High-temperature and High-pressure Sintering Method to Prepare Magnetite Reference Material for In-situ Microanalysis

Matrix-matched reference materials are important for in situ trace and isotope analyses. In this study, we developed an efficient method for preparing chemically homogeneous magnetite (MtTR-1) without adding a binder. The initial magnetite powder (d(90) = similar to 80 mu m) was milled to form micron (d(90) = 3.0 mu m) particles in an anhydrous ethanol suspension and argon environment using a high-energy vibration ball mill. The obtained particles were pressed into a magnetite cylinder (10 mm in diameter and 9.2 mm in height), sealed into a silver tube, and sintered at 500 degrees C and 1.2 GPa for 2 h. Laser Raman spectroscopy results indicated that high-temperature and high-pressure sintering did not induce any phase transformation. The smooth surface after polishing was subjected to repeated analyses via electron probe microanalyzer using a spot size of 5 mu m and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) using spot sizes of 38-60 mu m. The results indicated that the texture and chemical composition are homogeneous in MtTR-1. The smooth time-resolved signal intensities of elements, steep wall, and flat bottom of ablation craters also suggested homogeneity in the depth profile. Line profile analyses across the entire pressed pellet further demonstrated that the pellet is homogenous for the investigated elements. The concentrations of major and trace elements in MtTR-1 determined by ICP-OES and ICP-MS are used as the preferred values. The MtTR-1 can be cut into slices, repeatedly polished and used for in situ analyses. The proposed technique for producing magnetite can also be applicable to other minerals or rocks by optimizing the conditions, thus providing a new method for preparing reference materials for in-situ microanalysis.