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: (M1<2+>)a-x(M2<2+>)xAl<3+>4(OH)b(A)c . mH2O (in which M1<2+> stands for Zn<2+> or Cu<2+>, M2<2+> is at least one divalent metal ion selected from Ni <2+>, Co<2+>, Cu<2+>, Zn<2+> and Mg<2+>, a is 0.3 < a < 2.0 (with the proviso that M1 and M2 are not the same), x is 0 is at least one selected from SO4<2->, HPO4<2->, CO3<2->, SO3<2->, HPO3<2->, NO3<->, H2PO4<->, Cl<->, OH<-> and silicic ion, c is 0.4 < c < 2.0 and m is 0-4), which has an average secondary particle diameter of not more than about 3 mu m and a BET specific surface area of not more than about 30 m<2>/g; resin compositions containing the same; and agricultural film composed of said resin compositions.
