Frontispiece: Hydrothermal Formation of Calcium Copper Tetrasilicate

Abstract

Crystal Engineering CaCuSi4O10 is a famous pigment (Egyptian blue), rare mineral (cuprorivaite), and unique near‐infrared luminescent material. Now CaCuSi4O10 can be prepared hydrothermally using conditions vastly different from the historic salt‐flux and solid‐state routes. But mind the details—too hot a reaction and you get calcium silicate, too cold and you get a porous calcium copper silicate. 350 °C is just right. Additional parameters that strongly influence the hydrothermal chemistry leading to CaCuSi4O10 include the copper precursor, the mineralizer, and reaction duration. This new synthesis of CaCuSi4O10 sheds light on the formation of cuprorivaite in select mineral deposits around the world. For more information, see the Communication by T. T. Salguero and co‐workers on