Trace-element analysis is a powerful tool for studying numerous processes of research interest in the Earth Sciences; as well, it has important applications of interest to the mineral exploration community. Here, we present the results of a comprehensive study of five internationally recognized silicate reference standards (BHVO-1, GXR-5, GSR-5, MAG-1 and GSD-50) utilizing micro-PIXE analysis. Results of trace analyses for Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Y, Zr, Nb, Mo and Pb are presented. Measured data for the five standards exhibit agreement within + or -10%, and commonly better, with recommended levels of concentration in the 50-400 ppm range. Detection limits (for a typical 3- to 4-minute analysis of an accumulated charge of 2.5 microCoulombs) are in the range of 2-10 ppm, with precisions of 1-10%. A detailed study of the X-ray spectra in the vicinity of the Fe, Ni and Co lines illustrates that the GUPIX software is able to correctly fit the complex spectra and thus provide accurate Ni concentrations. Further tests on Ni in garnet are reported, in the context of current application of the Ni-in-garnet geothermometer in diamond exploration; it appears important to resolve the difference between the two published calibrations of this important geothermometer.