Transformation Series Parakeldyshite–Keldyshite: Mechanism of Transformation, Chemical Formula Revision, and Crystal Structure of Keldyshite

Abstract

A holotype sample of keldyshite has been studied. It was found to consist of aggregates of partially protonated, Na-deficient parakeldyshite (which is dominant) and keldyshite itself. A series of laboratory experiments on the hydrolysis of parakeldyshite was performed, as a result of which the crystal structure of the keldyshite formed during this process was refined. A refined idealized formula was proposed for it: (Na\(\square \))ZrSi₂O₆(OH). As a result of the protonation of the oxygen atom at the О5 position and formation of strong hydrogen bonds (with an O5…O5 distance of 2.458 Å in keldyshite), a series of hinge transformations occurs, causing a significant distortion of the Zr–Si–O framework within the crystal structure. The existence of the transformation series from parakeldyshite to keldyshite in nature is confirmed. The mechanism of keldyshite formation can be described by the reaction scheme Na⁺ + O²⁻ → \(\square \) + (OH)⁻. The substitution reaction of parakeldyshite by keldyshite most likely occurs via a transition from single crystal to single crystal.