The Crystal Chemistry of Holtite

Holtite, approximately (Al,Ta,square)Al(6)(BO(3))(Si,Sb(3+),As(3+))(Sigma 3)O(12)(O,OH,square)(Sigma 3), 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\u27i Tundry, Kola Peninsula, Russia; and Szklary, Lower Silesia, Poland. Holtite can contain \u3e30 wt.% Sb(2)O(3), As(2)O(3), Ta(2)O(5), Nb(2)O(5), and TiO(2) (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 R(1) = 0.02-0.05. The results show dominantly: Al, Ta, and vacancies at the Al(1) 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: Al(7-[5x+y+z]/3)(Ta,Nb)(x)square([2x+y+z]/3)BSi(3-y)(Sb,As)(y)O(18-y-z)(OH)(z), where x is the total number of pentavalent cations, y is the total amount of Sb + As, and z \u3c= y is the total amount of OH. Comparison with the electron microprobe compositions suggests the following approximate general formulae Al(5.83)(Ta,Nb)(0.50)square(0.67)BSi(2.50)(Sb,As)(0.50)O(17.00)(OH)(0.50) and Al(5.92)(Ta,Nb)(0.25)square(0.83)BSi(2.00)(Sb,As)(1.00) O(16.00)(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

Structural phase transitions in the Ag2Nb4O11 – Na2Nb4O11 solid solution

The phase transitions between various structural modifications of the natrotantite-structured system xAg2Nb4O11 – (1-x)Na2Nb4O11 have been investigated and a phase diagram constructed as a function of temperature and composition. This shows three separate phase transition types: (1) paraelectric – ferroelectric, (2) rhombohedral – monoclinic and (3) a phase transition within the ferroelectric rhombohedral zone between space groups R3c and R3. The parent structure for the entire series has space group R-3c. Compositions with x > 0.75 are rhombohedral at all temperatures whereas compositions with x < 0.75 are all monoclinic at room temperature and below. At x = 0.75, rhombohedral and monoclinic phases coexist with the phase boundary below room temperature being virtually temperature-independent. The ferroelectric phase boundary extends into the monoclinic phase field. No evidence was found for the R3–R3c phase boundary extending into the monoclinic phase field and it is concluded that a triple point is formed

Ferroelectricity in the xAg2Nb4O11–(1−x)Na2Nb4O11 solid solution

Compositions in the (AgxNa1-x)2Nb4O11 solid solution have been prepared by a conventional solid state method. Composites containing Ag2Nb4O11 have been shown to be ferroelectric and the Curie temperature shown to decrease from 149 °C at x = 1 to 62 °C at x = 0.7. Roomtemperature compositions with x ≤ 0.7 are monoclinic, while those with x ≥ 0.8 are rhombohedral with structures consistent with the relevant end-members. At x = 0.75, the structure was mainly rhombohedral but with coexistence of the monoclinic structure, indicating the proximity of a phase boundary

Food availability and predation risk, rather than intrinsic attributes are the main factors shaping the reproductive decisions of a long-lived predator

Acknowledgements We thank B. Sheldon and two anonymous reviewers for all their helpful comments on a previous version of the manuscript. Our thanks also go to M. Davison, B. Little, P. Hotchin, D. Anderson and all other field assistants for their help with data collection and Forest Enterprise, particularly Tom Dearnley and Neville Geddes for facilitating work in Kielder Forest. We are also grateful to C. Sutherland for his help and advice on statistical analyses. This work was partly funded by Natural Research Limited and a Natural Environment Research Council studentship NE/J500148/1 to SH and grant NE/F021402/1 to XL. Forest Research funded all the fieldwork on goshawks, tawny owls and field voles during 1973-1996. In addition, we are grateful to English Nature and the BTO for issuing licences to visit goshawk nest sites.Peer reviewedPublisher PD

Sexual and Viability Selection on a Wild Population of Tree Crickets Experiencing Female-biased Predation

