Rectangular hydromagnesite microsheet is prepared by simple solution route through controlled hydrolysis-condensation of magnesium chloride in urea solution using ethylene glycol (EG) as a bifunctional cosolvent. Calcination of the hydromagnesite sheets at 450 degrees C produces MgO without any alteration in the morphology. The MgO sheets were of 2-3 mu, side length and 4-5 nm thickness, and made of highly crystalline MgO nano-particles, which show reasonably high BET surface area of 10(6) m(2) g(-1). These MgO microsheets showed very strong activity as heterogeneous base catalyst in the solvent free Claisen-Schmidt condensation of benzaldehyde with acetophenone giving 99.0% conversion in less than 4 h with almost no loss of activity in three successive cycles. The strong activity is correlated with its large surface area and presence of oxide ions in low coordination (LC) O(LC)(2-) (where LC= 5C. 4C and 3C for terrace, edge, corner and kink sites, respectively) along with defects, which was revealed by photoluminescence (PL) emission studies. Presence of basic sites of different strength, determined by temperature programmed desorption of carbon dioxide (TPD-CO(2)) also supports the strong catalytic activity of the MgO micro-sheets. Increase in calcination temperature to 650 degrees C destroys the open sheet morphology and enhances crystallite size, which causes decrease in catalytic activity due to low surface area, constrain in mass transport from the core structure and low concentration of active basic sites. (C) 2011 Published by Elsevier B.V.