10 results
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 3b superstructure
- A. G. Christy, E. S. Grew, S. C. Mayo, M. G. Yates, D. I. Belakovskiy
-
- Journal:
- Mineralogical Magazine / Volume 62 / Issue 1 / February 1998
- Published online by Cambridge University Press:
- 05 July 2018, pp. 77-92
-
- Article
- Export citation
-
Hyalotekite, a framework silicate of composition (Ba,Pb,K)4(Ca,Y)2Si8(B,Be)2 (Si,B)28F, is found in relatively high-temperature (⩾ 500°C) Mn skarns at Långban, Sweden, and peralkaline pegmatites at Dara-i-Pioz, Tajikistan. A new paragenesis at Dara-i-Pioz is pegmatite consisting of the Ba borosilicates leucosphenite and tienshanite, as well as caesium kupletskite, aegirine, pyrochlore, microcline and quartz. Hyalotekite has been partially replaced by barylite and danburite. This hyalotekite contains 1.29–1.78 wt.% Y2O3, equivalent to 0.172–0.238 Y pfu or 8–11% Y on the Ca site; its Pb/(Pb+Ba) ratio ranges 0.36–0.44. Electron microprobe F contents of Långban and Dara-i-Pioz hyalotekite range 1.04–1.45 wt.%, consistent with full occupancy of the F site. A new refinement of the structure factor data used in the original structural determination of a Långban hyalotekite resulted in a structural formula, (Pbl.96Bal.86K0.18)Ca2(B1.76Be0.24)(Sil.56B0.44)Si8O28F, consistent with chemical data and all cations with positive-definite thermal parameters, although with a slight excess of positive charge (+57.14 as opposed to the ideal +57.00). An unusual feature of the hyalotekite framework is that 4 of 28 oxygens are non-bridging; by merging these 4 oxygens into two, the framework topology of scapolite is obtained. The triclinic symmetry of hyalotekite observed at room temperature is obtained from a hypothetical tetragonal parent structure via a sequence of displacive phase transitions. Some of these transitions are associated with cation ordering, either Pb–Ba ordering in the large cation sites, or B–Be and Si–B ordering on tetrahedral sites. Others are largely displacive but affect the coordination of the large cations (Pb, Ba, K, Ca). High-resolution electron microscopy suggests that the undulatory extinction characteristic of hyalotekite is due to a fine mosaic microstructure. This suggests that at least one of these transitions occurs in nature during cooling, and that it is first order with a large volume change. A diffuse superstructure observed by electron diffraction implies the existence of a further stage of short-range cation ordering which probably involves both (Pb,K)–Ba and (BeSi,BB)–BSi.
Recommended nomenclature for the sapphirine and surinamite groups (sapphirine supergroup)
- E. S. Grew, U. Hålenius, M. Pasero, J. Barbier
-
- Journal:
- Mineralogical Magazine / Volume 72 / Issue 4 / August 2008
- Published online by Cambridge University Press:
- 05 July 2018, pp. 839-876
-
- Article
- Export citation
-
Minerals isostructural with sapphirine-1A, sapphirine-2M, and surinamite are closely related chain silicates that pose nomenclature problems because of the large number of sites and potential constituents, including several (Be, B, As, Sb) that are rare or absent in other chain silicates. Our recommended nomenclature for the sapphirine group (formerly aenigmatite group) makes extensive use of precedent, but applies the rules to all known natural compositions, with flexibility to allow for yet undiscovered compositions such as those reported in synthetic materials. These minerals are part of a polysomatic series composed of pyroxene or pyroxene-like and spinel modules, and thus we recommend that the sapphirine supergroup should encompass the polysomatic series. The first level in the classification is based on polysome, i.e. each group within the supergroup corresponds to a single polysome. At the second level, the sapphirine group is divided into subgroups according to the occupancy of the two largest M sites, namely, sapphirine (Mg), aenigmatite (Na), and rhönite (Ca). Classification at the third level is based on the occupancy of the smallest M site with most shared edges, M7, at which the dominant cation is most often Ti (aenigmatite, rhönite, makarochkinite), Fe3+ (wilkinsonite, dorrite, høgtuvaite) or Al (sapphirine, khmaralite); much less common is Cr (krinovite) and Sb (welshite). At the fourth level, the two most polymerized T sites are considered together, e.g. ordering of Be at these sites distinguishes høgtuvaite, makarochkinite and khmaralite. Classification at the fifth level is based on XMg = Mg/(Mg + Fe 2+) at the M sites (excluding the two largest and Ml). In principle, this criterion could be expanded to include other divalent cations at these sites, e.g. Mn. To date, most minerals have been found to be either Mg-dominant (XMg > 0.5), or Fe2+-dominant (XMg < 0.5), at these M sites. However, XMg ranges from 1.00 to 0.03 in material described as rhönite, i.e. there are two species present, one Mg-dominant, the other Fe2+-dominant. Three other potentially new species are a Mg-dominant analogue of wilkinsonite, rhönite in the Allende meteorite, which is distinguished from rhonite and dorrite in that Mg rather than Ti or Fe3+ is dominant at Ml, and an Al-dominant analogue of sapphirine, in which Al > Si at the two most polymerized T sites vs. Al < Si in sapphirine. Further splitting of the supergroup based on occupancies other than those specified above is not recommended.
