Hyalotekite, (Ba,Pb,K)4(Ca,Y)2Si8(B,Be)2(Si,B)2O28F, a tectosilicate related to scapolite: new structure refinement, phase transitions and a short-range ordered 3bsuperstructure

Christy, Andrew G.; Mayo, Sheridan C; Yates, Martin G.; Belakovskiy, Dimitriy I.

doi:10.1180/002646198547486

Cited by 16 publications

(17 citation statements)

References 14 publications

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“…The average z coordinate weighted by atomic number ($ scattering factor) is 0.102, close to the value of 0.0961 refined for the unsplit cation by Kampf et al (2010). The calculated split is much larger that those observed when lonepair cations substitute for large cations without stereoactive lone pairs, such as the 0.49-0.53 Å separation between the split Ba and Pb sites of hyalotekite (Christy et al, 1998).…”

Section: Discussionsupporting

confidence: 71%

Effect of lone-pair stereoactivity on polyhedral volume and structural flexibility: application to Te^IVO₆octahedra

Christy

Mills

2013

Acta Crystallogr Sect B

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The Distortion Theorem implies that the irregularity of bond distances in a distorted coordination polyhedron causes an increase of mean bond distance. Examination of 40 polyhedra containing the lone-pair cation Te(IV) shows that this does not imply an increase in polyhedral volume. Volumes of these polyhedra are 10.3-23.7 Å(3), compared with the 12.83 Å(3) expected for a hypothetical regular octahedron. There is little correlation between volume and measures of polyhedral distortion such as quadratic elongation, bond-angle variance or vector bond valence. However, the oxygens of our polyhedra lie very close to a sphere of best fit, centred at ~ 1 Å from the Te(IV) atom. The Te(IV)-centre distance is an index of lone-pair stereoactivity and is linearly related to the radius Rsph of the sphere; this is explained by a more localized lone pair repelling the anions more strongly, leading to a longer non-bonded distance between the lone pair and anions. Polyhedral volume still varies considerably for a given Rsph, because the oxygen ligands may be distributed over the whole sphere surface, or confined to a small portion of it. The uniformity of this distribution can be estimated from the distance between the sphere centre and the centroid of the O6 polyhedron. Te(IV)-centre and centroid-centre distances alone then account for 95% of the variation observed in volume for polyhedra which are topologically octahedral. Six of the polyhedra studied that are outliers are closer in shape to pentagonal pyramids than octahedra. These have short distances from the central Te(IV) cation to other Te(IV) and/or to large, polarizable cations, suggesting additional weak bonding interactions between these species and the central lone pair. The flexibility of lone-pair polyhedra is further enhanced by the ability of a single polyhedron to accommodate different cations with different degrees of lone-pair activity, which facilitates more diverse solid solution behaviour than would otherwise be the case.

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Section: Discussionsupporting

confidence: 71%

Effect of lone-pair stereoactivity on polyhedral volume and structural flexibility: application to Te^IVO₆octahedra

Christy

Mills

2013

Acta Crystallogr Sect B

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“…Moreover, Be substitutes for B and B substitutes for Si, as is observed in many other structures, rather than Be substituting for Si, which tends not to occur. Christy et al (1998) rerefined the data of Moore et al (1982) and came to this same conclusion.…”

Section: Hyalotekitesupporting

confidence: 59%

“…Consider the possible end-member compositions of hyalotekite from the viewpoint of the above discussion of the site occupancies of Be, B and Si. Christy et al (1998) wrote the general formula of hyalotekite as (Ba,Pb,K) 4 (Ca,Y) 2 Si 8 (Be,B) 2 (Si,B) 2 O 28 F Their chemical data show that Ba is in many cases dominant over Pb 2+ , and Ca is dominant over Y, and hence a simplified formula can be written as…”

Section: Hyalotekitementioning

confidence: 99%

The Use of End-Member Charge-Arrangements in Defining New Mineral Species and Heterovalent Substitutions in Complex Minerals

