Stability and phase relations of Ca[ZnSi3]O8, a new phase with feldspar structure in the system CaO-ZnO-SiO2
-
Karl Thomas Fehr
and
Alexandra L. Huber
Abstract
Zinc feldspar [Ca(ZnSi3)O8] has been synthesized hydrothermally and its phase relations have been investigated in the system CaO-ZnO-SiO2 up to pressures of 1.0 GPa. Single-phase Zn feldspar grew from an oxide mixture at 600 °C and 0.7 GPa within 6 days. It crystallizes in space group P - 1 displaying lattice constants of a = 8.121(1) Å, b = 12.927(1) Å, c =7.206(1) Å, α = 93.76 (5)°, β = 116.120(7)°, γ = 84.368(7)°, V = 675.7 Å3, Z = 2 (Heuer et al. 1998). In Zn feldspar, Zn and Si are ordered on the tetrahedral sites and the structure of Zn feldspar is more similar to the low albite structure than to that of anorthite due to the tetrahedral ordering scheme. Zinc feldspar does not melt congruently but decomposes at subsolidus conditions according to the reaction: 4 Zn feldspar = willemite + 2 hardystonite + 7 quartz (1). Reaction 1 was determined by means of reversed experiments and its negative P-T slope can be expressed by the equation P (GPa) = 2.797(0.325) - 0.0031 (0.0004) T (°C). Introducing the stability relations of the SiO2 system, two invariant points, Iqtz and Itrid, are generated at 720 °C, 0.57 GPa and 896 °C, 0.016 GPa projected on the low/high-quartz and high-quartz/tridymite phase transitions, respectively. The phase assemblage willemite+ hardystonite+quartz is not stable at pressures above 1.2 GPa where petedunnite is formed according to the reaction: willemite + 2 hardystonite + 3 quartz = 4 petedunnite (2) (Rothkopf and Fehr 1998). The stability field of Zn feldspar is restricted to pressures <1.2 GPa and temperatures <530 °C, where Zn feldspar decomposes according to the doubly degenerated reaction: Zn feldspar = petedunnite + quartz (3). The intersection of reactions 1 to 3 generates an invariant point at ca. 530 °C and 1.2 GPa. Zinc usually is not determined routinely in electron microprobe analyses of feldspars and therefore data on the Zn contents of natural feldspar are scarce. Nevertheless, some feldspars of Pb-Zn hydrothermal vein deposits and skarns display deficiencies of tetrahedral cations and contain up to 0.18 wt% ZnO in combination with appreciable BaO contents, implying the existence of a Ba(ZnSi3)O8 component in addition to Ca(ZnSi3)O8. We recommend inclusion of Zn in analysis of feldspars from such paragenesis.
© 2015 by Walter de Gruyter Berlin/Boston
Articles in the same Issue
- Self diffusion of Si and O in dacitic liquid at high pressures
- The effect of anhydrous composition on water solubility in granitic melts
- Stability and phase relations of Ca[ZnSi3]O8, a new phase with feldspar structure in the system CaO-ZnO-SiO2
- Magmatic Na-rich phlogopite in a suite of gabbroic crustal xenoliths from Volcán San Pedro, Chilean Andes: Evidence for a solvus relation between phlogopite and aspidolite
- The influence of T, aSiO₂, and fO₂ on exsolution textures in Fe-Mg olivine: An example from augite syenites of the Ilimaussaq Intrusion, South Greenland
- An experimental study of the external reduction of olivine single crystals
- Determination of site population in olivine: Warnings on X-ray data treatment and refinement
- Structural properties of ferromagnesian cordierites
- A calorimetric study of zoisite and clinozoisite solid solutions
- Mineralogy of lead in a soil developed on a Pb-mineralized sandstone (Largentière, France)
- Experimental mixtures of smectite and rectorite: Re-investigation of “fundamental particles” and “interparticle diffraction”
- Hydrothermal reactivity of Lu-saturated smectites: Part I. A long-range order study
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- Pulsed field gradient proton NMR study of the self-diffusion of H2O in montmorillonite gel: Effects of temperature and water fraction
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- Structure, compressibility, hydrogen bonding, and dehydration of the tetragonal Mn3+ hydrogarnet, henritermierite
- Electric field gradient tensors at the aluminum sites in the Al2SiO5 polymorphs from CCD high-resolution X-ray diffraction data: Comparison with 27Al NMR results
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- O-D…O bond geometry in OD-chondrodite
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- In situ dehydration of yugawaralite
- Molecular dynamics simulation of phase transitions and melting in MgSiO3 with the perovskite structure—Comment
- Reply to Comment on “Molecular dynamics simulation of phase transitions and melting in MgSiO3 with the perovskite structure”
Articles in the same Issue
- Self diffusion of Si and O in dacitic liquid at high pressures
- The effect of anhydrous composition on water solubility in granitic melts
- Stability and phase relations of Ca[ZnSi3]O8, a new phase with feldspar structure in the system CaO-ZnO-SiO2
- Magmatic Na-rich phlogopite in a suite of gabbroic crustal xenoliths from Volcán San Pedro, Chilean Andes: Evidence for a solvus relation between phlogopite and aspidolite
- The influence of T, aSiO₂, and fO₂ on exsolution textures in Fe-Mg olivine: An example from augite syenites of the Ilimaussaq Intrusion, South Greenland
- An experimental study of the external reduction of olivine single crystals
- Determination of site population in olivine: Warnings on X-ray data treatment and refinement
- Structural properties of ferromagnesian cordierites
- A calorimetric study of zoisite and clinozoisite solid solutions
- Mineralogy of lead in a soil developed on a Pb-mineralized sandstone (Largentière, France)
- Experimental mixtures of smectite and rectorite: Re-investigation of “fundamental particles” and “interparticle diffraction”
- Hydrothermal reactivity of Lu-saturated smectites: Part I. A long-range order study
- Hydrothermal reactivity of Lu-saturated smectites: Part II. A short-range order study
- Pulsed field gradient proton NMR study of the self-diffusion of H2O in montmorillonite gel: Effects of temperature and water fraction
- Structural environment and oxidation state of Mn in goethite-groutite solid-solutions
- Structure, compressibility, hydrogen bonding, and dehydration of the tetragonal Mn3+ hydrogarnet, henritermierite
- Electric field gradient tensors at the aluminum sites in the Al2SiO5 polymorphs from CCD high-resolution X-ray diffraction data: Comparison with 27Al NMR results
- Sodium cation dynamics in nitrate cancrinite: A low and high temperature 23Na and 1H MAS NMR study and high temperature Rietveld structure refinement
- O-D…O bond geometry in OD-chondrodite
- Refinement of hydrogen positions in synthetic hydroxyl-clinohumite by powder neutron diffraction
- In situ dehydration of yugawaralite
- Molecular dynamics simulation of phase transitions and melting in MgSiO3 with the perovskite structure—Comment
- Reply to Comment on “Molecular dynamics simulation of phase transitions and melting in MgSiO3 with the perovskite structure”