The crystal chemistry of holtite
- L. A. Groat, E. S. Grew, R. J. Evans, A. Pieczka, T. S. Ercit
-
- Journal:
- Mineralogical Magazine / Volume 73 / Issue 6 / December 2009
- Published online by Cambridge University Press:
- 05 July 2018, pp. 1033-1050
-
- Article
- Export citation
-
Holtite, approximately (Al,Ta,□)Al6(BO3)(Si,Sb3+,As3+)Σ3O12(O,OH,□s)Σ3, is a member of the dumortierite group that has been found in pegmatite, or alluvial deposits derived from pegmatite, at three localities: Greenbushes, Western Australia; Voron'i Tundry, Kola Peninsula, Russia; and Szklary. Lower Silesia, Poland. Holtite can contain >30 wt.% Sb2O3, As2O3, Ta2O5, Nb2O5, and TiO2 (taken together), but none of these constituents is dominant at a crystallographic site, which raises the question whether this mineral is distinct from dumortierite. The crystal structures of four samples from the three localities have been refined to R1 = 0.02—0.05. The results show dominantly: Al, Ta, and vacancies at the Al(l) position; Al and vacancies at the Al(2), (3) and (4) sites; Si and vacancies at the Si positions; and Sb, As and vacancies at the Sb sites for both Sb-poor (holtite I) and Sb-rich (holtite II) specimens. Although charge-balance calculations based on our single-crystal structure refinements suggest that essentially no water is present, Fourier transform infrared spectra confirm that some OH is present in the three samples that could be measured. By analogy with dumortierite, the largest peak at 3505-3490 cm-1 is identified with OH at the O(2) and O(7) positions. The single-crystal X-ray refinements and FTIR results suggest the following general formula for holtite: Al7-[5x+y+z]/3 (Ta,Nb)x□[2x+y+z]\3,BSi3-y(Sb,As)yO18-y-z(OH)z, where x is the total number of pentavalent cations, y is the total amount of Sb + As, and z ⩽ y is the total amount of OH. Comparison with the electron microprobe compositions suggests the following approximate general formulae Al5.83(Ta,Nb)0.50□0.67BSi2.50(Sb,As)0.50O17.00(OH)0.50 and Al5.92(Ta,Nb)0.25□0.83BSi2.00(Sb,As)1.00O16.00(OH)1.00 for holtite I and holtite II respectively. However, the crystal structure refinements do not indicate a fundamental difference in cation ordering that might serve as a criterion for recognizing the two holtites as distinct species, and anion compositions are also not sufficiently different. Moreover, available analyses suggest the possibility of a continuum in the Si/(Sb + As) ratio between holtite I and dumortierite, and at least a partial continuum between holtite I and holtite II. We recommend that use of the terms holtite I and holtite II be discontinued.
