The structure of kaliophilite KAlSiO₄, a long-lasting crystallographic problem

Abstract

Kaliophilite is a feldspathoid mineral found in two Italian magmatic provinces and represents one of the 12 known phases with composition close to KAlSiO4. Despite its apparently simple formula, the structure of this mineral revealed extremely complex and resisted structure solution for more than a century. Samples from the Vesuvius–Monte Somma and Alban Hills volcanic areas were analyzed through a multi-technique approach, and finally the crystal structure of kaliophilite was solved using 3D electron diffraction and refined against X-ray diffraction data of a twinned crystal. Results were also ascertained by the Rietveld method using synchrotron powder intensities. It was found that kaliophilite crystallizes in space group P3 with unit-cell parameters a = 27.0597 (16), c = 8.5587 (6) Å, V = 5427.3 (7) Å3 and Z = 54. The kaliophilite framework is a variant of the tridymite topology, with alternating SiO4 and AlO4 tetrahedra forming sheets of six-membered rings (63 nets), which are connected along [001] by sharing the apical oxygen atoms. Considering the up (U) and down (D) orientations of the linking vertex, kaliophilite is the first framework that contains three different ring topologies: nine (1-3-5) (UDUDUD) rings, six (1-2-3) (UUUDDD) rings and twelve (1-2-4) (UUDUDD) rings. This results in a relatively open (19.9 tetrahedra nm−3) channel system with multiple connections between the double six-ring cavities. Such a framework requires a surprisingly large unit cell, 27 times larger than the cell of kalsilite, the simplest phase with the same composition. The occurrence of some Na for K substitution (3–10%) may be related to the characteristic structural features of kaliophilite. Micro-twinning, pseudo-symmetries and anisotropic hkl-dependent peak broadening were also detected, and they may account for the elusive character of the kaliophilite crystal structure.

An Orthorhombic Modification of KCoPO4 Stabilized under Hydrothermal Conditions: Crystal Chemistry and Magnetic Behavior

“…32,33 The currently known 13 types of 3D crystal structures of tridymite derivatives have various combinations of the ring topologies. 34 In the literature, there are heterogeneous data on the unitcell parameters and symmetry of polymorphs of potassium− cobalt orthophosphate KCoPO 4 (Table 2). According to ref 20, this controversial information may arise from the difficulties of growing single domain crystals.…”

“…All of them can be interpreted on the basis of a framework built of layers of hexagonal rings of tetrahedra sharing vertices; these layers are joined together through oxygen bridges. The different structure types may be distinguished within this family in accordance with the orientation of the tetrahedra in six-membered rings regarding plane grids. , The currently known 13 types of 3D crystal structures of tridymite derivatives have various combinations of the ring topologies …”

3D electron diffraction study of terrestrial iron oxide alteration in the Mineo pallasite

“…In Earth sciences, 3D ED has been used for the structure elucidation of nanoscopic inclusions (Xiong et al , 2020), mineral seeds (Németh et al , 2018), hydrated phases (Mugnaioli et al , 2020a; Krysiak et al , 2021) and modulated systems (Lanza et al , 2019; Steciuk et al , 2020), even when characterised by very large asymmetric units (Rozhdestvenskaya et al , 2010; Mugnaioli et al , 2020b). 3D ED has been applied to geological samples in different contexts (Mugnaioli and Gemmi, 2018), and in particular for the characterisation of meteorites (Pignatelli et al , 2017, 2018; Suttle et al , 2021), impactites (Campanale et al , 2021), cryptocrystalline oxides (Koch-Müller et al , 2014) and hydroxides (Viti et al , 2016).…”

Structural and chemical complexity of minerals: an update

“…The complexity of chemical composition and crystal structure are therefore fundamental characteristics of minerals that have high relevance to the understanding of their stability, occurrence and evolution. The universal quantitative measures of structural and chemical complexity were proposed in Krivovichev (2012, 2013, 2014; Krivovichev et al , 2018) on the basis of Shannon information theory and have been recognised as useful parameters for the analysis of various mineralogical and crystal chemical phenomena, from crystal-structure classifications to metastable crystallisation and mineral evolution (Tambornino et al , 2015; Christy et al , 2016; Varn and Crutchfield, 2016; Plášil et al , 2017; Plášil, 2018a, 2018b, 2018c; Ovchinnikov et al , 2018; Majzlan et al , 2018; Colmenero et al , 2019; Gurzhiy and Plášil, 2019; Gurzhiy et al , 2019, 2021; Hornfeck, 2020; Bindi et al , 2020; Majzlan, 2020; Mugnaioli et al , 2020; Sabirov and Shepelevich, 2021; Kaußler and Kieslich, 2021, etc.). The aim of this review is to summarise the recent findings in the field of structural and chemical complexity of minerals and to outline possible directions for its further development.…”