The incommensurately modulated structures of volcanic plagioclase: displacement, ordering and phase transition

Abstract

Four basaltic phenocryst samples of plagioclase, with compositions ranging from An 48 (andesine) to An 64 (labradorite), have been studied with single-crystal X-ray and neutron diffraction techniques. The samples were also subjected to a heating experiment at 1100 C for two weeks in an effort to minimize the Al-Si ordering in their structures. The average and the modulated structures of the samples (before and after the heating experiment) were compared, in order to understand the mechanism of the phase transition from the disordered C " 1 1 structure to the e-plagioclase structure. A comparison between the structures from neutron and X-ray diffraction data shows that the hT-Oi distance does not solely depend on the Al occupancy as previously thought. A dramatic decrease of the Al-Si ordering is observed after heating at 1100 C for two weeks for all four samples, with an obvious change in the intensities of the satellite reflections (e-reflections) in the diffraction pattern. Evident changes in the modulation period were also observed for the more calcic samples. No obvious change in the Ca-Na ordering was observed after the heating experiment. An in situ heating X-ray diffraction experiment was carried out on the andesine sample (An 48 ) to study the change in the satellite intensity at high temperature.A dramatic weakening of the satellite peaks was observed between 477 C and 537 C, which strongly supports the displacive nature of the initiation of e2 ordering. Rigid-Unit Mode (RUM) analysis of the plagioclase structure suggests the initial position of the e-reflections is determined by the anti-RUMs in the framework.

The incommensurately modulated structures of low-temperature labradorite feldspars: a single-crystal X-ray and neutron diffraction study

“…The e-reflections in the volcanic phenocrysts are normally weak and diffuse; therefore, the q-vectors are relatively poorly defined. The q-vectors also depend on the ordering states of the structure as discovered by Jin, Xu et al (2019). We suggest calling these volcanic phenocryst e-plagioclases with diffuse and 'moving' satellite peaks e , to separate them from the e and e structures where the q-vectors do not change with research papers Acta Cryst.…”

“…B76, 93-107 ordering states. Jin, Xu et al (2019) speculated that all disordered C " 1 1 structures start showing e-reflections around the same positions in reciprocal space, independent of the composition, which are governed by the Anti-Rigid Unit Modes (ARUM) of the tetrahedral framework. The q-vector moves towards the e line as the modulation gets stronger in the structure.…”

Revisiting the Bøggild Intergrowth in Iridescent Labradorite Feldspars: Ordering, Kinetics, and Phase Equilibria

“…The subsolidus phase relations of plagioclase feldspar solution has been most comprehensively summarized by Carpenter [13] (Figure S16), taking into consideration all the reported experimental data and natural samples available at the time. There are, however, some inconsistencies between the labradorite region in Carpenter's phase diagram and some recent discoveries regarding the incommensurately modulated structures of e-plagioclase [14,15,23,29,30,32]. The new data reported in this paper regarding the Bøggild intergrowth also provide some new insights to the subsolidus phase relations of the low-temperature intermediate plagioclase, as is discussed below.…”

“…The first phase relation to be discussed is that between the e1 and e2 phases. Recent studies [15,23,30,32] have shown that the e2 structure can be ordered into the e1 structure…”

“…The first phase relation to be discussed is that between the e1 and e2 phases. Recent studies [15,23,30,32] have shown that the e2 structure can be ordered into the e1 structure continuously, and a second-order phase boundary has been proposed between An 40 and An 44 at below 550 • C. A small discontinuity is also possible, as suggested by the abrupt change in the q-vectors in sample H4-04 between An 44 and An 45 [29,30]. The homogeneous metamorphic e1 structure with a composition of An 52 [14] and the igneous e1 structures with compositions as sodic as An 42 [30] all suggest a continuous region of the e1 phase bounded by a inclined gap with the C1 phase, which was first proposed by McConnell [12,26] and was adopted by the phase relations in Jin et al [23].…”