Chevkinite-group minerals from granulite-facies metamorphic rocks and associated pegmatites of East Antarctica and South India
- H. E. Belkin, R. Macdonald, E. S. Grew
-
- Journal:
- Mineralogical Magazine / Volume 73 / Issue 1 / February 2009
- Published online by Cambridge University Press:
- 05 July 2018, pp. 149-164
-
- Article
- Export citation
-
Electron microprobe data are presented for chevkinite-group minerals from granulite-facies rocks and associated pegmatites of the Napier Complex and Mawson Station charnockite in East Antarctica and from the Eastern Ghats, South India. Their compositions conform to the general formula for this group, viz. A4BC2D2Si4O22 where, in the analysed specimens A = (rare-earth elements (REE), Ca, Y, Th), B = Fe2+, Mg, C = (Al, Mg, Ti, Fe2+, Fe3+, Zr) and D = Ti and plot within the perrierite field of the total Fe (as FeO) (wt.%) vs. CaO (wt.%) discriminator diagram of Macdonald and Belkin (2002). In contrast to most chevkinite-group minerals, the A site shows unusual enrichment in the MREE and HREE relative to the LREE and Ca. In one sample from the Napier Complex, Y is the dominant cation among the total REE + Y in the A site, the first reported case of Y-dominance in the chevkinite group. The minerals include the most Al-rich yet reported in the chevkinite group (≤9.15 wt.% Al2O3), sufficient to fill the C site in two samples. Conversely, the amount of Ti in these samples does not fill the D site, and, thus, some of the Al could be making up the deficiency at D, a situation not previously reported in the chevkinite group. Fe abundances are low, requiring Mg to occupy up to 45% of the B site. The chevkinite-group minerals analysed originated from three distinct parageneses: (1) pegmatites containing hornblende and orthopyroxene or garnet; (2) orthopyroxene-bearing gneiss and granulite; (3) highly aluminous paragneisses in which the associated minerals are relatively magnesian or aluminous. Chevkinite-group minerals from the first two parageneses have relatively high FeO content and low MgO and Al2O3 contents; their compositions plot in the field for mafic and intermediate igneous rocks. In contrast, chevkinite-group minerals from the third paragenesis are notably more aluminous and have greater Mg/Fe ratios.
Kornerupine in a sapphirine-spinel granulite from Labwor Hills, Uganda
- P. H. Nixon, E. S. Grew, E. Condliffe
-
- Journal:
- Mineralogical Magazine / Volume 48 / Issue 349 / December 1984
- Published online by Cambridge University Press:
- 05 July 2018, pp. 550-552
-
- Article
- Export citation
The dumortierite supergroup. II. Three new minerals from the Szklary pegmatite, SW Poland: Nioboholtite, (Nb0.6□0.4)Al6BSi3O18, titanoholtite, (Ti0.75□0.25)Al6BSi3O18, and szklaryite, □Al6BAs3+3O15
- A. Pieczka, R. J. Evans, E. S. Grew, L. A. Groat, C. Ma, G. R. Rossman
-
- Journal:
- Mineralogical Magazine / Volume 77 / Issue 6 / August 2013
- Published online by Cambridge University Press:
- 05 July 2018, pp. 2841-2856
-
- Article
- Export citation
-
