Background <p>Characterizing deformation and failure mechanisms through small-scale testing has helped in the fundamental understanding of material response, and direct loading in a transmission electron microscope (TEM) has played a large role in this effort. However, crystalline materials exhibit incoherent scattering within the TEM and the resulting intensity variations inhibit direct optical metrology.</p> Objective <p>In this work, we seek to both validate an<i> in situ</i> optical full-field metrology method in the TEM for use with crystalline materials, and measure fracture properties of a MgAl₂O₄spinel single crystal at the microscale.</p> Methods <p>Microscale single edge notch bend beams were machined from a spinel single crystal and loaded in the TEM. <i>In situ</i> imaging of a nanoscale speckle pattern allowed use of particle tracking (PT) to extract full-field measurements of the displacement field. A numerical analysis methodology was then used to obtain mixed mode stress intensity factor values.</p> Results <p>A discrepancy between PT and far-field actuator measurements of applied displacement was found (about a maximum of 35% difference), indicating the advantage of using near-field optical measurements in the TEM. For such small-scale testing it is also generally unavoidable to introduce asymmetry in loading. However, the PT results allowed measurement of both <i>K</i>_I and <i>K</i>_II, which were found to be at the time of crack initiation <i>K</i>_IC = 1.51± 0.03 MPa∙m^0.5, K_IIC = 0.04± 0.002 MPa∙m^0.5, respectively.</p> Conclusions <p>The application of PT enables full-field deformation measurements on crystalline materials deformed in the TEM. The effectiveness of the inverse property extraction was demonstrated by good agreement between the full-field PT measurements and FEM results. The MgAl₂O₄ spinel toughness values extracted also agreed well with previous literature results.</p>