The nucleation and growth rate of an amorphous mineral film at a Langmuir monolayer interface has been observed in situ with X-ray scattering techniques for the first time. We present measurements of a calcium carbonate film, nucleating at a fatty acid monolayer from a supersaturated calcium carbonate subphase containing a dilute poly(acrylic acid) additive, in emulation of biogenic mineralizing systems. The arachidic acid molecules were observed to self-assemble on the subphase into an untilted structure, with one bound Ca2+ ion per 14-25 surfactant molecules, and with a two-molecule planar centered rectangular unit cell with lattice constants a = 4.38 Angstrom and b = 8.51 Angstrom. Subsequently, an amorphous mineral phase forms beneath the monolayer, growing at a,rate of 22 Angstrom/h to a thickness of 300 A after about 15 h. The electron density of the film was 0.7 times that of crystalline calcium carbonate hexahydrate, suggesting that the precursor is a hydrated phase with an open structure. During film growth, no changes were observed in the in-plane scattering peaks from the arachidic acid monolayer. No intensity was observed at Bragg peak positions for anhydrous calcium carbonate minerals. These observations indicate that nucleation of the calcium carbonate film is not directed by atomic scale templating at the fatty acid, but is instead dictated by the redistribution of ions near the interface. We discuss the relevance of our study to template-directed nucleation in the contexts of biomineralization and biomimetic materials science.