Three new minerals in the dumortierite supergroup were discovered in the Szklary pegmatite, Lower Silesia, Poland. Nioboholtite, endmember (Nb0.6☐0.4)Al6B3Si3O18, and titanoholtite, endmember (Ti0.75☐0.25)Al6B3Si3O18, are new members of the holtite group, whereas szklaryite, endmember ☐Al6BAs3+3O15, is the first representative of a potential new group. Nioboholtite occurs mostly as overgrowths not exceeding 10 μm in thickness on cores of holtite. Titanoholtite forms patches up to 10 μm across in the holtite cores and streaks up to 5 μm wide along boundaries between holtite cores and the nioboholtite rims. Szklaryite is found as a patch ∼2 μm in size in As- and Sb- bearing dumortierite enclosed in quartz. Titanoholtite crystallized almost simultaneously with holtite and other Ta-dominant minerals such as tantalite-(Mn) and stibiotantalite and before nioboholtite, which crystallized simultaneously with stibiocolumbite during decreasing Ta activity in the pegmatite melt. Szklaryite crystallized after nioboholtite during the final stage of the Szklary pegmatite formation. Optical properties could be obtained only from nioboholtite, which is creamy-white to brownish yellow or grey-yellow in hand specimen, translucent, with a white streak, biaxial (–), nα = 1.740 – 1.747, nβ ∼ 1.76, nγ ∼ 1.76, and Δ < 0.020. Electron microprobe analyses of nioboholtite, titanoholtite and szklaryite give, respectively, in wt.%: P2O5 0.26, 0.01, 0.68; Nb2O5 5.21, 0.67, 0.17; Ta2O5 0.66, 1.18, 0.00; SiO2 18.68, 21.92, 12.78; TiO2 0.11, 4.00, 0.30; B2O3 4.91, 4.64, 5.44; Al2O3 49.74, 50.02, 50.74; As2O3 5.92, 2.26, 16.02; Sb2O3 10.81, 11.48, 10.31; FeO 0.51, 0.13, 0.19; H2O (calc.) 0.05, –, –, Sum 96.86, 96.34, 97.07, corresponding on the basis of O = 18–As–Sb to {(Nb0.26Ta0.02☐0.18)(Al0.27Fe0.05Ti0.01)☐0.21}Σ1.00Al6B0.92{Si2.03P0.02(Sb0.48As0.39Al0.07}Σ3.00(O17.09OH0.04☐0.87)Σ18.00, {(Ti0.32 Nb0.03 Ta0.03☐0.10)(Al0.35 Ti0.01 Fe0.01)☐0.15 }Σ1.00 Al6 B0.86 {Si2 . 3 6 (Sb0.5 1 As0.14 )}Σ3.01(O17.35☐0.65)Σ18.00 and {☐0.53 (Al0.41 Ti0.02 Fe0.02 )(Nb0.01☐0.01 )}Σ1.00Al6 B1.01 {(As1.07 Sb0.47 Al0.03 ) Si1.37 P0.06 }Σ3.00(O16.46☐1.54 )Σ18.00. Electron backscattered diffraction indicates that the three minerals are presumably isostructural with dumortierite, that is, orthorhombic symmetry, space group Pnma (no. 62), and unit-cell parameters close to a = 4.7001, b = 11.828, c = 20.243 Å, with V = 1125.36 Å3 and Z = 4; micro-Raman spectroscopy provided further confirmation of the structural relationship for nioboholtite and titanoholtite. The calculated density is 3.72 g/cm3 for nioboholtite, 3.66 g/cm3 for titanoholtite and 3.71 g/cm3 for szklaryite. The strongest lines in X-ray powder diffraction patterns calculated from the cell parameters of dumortierite of Moore and Araki (1978) and the empirical formulae of nioboholtite, titanoholtite and szklaryite are [d, Å, I (hkl)]: 10.2125, 67, 46, 19 (011); 5.9140, 40, 47, 57 (020); 5.8610, 66, 78, 100 (013); 3.4582, 63, 63, 60 (122); 3.4439, 36, 36, 34 (104); 3.2305, 100, 100, 95 (123); 3.0675, 53, 53, 50 (105); 2.9305, 65, 59, 51 (026); 2.8945, 64, 65, 59 (132), respectively. The three minerals have been approved by the IMA CNMNC (IMA 2012-068, 069, 070) and were named for their relationship to holtite and occurrence in the Szklary pegmatite, respectively.
