Samples in the anorthite primary crystallization field were prepared in the CaO-MgO-Al2O3-SiO2 quaternary system by sol-gel techniques. The homogeneous dried gels were characterized by infrared spectroscopy, thermal analysis and electron microscopy, The crystallization of gel-like glass as a function of temperature was followed by X-ray diffraction. Anorthite was detected as the only crystalline phase. Thermal analysis results indicated a mechanism of crystal growth dominated by surface nucleation. This fact could be related to the small size of the primary particles observed by electron microscopy.