Linking structure and chemistry in the schorl-dravite series

Bloodaxe, Erich S.; Hughes, John M.; Dyar, M. D.; Grew, Edward S.; Guidotti, Charles V.

doi:10.2138/am-1999-5-627

Cited by 103 publications

(65 citation statements)

References 3 publications

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“…Measured data were evaluated with the DIFFRAC plus EVA software package. Analyzed scans were fitted by, and lattice parameters refined with, the DIFFRAC plus TOPAS software on a basis of dravite structure (sample 32008, Bloodaxe et al 1999). Average bond lengths in octahedra were calculated from lattice parameters according to empirical formulae: a = 13.045 + 1.437 <Y-O> Å; c = 0.886 + 3.272 <Z-O> Å (Bosi and Lucchesi 2004).…”

Section: Methodsmentioning

confidence: 99%

“…The <Z-O> bond length in octahedra entirely occupied by Al, as in elbaite, rossmanite, olenite and foitite, varies between 1.90 and 1.91 Å (e.g., Grice and Ercit 1993;MacDonald et al 1993;Burns et al 1994;Selway et al 1998, Cempírek et al 2006. In contrast, the <Z-O> bond length in Mg-bearing tourmalines is in the range of 1.92 to 1.93 Å (e.g., Bloodaxe et al 1999;Hawthorne et al 1999;Bosi and Lucchesi 2004;Ertl et al 2010;Bačík et al 2011a, in print). There is a good correlation between Z Mg content and the <Z-O> bond length (Ertl et al 2010): the <Z-O> bond length increases with the Z Mg (or other cations with ionic radius larger than Al, if present).…”

Section: Crystal Chemistry Of Tourmalinementioning

confidence: 99%

“…There is a good correlation between Z Mg content and the <Z-O> bond length (Ertl et al 2010): the <Z-O> bond length increases with the Z Mg (or other cations with ionic radius larger than Al, if present). Significant amounts of Mg may enter the Z site due to Al-Mg disorder even if the Al content exceeds 6 apfu and could be sufficient to fill the Z site entirely (e.g., Grice and Ercit 1993;Hawthorne et al 1993Hawthorne et al , 1999Bloodaxe et al 1999;Bosi and Lucchesi 2004;Ertl et al 2010). The studied tourmalines have affinity to magnesio-foitite and oxy-dravite with a relatively high value of the <Z-O> bond length (1.929 Å), despite of their Al-rich composition (up to 6.72 Al apfu).…”

Section: Crystal Chemistry Of Tourmalinementioning

confidence: 99%

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Magnesian tourmalines from plagioclase-muscovite-scapolite metaevaporite layers in dolomite marble near Prosetín (Olešnice Unit, Moravicum, Czech Republic)

Bačík¹,

Uher²,

Cempírek

et al. 2012

Jour. Geosci.

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Radial aggregates of blue-grey tourmaline were found in plagioclase-muscovite-scapolite metaevaporite layers in dolomite marble near Prosetín (Olešnice Unit, Moravicum, Czech Republic). It occurs in association with plagioclase (An 15-37 ), muscovite, scapolite, phlogopite, vermiculite, pumpellyite-(Al), and clinozoisite. Electron-microprobe analyses of tourmaline show dravitic composition with very high content of Mg (1.92 to 2.77 apfu), Al (up to 6.71 apfu), low content of Fe (up to 0.39 apfu) and variable amounts of vacancies (0.09 to 0.47) and Ca (0.03 to 0.29 apfu) in the X-site. Some analyses correspond to "oxy-dravite" and some others almost attain magnesio-foitite compositions. The proportion of X-site vacancy decreases from the crystal cores to their rims while Ca content increases. Generally, charge excess due to the high Al-contents is balanced either by an increasing X-site vacancy or by deprotonization of W OH; the W O 2-content calculated from charge-balanced formula attains 0.71 apfu. Lattice parameters [a = 15.9116(6) Å; c = 7.1987(4) Å] and deduced average bond lengths ( = 1.995 Å; = 1.929 Å) indicate a relatively high Al-Mg disorder. Three main substitution mechanisms are inferred to operate in the studied magnesian tourmalines: (1) CaMg(NaAl) -1 , mainly in Ca-enriched dravite, (2) X □Al(NaMg) -1 in nearly magnesio-foititic compositions, and (3) AlO(MgOH) -1 in "oxydravitic" members. The tourmaline is relatively poor in trace elements; only Ti, Sr, and Ga exceed 100 ppm according to LA-ICP-MS study. There is a pronounced positive correlation between Sr and Ca (r 2 = 0.77), which suggests that Sr accumulated in Ca-enriched zones of dravite. The pale blue-grey color of the studied tourmalines is most likely a result of Fe 2+ crystal field transitions along with the absence of significant amounts of other chromophores. Trace-element contents, mineral assemblage and compositional zoning of tourmalines as well as host-rock mineral association suggest prograde metamorphic growth and support metaevaporitic origin of the plagioclase-muscovite-scapolite rocks.

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Section: Methodsmentioning

confidence: 99%

Section: Crystal Chemistry Of Tourmalinementioning

confidence: 99%

Section: Crystal Chemistry Of Tourmalinementioning

confidence: 99%

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Magnesian tourmalines from plagioclase-muscovite-scapolite metaevaporite layers in dolomite marble near Prosetín (Olešnice Unit, Moravicum, Czech Republic)

Bačík¹,

Uher²,

Cempírek

et al. 2012

Jour. Geosci.

