordinate apical anion sites. We discuss various structural and crystal chemical properties of DLMs, illustrating their commonalities as variations on the theme described by the structure-generating function. We also discuss three closely related structures which involve modifications of different geometrical or topological aspects of the DLM structure: the structure of lyonsite and its synthetic analogues, the structure of satterlyite and holtedahlite, and the structure of the AB polytype of the cancrinite group of feldspathoids.

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Section: Review Of Dumortierite-like Materialsmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

Structure and Topology of Dumortierite and Dumortierite-Like Materials

Evans

Groat

2012

The Canadian Mineralogist

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“…Chopin, 1986; Schertl et al. , 1991; Chopin et al. , 1986, 1995; Chopin & Schertl, 1999; Chopin & Ferraris, 2003 and references therein).…”

Section: Introductionmentioning

confidence: 99%

“…bearthite, ellenbergerite, Mg-dumortierite, wagnerite) minerals have been characterized (e.g. Chopin, 1986;Schertl et al, 1991;Chopin et al, 1986Chopin et al, , 1995Chopin & Schertl, 1999;Chopin & Ferraris, 2003 and references therein). Schertl et al (1991) distinguished prograde pyrope megablasts and peak smaller pyrope.…”

Section: Introductionmentioning

confidence: 99%

Metasomatism of continental crust during subduction: the UHP whiteschists from the Southern Dora‐Maira Massif (Italian Western Alps)

Ferrando¹,

Frezzotti

Petrelli

et al. 2009

Journal Metamorphic Geology

103

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The ultrahigh‐pressure pyrope whiteschists from the Brossasco‐Isasca Unit of the Southern Dora‐Maira Massif represent metasomatic rocks originated at the expense of post‐Variscan granitoids by the influx of fluids along shear zones. In this study, geochemical, petrological and fluid‐inclusion data, correlated with different generations of pyrope‐rich garnet (from medium, to very‐coarse‐grained in size) allow constraints to be placed on the relative timing of metasomatism and sources of the metasomatic fluid. Geochemical investigations reveal that whiteschists are strongly enriched in Mg and depleted in Na, K, Ca and LILE (Cs, Pb, Rb, Sr, Ba) with respect to the metagranite. Three generations of pyrope, with different composition and mineral inclusions, have been distinguished: (i) the prograde Prp I, which constitutes the large core of megablasts and the small core of porphyroblasts; (ii) the peak Prp II, which constitutes the inner rim of megablasts and porphyroblasts and the core of small neoblasts; and (iii) the early retrograde Prp III, which locally constitutes an outer rim. Two generations of fluid inclusions have been recognized: (i) primary fluid inclusions in prograde kyanite that represent a NaCl‐MgCl2‐rich brine (6–28 wt% NaCleq with Si and Al as other dissolved cations) trapped during growth of Prp I (type‐I fluid); (ii) primary multiphase‐solid inclusions in Prp II that are remnants of an alumino‐silicate aqueous solution, containing Mg, Fe, alkalies, Ca and subordinate P, Cl, S, CO32‐, LILE (Pb, Cs, Sr, Rb, K, LREE, Ba), U and Th (type‐II fluid), at the peak pressure stage. We propose a model that illustrates the prograde metasomatic and metamorphic evolution of the whiteschists and that could also explain the genesis of other Mg‐rich, alkali‐poor schists of the Alps. During Alpine metamorphism, the post‐Variscan metagranite of the Brossasco‐Isasca Unit experienced a prograde metamorphism at HP conditions (stage A: ∼1.6 GPa and ≤ 600 °C), as indicated by the growth of an almandine‐rich garnet in some xenoliths. At stage B (1.7–2.1 GPa and 560–590 °C), the influx of external fluids, originated from antigorite breakdown in subducting oceanic serpentinites, promoted the increase in Mg and the decrease of alkalies and Ca in the orthogneiss toward a whiteschist composition. During stage C (2.1 < P < 2.8 GPa and 590 < T < 650 °C), the metasomatic fluid influx coupled with internal dehydration reactions involving Mg‐chlorite promoted the growth of Prp I in the presence of the type‐I MgCl2‐brine. At the metamorphic peak (stage D: 4.0–4.3 GPa and 730 °C), Prp II growth occurred in the presence of a type–II alumino‐silicate aqueous solution, mostly generated by internal dehydration reactions involving phlogopite and talc. The contribution of metasomatic external brines at the metamorphic climax appears negligible. This fluid, showing enrichment in LILE and depletion in HFSE, could represent a metasomatic agent for the supra‐subduction mantle wedge.

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“…In other cases, B contents less deviant from stoichiometry, or even approaching it, were reported using wet chemical (e.g. Claringbull and Hey, 1952;Voloshin et al, 1977;Chopin et al, 1995), ion microprobe (Willner and Schreyer, 1991), and electron microprobe analyses (Vrána, 1979;Chopin et al, 1995;Fuchs et al, 2005;Pieczka et al, 2013), but the precision was too low to properly assess whether the observed deviations are significant.…”

Section: Crystal Structure Of Dumortierite and Related Mineralsmentioning

confidence: 99%

The dumortierite supergroup. I. A new nomenclature for the dumortierite and holtite groups

Pieczka¹,

Evans²,

Groat³

et al. 2013

Mineral. mag.

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Although the distinction between magnesiodumortieite and dumortierite, i.e. Mg vs. Al dominance at the partially vacant octahedral Al1 site, had met current criteria of the IMA Commission on New Minerals, Nomenclature and Classification (CNMNC) for distinguishing mineral species, the distinction between holtite and dumortierite had not, since Al and Si are dominant over Ta and (Sb,As) at the Al1 and two Si sites, respectively, in both minerals. Recent studies have revealed extensive solid solution between Al, Ti, Ta and Nb at Al1 and between Si, As and Sb at the two Si sites or nearly coincident (As,Sb) sites in dumortierite and holtite, further blurring the distinction between the two minerals.This situation necessitated revision in the nomenclature of the dumortierite group. The newly constituted dumortierite supergroup, space group Pnma (no. 62), comprises two groups and six minerals, one of which is the first member of a potential third group, all isostructural with dumortierite. The supergroup, which has been approved by the CNMNC, is based on more specific end-member compositions for dumortierite and holtite, in which occupancy of the Al1 site is critical.( , and thus this mineral does not belong in either the dumortierite or the holtite group. Although szklaryite is distinguished by the mechanism introducing vacancies at the Al1 site, the primary criterion for identifying it is based on occupancy of the Si/As,Sb sites: (As 3+ + Sb 3+ ) > Si 4+ consistent with the dominant-valency rule. A Sb 3+ analogue to szklaryite is possible.

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Magnesiodumortierite, a new mineral from very-high-pressure rocks (western Alps). II. Crystal chemistry and petrological significance

Cited by 23 publications

References 12 publications

Structure and Topology of Dumortierite and Dumortierite-Like Materials

Structure and Topology of Dumortierite and Dumortierite-Like Materials

Metasomatism of continental crust during subduction: the UHP whiteschists from the Southern Dora‐Maira Massif (Italian Western Alps)

The dumortierite supergroup. I. A new nomenclature for the dumortierite and holtite groups

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