Holtite and dumortierite from the Szklary Pegmatite, Lower Silesia, Poland
- A. Pieczka, E. S. Grew, L. A. Groat, R. J. Evans
-
- Journal:
- Mineralogical Magazine / Volume 75 / Issue 2 / April 2011
- Published online by Cambridge University Press:
- 05 July 2018, pp. 303-315
-
- Article
- Export citation
-
The Szklary holtite is represented by three compositional varieties: (1) Ta-bearing (up to 14.66 wt.% Ta2O5), which forms homogeneous crystals and cores within zoned crystals; (2) Ti-bearing (up to 3.82 wt.% TiO2), found as small domains within the core; and (3) Nb-bearing (up to 5.30 wt.% Nb2O5,) forming the rims of zoned crystals. All three varieties show variable Sb+As content, reaching 19.18 wt.% Sb2O3 (0.87 Sb a.p.f.u.) and 3.30 wt.% As2O3 (0.22 As a.p.f.u.) in zoned Ta-bearing holtite, which constitutes the largest Sb+As content reported for the mineral. The zoning in holtite is a result of Ta-Nb fractionation in the parental pegmatite-forming melt together with contamination of the relatively thin Szklary dyke by Fe, Mg and Ti. Holtite and the As- and Sb-bearing dumortierite, which in places overgrows the youngest Nb-bearing zone, suggest the following crystallization sequence: Ta-bearing holtite → Ti-bearing holtite → Nb-bearing holtite → As- and Sb-bearing, (Ta,Nb,Ti)-poor dumortierite → As- and Sb-dominant, (Ta,Nb,Ti)-free dumortierite-like mineral (16.81 wt.% As2O3 and 10.23 wt.% Sb2O3) with (As+Sb) > Si. The last phase is potentially a new mineral species. Al6□☐(Sb,As)3O15, or Al5☐2B(Sb,As)3O12(OH)3, belonging to the dumortierite group. The Szklary holtite shows no evidence of clustering of compositions around ‘holtite I’ and ‘holtite II'. Instead, the substitutions of Si4+ by Sb3++As3+ at the Si/Sb sites and of Ta5+ by Nb5+ or Ti4+ at the Al(l) site suggest possible solid solutions between: (1) (Sb,As)-poor and (Sb,As)-rich holtite; (2) dumortierite and the unnamed (As+Sb)-dominant dumortierite-like mineral; and (3) Ti-bearing dumortierite and holtite. i.e. our data provide further evidence for miscibility between holtite and dumortierite, but leave open the question of defining the distinction between them. The Szklary holtite crystallized from the melt along with other primary Ta-Nb-(Ti) minerals such as columbite-(Mn), tantalite-(Mn), stibiotantalite and stibiocolumbite as the availability of Ta decreased. The origin of the parental melt can be related to anatexis in the adjacent Sowie Mountains complex, leading to widespread migmatization and metamorphic segregation in pelitic-psammitic sediments metamorphosed at ∼390—380 Ma.
New data on welshite, e.g. Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2[Si2.8Be1.7Fe0.653+Al0.7As0.17O18], an aenigmatite-group mineral
- E. S. Grew, U. Hålenius, M. Kritikos, C. K. Shearer
-
- Journal:
- Mineralogical Magazine / Volume 65 / Issue 5 / October 2001
- Published online by Cambridge University Press:
- 05 July 2018, pp. 665-674
-
- Article
- Export citation
-
Electron and ion microprobe data on two samples of welshite from the type locality of Långban, Sweden, gave analytical totals of 99.38–99.57 wt.% and BeO contents of 4.82–5.11 wt.%, corresponding to 1.692–1.773 Be/20 O. Mössbauer and optical spectra of one of these samples gave [iv]Fe3+/ΣFe = 0.91, [vi]Fe2+/ΣFe = 0.09, and no evidence of Mn3+. The resulting formula for this sample is Ca2Mg3.8Mn0.62+Fe0.12+Sb1.55+O2[Si2.8Be1.7Fe0.653+Al0.7As0.17O18, and that for the second sample, Ca2Mg3.8Mn0.12+Fe0.12+Fe0.83+Sb1.25+O2[Si2.8Be1.8Fe0.653+Al0.5As0.25O18], is related by the substitution involving tetrahedral and octahedral sites: 0.59[vi,iv](Fe,Al)3+ ≈ 0.42[vi](Mg,Mn,Fe)2+ + 0.21([vi]Sb,[iv]As)5+, i.e. 3[vi,iv]M3+ = 2[vi]M2+ + [vi,iv]M5+. Welshite is distinctive among aenigmatite-group minerals in the high proportion of Fe3+ in tetrahedral coordination and is unique in its Be content, substantially exceeding 1Be per formula unit. Given the cation distributions in other minerals related to aenigmatite, we think it is reasonable to assume that at least one tetrahedral site is >50% occupied by Be and that one octahedral site is >50% occupied by Sb, so that welshite should be retained as a distinct species with its own name in the aenigmatite group.
Prismatine and ferrohögbomite-2N2S in granulite-facies Fe-oxide lenses in the Eastern Ghats Belt at Venugopalapuram, Vizianagaram district, Andhra Pradesh, India: do such lenses have a tourmaline-enriched lateritic precursor?
