Abstract: Novel precipitated calcium carbonate particles in clustered form, which when used as fillers impart improved strength, opacity and other advantages to paper, are prepared by a process involving adding lime and carbon dioxide to a reaction mixture containing seed material having a scalenohedral morphology, the lime and carbon dioxide being added simultaneously. The flow rates of the lime and carbon dioxide are adjusted to control the solution conductivity of the reaction minute to between about 2.0 and about 4.0 mS to form the clusters of calcite particles, the clusters having an average equivalent spherical diameter of between about 1.5 and 3.5 microns and a specific surface area of between about 2.0 and 9.0 m.sup.2 /g. At least about 25% of the particles of said clusters have a prismatic form.