Anomalous thermal response of a fibrolite sample near room temperature is interpreted as a slow structural change in fibrolite. The heat effect would appear small for the completed transition; however, failure of fibrolite to transform could result in larger measured heat capacities for the sample at temperatures above room temperature than those measured for the transformed sample. This may explain why the heat capacities reported by Salje (1986) for fibrolite are about 2% larger than those reported here.
Lack ofa demonstrated excess heat capacity for novaculite at low temperatures suggests that aggregates of cemented or intergrown small crystals do not have the same degree of surface lattice distortion and degrees of freedom for surface atoms and consequently have smaller surface energies than powders of the same mineral. The chemically similar qvartz grains and quartz cement of the novaculite represent one limit on the excess properties that fine-grained geologic materials may exhibit.