Lightweight concrete has a lower unit weight than conventional concrete and a high thermal insulation capacity. It is manufactured with either natural or artificial lightweight aggregates. Vermiculite, one of the natural lightweight aggregates used, is a micaceous material that when heated increases its original volume due to an exfoliation process, so its incorporation in concrete or mortar imparts very low density. These natural materials usually contain impurities such as asbestiform minerals that can be released during the expansion process, causing a harmful environmental effect. Commercial asbestos-containing materials are currently banned due to the asbestos-related lung diseases they cause. In the province of Córdoba, Argentina, there are vermiculite mines where impurities such as asbestos group minerals have been identified. Studies using stereomicroscope, petrographic microscope, XRD, SEM, DTA and chemical analyses were conducted to characterize the different types of asbestos present and determine their particle size to assess whether they could be harmful to human health. Mica group minerals, abundant quartz and amphiboles were identified. The latter occur as isolated green crystals and their average length is 900 μm. Tremolite was determined by XRD.