Dana medal lecture: Tilts and tetrahedra: The origin of the anisotropy of feldspars
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Ross J. Angel
,
Lindsay M. Sochalski-Kolbus
Abstract
Following the ideas of Helen Megaw, we describe the changes in the conformation of the tetrahedral framework of feldspars in terms of just four distinct tilt systems of rigid tetrahedra. These systems are based on the four allowed tilts of a ring of four corner-linked tetrahedra with point symmetry 2. Of the four tilt systems, only two result in significant volume change of the unit cell. We show that all of the essential features of the structures, unit-cell parameters and volumes of the AlSi3 feldspars, and their expansion and compression induced by changes in pressure, temperature, and composition at crustal pressures, are generated by the simultaneous application of these two tilts to an un-tilted framework of regular tetrahedra. In combination these two tilts impose significant anisotropy upon the expansion of the unit cell of the feldspar with the majority of the expansion accommodated by the expansion of d(100). This demonstrates that the fundamental reason for the anisotropy of feldspars lies in the topology of the tetrahedral framework. A comparison of the actual tilts observed in alkali feldspars with the model shows that the tilts maximize the shortest O-O distances in the structure, and therefore O-O repulsions along with the volume requirement control the values of the tetrahedral tilts in alkali feldspars and thus the anisotropy of the structure. Therefore, the bonding requirements of the bridging O atoms and the directionality of the bonding to the extraframework cations only play a secondary role in modifying this basic pattern of anisotropy, which is intrinsic to the common topology of the framework of all feldspars.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Dana medal lecture: Tilts and tetrahedra: The origin of the anisotropy of feldspars
- Amorphous Materials: Determination of water content in silicate glasses using Raman spectrometry: Implications for the study of explosive volcanism
- Density functional calculations of the enthalpies of formation of rare-earth orthophosphates
- U-Pb ID-TIMS dating applied to U-rich inclusions in garnet
- Wassonite: A new titanium monosulfide mineral in the Yamato 691 enstatite chondrite
- Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy
- The morphology of the reaction front of the dissolutionprecipitation reaction rutile + wollastonite = titanite in time series experiments at 600 °C/400 MPa
- Precipitation of rutile and ilmenite needles in garnet: Implications for extreme metamorphic conditions in the Acadian Orogen, U.S.A.
- The genesis of mantle-derived sapphirine
- Experimental investigation of smectite interaction with metal iron at 80 °C: Structural characterization of newly formed Fe-rich phyllosilicates
- High-pressure study of a natural cancrinite
- Investigation of cation ordering in triclinic sodium birnessite via 23Na MAS NMR spectroscopy
- The dual behavior of the β-As4S4 altered by light
- Pressure-induced changes in local electronic structures of SiO2 and MgSiO3 polymorphs: Insights from ab initio calculations of O K-edge energy-loss near-edge structure spectroscopy
- Structural stability, cation ordering, and local relaxation along the AlNbO4-Al0.5Cr0.5NbO4 join
- Europium oxidation state and local structure in silicate glasses
- Kinetics of partial dehydroxylation in dioctahedral 2:1 layer clay minerals
- Determination of Fe3+/Fe using the electron microprobe: A calibration for amphiboles
- Characterization of smectite to NH4-illite conversion series in the fossil hydrothermal system of Harghita Bãi, East Carpathians, Romania
- Allanite-(Nd), CaNdAl2Fe2+(SiO4)(Si2O7)O(OH), a new mineral from Åskagen, Sweden
- Tsilaisite, NaMn3Al6(Si6O18)(BO3)3(OH)3OH, a new mineral species of the tourmaline supergroup from Grotta d’Oggi, San Pietro in Campo, island of Elba, Italy
- Letter. A nanoscopic approach to the kinetics of anhydrite (100) surface growth in the range of temperatures between 60 and 120 °C
- Letter. High-pressure phase transitions in Ca-Mn carbonates (Ca,Mn)CO3 studied by Raman spectroscopy
- Letter. Actinides in Geology, Energy, and the Environment: Thermally induced transformation of vorlanite to “protovorlanite”: Restoration of cation ordering in self-irradiated CaUO4
