Novel, fine particulate synthetic chalcoalumite, process for preparation thereof, and a heat insulating agent and agricultural film containing said fine, particulate synthetic chalcoalumite. The above objects are met by synthetic chalcoalumite represented by formula (1) below: (M.sub.1.sup.2+).sub.a-x (M.sub.2.sup.2+).sub.Al.sup.3+.sub.4 (OH).sub.b (A.sup.n-).sub.c. mH.sub.2 O (1) (in which M.sub.1.sup.2+ stands for Zn.sup.2+ or Cu.sup.2+, M.sub.2.sup.2+ is at least one divalent metal ion selected from Ni.sup.2+, Co.sup.2+, Cu.sup.2+, Zn.sup.2+ and Mg.sup.2+, a is 0.3