Hawthorne

2002

The Canadian Mineralogist

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The characteristics of an end-member formula are defined as follows: (1) the chemical formula must be fixed; no variable chemical components are possible; (2) the end-member formula must be compatible with the crystal structure of the mineral (or putative mineral); (3) the chemical composition at each site in the crystal structure must be fixed; an end-member formula may have two types of cation or anion (in a fixed ratio) at one site in the structure if required for electroneutrality; two cations or anions at more than one site are not allowed. Combining these characteristics of end-member formulae with aspects of their crystal structures can lead to unambiguous definition of end-member compositions of complex minerals, and can give considerable insight into coupled heterovalent substitutions. Several examples are given. The end-member formula of the tourmaline-group mineral povondraite was originally written as Na Fe 3+ 3 Fe 3+ 6 (Si 6 O 18) (BO 3) 3 (O,OH) 4 , whereas the correct end-member formula is Na Fe 3+ 3 (Fe 3+ 4 Mg 2) (Si 6 O 18) (BO 3) 3 (OH) 3 O. The yttrium-rich milarite described by Č erný et al. (1991) is shown to be a new mineral of the milarite group, with the end-member formula K (CaY) [Be 3 (Si 12 O 30)]. There are seven accredited [4] (Li,Zn)bearing minerals of the milarite group, and there has been some ambiguity over the end-member compositions of darapiosite, dusmatovite and sogdianite; end-member formulae for these minerals are unambiguously defined with the approach used here. The complex Be-bearing borosilicate mineral hyalotekite has been somewhat elusive with regard to unambiguous definition of its solid-solution behavior. The ideal formula of Christy et al. (1998) can be resolved into two distinct end-member formulae: (1) Ba 4 Ca 2 Si 8 (BeB) (Si) 2 O 28 F, and (2) Ba 4 Ca 2 Si 8 (B 2) (SiB) O 28 F; note that one end-member has essential Be and the other endmember does not. Thus the competing views on whether hyalotekite is a Be mineral or not are both correct, and there are two distinct minerals buried in the chemical compositional data for hyalotekite. "Makarochkinite" is a non-accredited phase described by Yakubovich et al. (1990) that has been identified as identical to høgtuvaite by Grauch et al. (1994). Application of the approach developed here shows that, according to the published chemical data, "makarochkinite" and høgtuvaite have distinct end-member formulae, Ca 2 (Fe 2+ 4 Fe 3+ Ti) (Si 4 Be Al) O 20 and Ca 2 (Fe 2+ 4 Fe 3+ 2) (Si 5 Be) O 20 , respectively. These examples show that the very simple approach developed here, using the characteristics of an end-member formula with aspects of the crystal structure, can simply and easily resolve many quite complicated problems in mineral chemistry.

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“…The inherent underbonding of these bridging O atoms is expected to make such linkages less likely than Be-O-Si or Al-O-Si linkages. An analogous preference for Be/B mixing over Be/Si mixing became apparent in a new refinement of hyalotekite, (Ba,Pb,K) 4 (Ca,Y) 2 Si 8 (B,Be) 2 (Si,B) 2 O 28 F, a tectosilicate related to scapolite (Christy et al 1998).…”

Section: Cation Ordering In Khmaralitementioning

confidence: 93%

Khmaralite, a new beryllium-bearing mineral related to sapphirine; a superstructure resulting from partial ordering of Be, Al, and Si on tetrahedral sites

