Atomic movements in plagioclase feldspars: Kinetic interpretation
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Anorthite
Orthoclase
Anorthite
Alkali feldspar
Orthoclase
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(1979). The albite-anorthite-orthoclase system. International Geology Review: Vol. 21, No. 12, pp. 1416-1426.
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The models of the modulated structure of the plagioclase feldspar based on X-ray diffraction and electron microscopy have been reviewed. Important models so far presented can be divided into two categories due to difference in evaluation of the second harmonic of the (Ca, Na) modulation wave. The first type (TF***, HTKJ**** and KHNE***** models), where the second harmonic is assumed to be negligible, gives the ordered arrangement of Na and Ca among the neighbouring subcells. On the other hand, in the second type (KM** model) the second harmonic is strong enough to give the alternating bands of the albite-like and anorthite-like structures. Because of the weak intensities of the second satellite reflections corresponding to the second harmonic, the final answer between the model types has not been obtained using only the X-ray diffraction method. However, the fine textures with the antiphase domain boundaries and crystal-chemical considerations of the plagioclase feldspars support the existence of the second harmonic, or, of the alternating bands of the albite-like and anorthite-like structures.
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The modulated structures of plagioclase feldspars have been extensively studied by the X‐ray diffraction method and high resolution electron microscopy. The results on bytownite (An73) and labradorite (An52) indicate that the modulated structure of the e‐plagioclase consist of the periodic antiphase domains of the anorthite‐like structure separated by the antiphase boundaries which form bands of albite‐like structure. Orientation and width of the antiphase domains change with composition of the e‐plagioclase. This results in the movement of the e satellites for the e‐plagioclase.The modulated structures in the e‐plagioclase are considered to appear as a possible precursor to unmixing to two stable phases of anorthite and albite in the process of ordering of substituting atoms in the high‐temperature form. The periodicity of the antiphase boundaries observed in the e‐plagioclase becomes irregular as the Na content of plagioclass becomes less than An75, resulting in the b‐type antiphase boundaries.
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A new equilibrium diagram for the system Or: Ab: An has been constructed on the basis of the feldspar content of a large number of rocks. It indicates that an increase in the anorthite content of the plagioclase present corresponds to an increase in the possible maximum of orthoclase in the rocks. Thus the eutectic point between ortho-clase and albite lies at about 45 Or: 55 Ab, whereas the boundary between the orthoclase and plagioclase fields in the vicinity of andesine is near 70 Or: 30 ().
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Feldspars are one of the archetypical families of framework silicates. They not only comprise around 60% volumetrically of the Earth’s crust, but are among some of the most structurally complicated minerals. Investigation into the structural behavior of various intermediate plagioclases at pressure has been undertaken with the intent of categorizing the elastic behavior with pressure across the solid solution series and establishing a conceptual model to characterize feldspar compression. Complex behavior has been observed in the Equation of State for plagioclase feldspars in excess of 3 GPa, including an anomalous softening of ordered albite in excess of 8.4 GPa (Benusa et al 2005: Am Min 90:1115-1120). This softening was not observed in the EoS for the more intermediate plagioclase compositions containing between 20 and 40 mol% of end-member anorthite. The calculated elastic compliance tensor sums at room pressure show a general stiffening with increasing anorthite component, small elastic changes at the C-1 to I-1 transition, and a dominantly first-order response at the P-1 to I-1 transition near end-member anorthite. The crystal structure of An37 plagioclase was determined by single-crystal X-ray diffraction. The compression mechanisms in An37 are similar to those in albite at lower pressures. The softening in albite at higher pressures is therefore attributed to the structural shearing in albite that is absent in An37 plagioclase up to 9.5 GPa.
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