Transformation series parakeldyshite-keldyshite: mechanism of transformation, chemical formula revision and crystal structure of keldyshite

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