- E. S. Grew, A. T. Rao, K. K. V. S. Raju, C. Hejny, J. M. Moore, D. J. Waters, M. G. Yates, C. K. Shearer
-
- Journal:
- Mineralogical Magazine / Volume 67 / Issue 5 / October 2003
- Published online by Cambridge University Press:
- 05 July 2018, pp. 1081-1098
-
- Article
- Export citation
-
Fluorine-rich prismatine, (□,Fe,Mg)(Mg,Al,Fe)5Al4(Si,B,Al)5O21(OH,F), with F/(OH+F) = 0.36–0.40 and hercynite are major constituents of a Fe-Al-B-rich lens in ultrahigh-temperature granulite-facies quartz-sillimanite-hypersthene-cordierite gneisses of the Eastern Ghats belt, Andhra Pradesh, India. Hemo-ilmenite, sapphirine, magnetite, biotite and sillimanite are subordinate. Lithium, Be and B are concentrated in prismatine (140 ppm Li, 170 ppm Be, and 2.8 –3.0 wt.% B2O3). Another Fe-rich lens is dominantly magnetite, which encloses fine-grained zincian ferrohögbomite-2N2S, (Fe2+,Mg,Zn,Al)6 (Al,Fe3+,Ti)16O30(OH)2, containing minor Ga2O3 (0.30 –0.92 wt.%). Fe-Al-B-rich lenses with prismatine (or kornerupine) constitute a distinctive type of B-enrichment in granulite-facies rocks and have been reported from four other localities worldwide. A scenario involving a tourmalineenriched lateritic precursor affected by dehydration melting is our preferred explanation for the origin of the Fe-Al-B-rich lenses at the five localities. Whole-rock analyses and field relationships at another of these localities, Bok se Puts, Namaqualand, South Africa, are consistent with this scenario. Under granulite-facies conditions, tourmaline would have broken down to give kornerupine-prismatine (±other borosilicates) plus a sodic melt containing H2O and B. Removal of this melt depleted the rock in Na and B, but the formation of ferromagnesian borosilicate phases in the restite prevented total loss of B.
The dumortierite supergroup. I. A new nomenclature for the dumortierite and holtite groups
- A. Pieczka, R. J. Evans, E. S. Grew, L. A. Groat, C. Ma, G. R. Rossman
-
- Journal:
- Mineralogical Magazine / Volume 77 / Issue 6 / August 2013
- Published online by Cambridge University Press:
- 05 July 2018, pp. 2825-2839
-
- Article
- Export citation
-
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.
(1) Dumortierite group, with Al1 = Al3+, Mg2+ and ☐, where ☐ denotes cation vacancy. Charge balance is provided by OH substitution for O at the O2, O7 and O10 sites. In addition to dumortierite, endmember composition AlAl6BSi3O18, and magnesiodumortierite, endmember composition MgAl6BSi3O17(OH), plus three endmembers, “hydroxydumortierite”, ☐Al6BSi3O15(OH)3 and two Mg-Ti analogues of dumortierite, (Mg0.5Ti0.5)Al6BSi3O18 and (Mg0.5Ti0.5)Mg2Al4BSi3O16(OH)2, none of which correspond to mineral species. Three more hypothetical endmembers are derived by homovalent substitutions of Fe3+ for Al and Fe2+ for Mg.
(2) Holtite group, with Al1 = Ta5+, Nb5+, Ti4+ and ☐. In contrast to the dumortierite group, vacancies serve not only to balance the extra charge introduced by the incorporation of pentavalent and quadrivalent cations for trivalent cations at Al1, but also to reduce repulsion between the highly charged cations. This group includes holtite, endmember composition (Ta0.6☐0.4)Al6BSi3O18, nioboholite (2012-68), endmember composition (Nb0.6☐0.4)Al6BSi3O18, and titanoholtite (2012-69), endmember composition (Ti0.75☐0.25)Al6BSi3O18.
(3) Szklaryite (2012-70) with Al1 = ☐ and an endmember formula ☐Al6BAs3+3O15. Vacancies at Al1 are caused by loss of O at O2 and O7, which coordinate the Al1 with the Si sites, due to replacement of Si4+ by As3+ and Sb3+, 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: (As3+ + Sb3+) > Si4+ consistent with the dominant-valency rule. A Sb3+ analogue to szklaryite is possible.