We detail the synthesis and characterization of KAlSiO4-O1, a kalsilite polymorph which for years has been considered a material of technological interest. Our experimental protocol requires the preparation of a precursor, obtained by the mixing of potassium hydroxide, aluminium hydroxide and naturally derived amorphous silica (diatomitic rock from Crotone, southern Italy). The precursor was hydrothermally treated at 150 °C and then calcined at 1000 °C. Abundant KAlSiO4-O1 phase can be observed after 2 h and its maximum abundance (92%) is observed after 5 h; coexistence with leucite is attested during the time interval 6–100 h. The sample recovered after 5 h was fully characterised by powder X-ray diffraction, He-pycnometry, inductively coupled plasma optical emission spectrometry, infrared spectroscopy and 29Si magic-angle-spinning nuclear magnetic resonance. The amount of amorphous phase in the synthesis powders was estimated with quantitative phase analysis using the combined Rietveld and reference intensity ratio methods. Testing a diatomitic precursor as a source of amorphous silica in the synthesis of KAlSiO4-O1 presents both economic and environmental incentives. The high yields and the results of the characterization open the way for the transfer to an industrial production scale.

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