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“…It is difficult to analyze samples for boron by electron microprobe, and difficult to detect small amounts of substituent B in X-ray structure studies. Although the presence of tetrahedrally coordinated B has been demonstrated in tourmaline using magic-angle-spinning nuclear magnetic resonance (MAS NMR) (Tagg et al 1999) and substantial [4] B substitution for Si has been reported in synthetic olenite (Wodara & Schreyer 1997, 1998, Schreyer et al 2000, no structural study has unequivocally shown B to occur in amounts greater than 3 apfu (atoms per formula unit) in tourmaline (e.g., Burns et al 1994, Hawthorne 1996, Bloodaxe et al 1999, with a single exception. Ertl et al (1997) reported the presence in a sample of olenite of significant excess B by electron-microprobe analysis.…”

Section: Introductionmentioning

confidence: 99%

Tetrahedrally Coordinated Boron in a Tourmaline: Boron-Rich Olenite From Stoffhutte, Koralpe, Austria

Hughes¹,

Ertl²,

Dyar³

et al. 2000

The Canadian Mineralogist

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The substitution of [4] B for [4] Si in tourmaline has been the source of much discussion. A sample of olenite from Stoffhütte, Koralpe, Styria, Austria has been found to contain more than one [4] B per formula unit of 31 (O,OH,F), demonstrating that significant [4] B = [4] Si substitution does indeed occur in tourmaline. The composition of the structurally characterized crystal was determined using electron microprobe and SIMS data; in combination with refinement of the site scattering, the chemical data yield (3), c 7.0638(9) Å. The atomic arrangement was refined to R = 0.014 using X-ray data; is equal to 1.609 Å, reflecting substitution of the smaller boron atom for Si. Conjecture on the pattern of distribution of [4] B can be made on the basis of X-site occupancy.

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“…The bond valence sum increases monotonously from 2.5 vu in Al-poor tourmalines to 2.8 vu in elbaite and olenite samples. In Al-poor tourmalines, there is usually a proportion of Mg at the Z site present due to Al-Mg disorder (Grice and Ercit 1993;Hawthorne et al 1993;Bloodaxe et al 1999;Marschall et al 2004;Ertl et al 2010). Such a bond-valence sum brings relatively clear evidence of this disorder, although the sum would get higher, if disorder is considered and Mg-O values would be used.…”

Section: Bond Valence Estimationmentioning

confidence: 99%

The crystal-chemical autopsy of octahedral sites in Na-dominant tourmalines: octahedral metrics model unconstrained by the Y,Z-site disorder assignment

Bačík¹

2018

J. Geosci.

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The structure of tourmaline-supergroup minerals includes two types of octahedral sites: the ZO 6 octahedron is smaller and more distorted than the YO 6 octahedron. The octahedral sites metrics were studied and their dependency on the chemical composition unconstrained by Y,Z-site disorder assignment. Published chemical and structural data were collected from American Mineralogist Crystal Structure Database for tourmaline samples belonging to dravite-schorl, schorl-elbaite (including tsilaisites) and schorl (± dravite)-olenite series. Correlation analysis of this dataset provided the evidence of cation distribution between sites -Al and Mg are disordered between Z and Y sites, while Fe (mostly ferrous), Li and Mn strongly prefer Y site. Irregular cation distribution results in the variable metrics of both octahedra in tourmalines. It is the function of well-balanced relations between cations at octahedral and neighbouring sites based on bond-valence variations due to different ionic charges. Considering Z and Y cations, there is a dependence of the cation charge difference and the octahedral metrics. The most pronounced irregularity of both octahedra was observed in elbaite samples with the largest charge difference between Li and Al. In contrast, "buergerite" samples with trivalent Fe and Al at both octahedral sites have both octahedra almost isometric. Schorl and dravite samples display an increasing metric irregularity related to the Al and Mg content; increase in Mg reduces irregularity because Mg is distributed between both octahedral sites balancing charge difference. In contrast, Fe-rich and Al-rich schorl samples display larger irregularity which may result from selective incorporation of Fe 2+ to the Y site. In olenite samples, the irregularity of both octahedra decreases with an increasing Al content. These variations are related to the shared edge of ZO 6 and YO 6 octahedra including both O3 and O6 site where bonds of both anions are balancing bond-valence requirements of the stable electroneutral structure. In addition to the bond-valence relations, effects of the internal geometry of atomic shells should be also considered, i.e. Jahn-Teller distortion that can be decisive factor in cation occupancy. Especially Fe 2+ can strongly prefer YO 6 octahedron whose prolonged tetragonal dipyramidal geometry is more favourable for Fe 2+ in (t 2g ) 4 (e g ) 2 configuration. Received: 6 October 2017; accepted: 31 May 2018; handling editor: J. Cempírek The online version of this article (doi: 10.3190/jgeosci.260) contains supplementary electronic material. Keywords: tourmaline supergroup, octahedral metrics, bond-valence theory, Jahn-Teller effectvacancies (Henry et al. 2011), although Ti structural position is not clear yet.Although the site occupancy usually follows Goldschmidt rules (ionic radii of cations occupying the Z site are lower and therefore the ZO 6 octahedron is smaller), local structural and bond-valence requirements may result in various cation disorder. Along with the Al-Mg disorder (Hawthorne ...

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Linking structure and chemistry in the schorl-dravite series

Cited by 103 publications

References 3 publications

Magnesian tourmalines from plagioclase-muscovite-scapolite metaevaporite layers in dolomite marble near Prosetín (Olešnice Unit, Moravicum, Czech Republic)

Magnesian tourmalines from plagioclase-muscovite-scapolite metaevaporite layers in dolomite marble near Prosetín (Olešnice Unit, Moravicum, Czech Republic)

Tetrahedrally Coordinated Boron in a Tourmaline: Boron-Rich Olenite From Stoffhutte, Koralpe, Austria

The crystal-chemical autopsy of octahedral sites in Na-dominant tourmalines: octahedral metrics model unconstrained by the Y,Z-site disorder assignment

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