Barbier

Moore²,

1999

American Mineralogist

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Khmaralite, Ca 0.04 Mg 5.46 Fe 3+ 0.12 Fe 2+ 1.87 Al 14.26 Be 1.43 B 0.02 Si 4.80 O 40, is a new mineral closely related to sapphirine from Khmara Bay, Enderby Land, Antarctica. It occurs in a pegmatite metamorphosed at T ≥ 820 °C, P ≥ 10 kbar. The minerals surinamite, musgravite, and sillimanite associated with khmaralite at Casey Bay saturate it in BeO, and thus its BeO content could be close to the maximum possible. Optically, khmaralite is biaxial (-); at λ = 589 nm, α = 1.725(2), β = 1.740(2), γ = 1.741(2), 2V meas = 34.4 (1.8)°, v > r strong, and β  b. The weak superstructure reported using electron diffraction has been confirmed by single-crystal X-ray diffraction. The superstructure corresponds to a doubling of the a axis in monoclinic sapphirine-2M (P2 1 /c setting) with the following unit-cell parameters: a = 19.800(1), b = 14.371(1), c = 11.254(1) Å, β = 125.53(1)°, Z = 4, D calc = 3.61 g/cm 3 . Using a simplified chemical formula of Mg 5.46 Al 14.28 Fe 2.00 Si 4.80 Be 1.46 O 40 , the fully anisotropic structure refinement of all site occupancies, including Al vs. Mg and Al vs. Si on the 16 octahedral (M) and 12 tetrahedral (T) sites respectively, establishes that the 2×a superstructure results primarily from ordering in the doubled (T1 → T6, T7 → T12) tetrahedral chain, which is parallel to the a axis. The strongest contribution comes from Al-Si-Be ordering on the T2 (33-16-51%) vs. T8 (0-95-5%) and T3 (4-78-18%) vs. T9 (30-0-70%) sites. Thus, the sequence Al-Si-Al in sapphirine is replaced by the sequence SiBe-Si in khmaralite; i.e., Be replaces Si on two sites and Si replaces Al on on four adjacent sites, resulting in an indirect replacement of Al by Be by the coupled substitution Be + Si = 2Al. The Be distribution in khmaralite and the strong preference for Be/Al mixing over Be/Si mixing appear to satisfy the bonding requirements of the bridging O atoms by minimizing the number of Be-O-Be and Be-O-Al linkages.

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Hyalotekite, (Ba,Pb,K)₄(Ca,Y)₂Si₈(B,Be)₂(Si,B)₂O₂₈F, a tectosilicate related to scapolite: new structure refinement, phase transitions and a short-range ordered 3bsuperstructure

Cited by 16 publications

References 14 publications

Effect of lone-pair stereoactivity on polyhedral volume and structural flexibility: application to Te^IVO₆octahedra

Effect of lone-pair stereoactivity on polyhedral volume and structural flexibility: application to Te^IVO₆octahedra

The Use of End-Member Charge-Arrangements in Defining New Mineral Species and Heterovalent Substitutions in Complex Minerals

Khmaralite, a new beryllium-bearing mineral related to sapphirine; a superstructure resulting from partial ordering of Be, Al, and Si on tetrahedral sites

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Hyalotekite, (Ba,Pb,K)4(Ca,Y)2Si8(B,Be)2(Si,B)2O28F, a tectosilicate related to scapolite: new structure refinement, phase transitions and a short-range ordered 3bsuperstructure

Cited by 16 publications

References 14 publications

Effect of lone-pair stereoactivity on polyhedral volume and structural flexibility: application to TeIVO6octahedra

Effect of lone-pair stereoactivity on polyhedral volume and structural flexibility: application to TeIVO6octahedra

The Use of End-Member Charge-Arrangements in Defining New Mineral Species and Heterovalent Substitutions in Complex Minerals

Khmaralite, a new beryllium-bearing mineral related to sapphirine; a superstructure resulting from partial ordering of Be, Al, and Si on tetrahedral sites

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Hyalotekite, (Ba,Pb,K)₄(Ca,Y)₂Si₈(B,Be)₂(Si,B)₂O₂₈F, a tectosilicate related to scapolite: new structure refinement, phase transitions and a short-range ordered 3bsuperstructure

Effect of lone-pair stereoactivity on polyhedral volume and structural flexibility: application to Te^IVO₆octahedra

Effect of lone-pair stereoactivity on polyhedral volume and structural flexibility: application to Te^IVO₆octahedra