Abstract We report Mossbauer spectra, magnetic hysteresis, acquisition and demagnetization of isothermal remanent magnetization (IRM), and low-temperature IRM warming curves of biotite crystals, as well as a summary of domain observations on magnetic extracts. The biotites are from 1.0 to 1.2 Ga gneisses and 1.85 Ga plutons of the Grenville, Churchill and Bear Provinces (Canada). Most have paramagnetic susceptibilities in the range (0.45–1.2) × 10− 3 SI, similar to reported values for iron-rich biotites. Magnetite, identified by its 120 K Verwey transition, is an inclusion in all the biotites. Sizes and domain states of inclusions correlate with magnetite content as measured by saturation magnetization Ms. The higher Ms is, the lower are the domain state indicators Mrs / Ms (Mrs is saturation IRM) and coercive force Hc. Added to a base population (0.003–0.1%) of pseudo-single-domain (PSD) grains with Mrs / Ms from 0.1 to 0.3 and Hc from 7 to 20 mT is ≤ 1% (in two exceptional cases, 7% and 14%) of coarse-grained multidomain (MD) magnetite with much softer hysteresis properties. The underlying PSD magnetite fraction in biotites potentially retains useful paleomagnetic remanence. In the more magnetic micas, this stable fraction may be swamped by MD magnetite with less stable behavior.