Sex-biased predation can have important evolutionary consequences for prey, however female-biased predation is uncommon in nature and understudied in science. Understanding female-biased predation is necessary to create a complete picture of the costs of sex-specific behaviours and phenotypes. In this thesis, I used a system of tree crickets, Oecanthus nigricornis, which suffer female-biased predation by wasp predators, Isodontia mexicana, to 1) test hypotheses about the causes of female-biased predation; 2) quantify how viability selection caused by such predation may affect male and female traits differently; and 3) test if sexual selection opposes or reinforces viability selection among each sex. To address the first objective, I compared tree cricket prey taken from I. mexicana nests to surviving tree crickets, and found that wasps took significantly more adult females only when females were larger than males. I also found that females with more ovarian eggs were more likely to be caught by wasps, and laboratory experiments showed that female jumping ability is negatively affected by egg load. To address the second objective, I compared viability selection (from wasp predation) on males and females over several years. Within single study seasons, male and female phenotypes were affected differently by viability selection, but temporal variation in selection across years resulted in overall weak nonlinear selection on both sexes, and no significant effect on sexual dimorphism. To address the third objective, I compared the direction and form of sexual selection (calculated using estimates of mating success) in males and females to those of viability selection, and found that sexually selected traits had viability costs in males but not in females. Thus, in this predator-prey system, I found that females pay a viability cost for investing in fecundity traits, and males pay a viability cost for investing in sexual traits.Ph.D

Some recent advances in the mineralogy and geochemistry of Nb and Ta in rare-element granitic pegmatites

A review of the current understanding of the chemistry and structural properties is provided for columbite-tantalite, ferrotapiolite-manganotapiolite, ixiolite, wodginite, tantalian and niobian rutile, tantalian-niobian cassiterite, and the pyrochlore group. Understanding of the crystal chemistry of some of the most common species appears to be rather spurious. Paragenetic assemblages of primary Nb, Ta-bearing minerals typical of different classes of rare-element pegmatites are summarized. They show extensive gaps in upgrading old and partly obsolete information. In contrast, a review of recently discovered "exotic" Nb, Ta-bearing minerals is complemented by a summary of late alterations and replacements which is well documented. Extensive variety in fractionation of columbite-tantalite is demonstrated on some pegmatites, pegmatite groups and fields. More data from different pegmatite types are required, in conjunction with experimental work, to understand the factors regulating the course of fractionation. Compositional and structural stabilities are poorly understood. They require thorough experimental investigation, under conditions comparable to those of crystallizing pegmatite melts and fluids.On a procédé à l'examen des données existantes sur la composition chimique et les propriétés structurales de columbite-tantalite, ferrotapiolite-manganotapiolite, ixiolite, wodginite, rutile tantalifère et niobifère, cassitérite niobo-tantalifère, et du groupe de pyrochlore. La compréhension de la cristallochimie de certaines de ces espèces apparaît quelque peu erronée. La synthèse des observations sur les paragenèses primaires des minéraux de Nb-Ta, caractéristiques des différents types de pegmatites à éléments rares, met en évidence des lacunes considérables lorsque l'on réinterprète les anciennes données partiellement désuètes. Par contre, les données sur les minéraux "exotiques" niobo-tantalifères, récemment découverts, sont complétées par un résumé bien documenté des phénomènes de remplacement et d'altérations tardives. On met en évidence des variations considérables du fractionnement chimique de la columbite-tantalite dans certaines pegmatites et districts pegmatitiques. Afin de comprendre les facteurs qui président à ce fractionnement, il est indispensable d'acquérir les données supplémentaires sur différents types de pegmatites, complétées par des études expérimentales. Les conditions de stabilité chimique et structurale sont mal connues. Leur compréhension nécessite une étude expérimentale détaillée, menée dans des conditions comparables à celles de la cristallisation des bains et fluides pegmatitiques .Černý Petr, Ercit T. Scott. Some recent advances in the mineralogy and geochemistry of Nb and Ta in rare-element granitic pegmatites. In: Bulletin de Minéralogie, volume 108, 3-4, 1985. Minéralogie dans les Sciences de la Terre et l'Industrie - Hommage à François Permingeat