Abstract <p>The influence of fibrous basalt content on the crystallization patterns of composites based on high-density polyethylene was investigated. The crystallization process was studied using the stepwise dilatometry method within the temperature range of 20–210°C. The fibrous basalt content in the HDPE composition was varied between 1 and 20 wt %. The dependence of the specific volume of the composites on temperature was examined by dilatometric analysis. The results of analysis allowed for the identification of first-order phase transitions, the second-order phase transitions (glass transition temperature), and the free and occupied specific volumes of the composites. The studies showed that fibrous basalt is distributed within the free volume of the polymer matrix, which is primarily located in the interspherulitic amorphous region. The kinetic patterns of crystallization of the composites were studied in Kolmogorov–Avrami coordinates, establishing that the mechanism of crystalline formation growth during continuous nucleation largely depends on the content of fibrous filler. In particular, it was determined that the initial high-density polyethylene and composites with filler content of 1–5 wt % are characterized by a three-dimensional spherulitic type of crystal growth; composites with fibrous filler content of 10–15 wt % exhibit a lamellar type of crystal growth; and samples with filler content of 20 wt % show a rodlike type, which is the simplest type of crystalline growth.</p>