Composition, size, and phase are key properties that define the ability of an aerosol particle to initiate ice in cirrus clouds. Properties of cirrus ice nuclei (IN) have not been well constrained due to a lack of systematic measurements in the upper troposphere. We have analyzed the size and composition of sublimated cirrus particles sampled from a high altitude research aircraft using both in situ and offline techniques. Mineral dust and metallic particles are the most enhanced residue types relative to background aerosol. Using a combination of cirrus residue composition, relative humidity, and cirrus particle concentration measurements, we infer that heterogeneous nucleation is a dominant cirrus formation mechanism for the mid-latitude, subtropical, and tropical regions under study. Other proposed heterogeneous IN including biomass burning particles, elemental carbon, and biological material were not abundant in cirrus residuals.