Laser-Assisted Atom Probe Tomography of Deformed Minerals: A Zircon Case Study

Testing the Influence of Laser Pulse Energy and Rate in the Atom Probe Tomography Analysis of Minerals

The use of atom probe tomography (APT) for mineral analysis is contributing to fundamental studies in Earth Sciences. Meanwhile, the need for standardization of this technique is becoming evident. Pending the use of mineral standards, the optimization of analysis parameters is needed to facilitate the study of different mineral groups in terms of data collection and quality. The laser pulse rate and energy are variables that highly affect the atom evaporation process occurring during APT analysis, and their testing is important to forecast mineral behavior and obtain the best possible data. In this study, five minerals representative of major groups (albite, As-pyrite, barite, olivine, and monazite) were analyzed over a range of laser pulse energies (10-50 pJ) and rates (100-250 kHz) to assess output parameter quality and evaluate compositional estimate stoichiometry. Among the studied minerals, As-pyrite, with the higher thermal conductivity and lower band gap, was the most affected by the laser pulse variation. Chemical composition estimates equal or close to the general chemical formula were achieved for monazite and As-pyrite. The analysis of multihit events has proved to be the best strategy to verify the efficacy of the evaporation process and to evaluate the best laser pulse setting for minerals.

Atom Probe Tomography Analysis of Mica

Laser-assisted atom probe tomography (APT) is a relatively new, powerful technique for sub-nanometric mineral and biomineral analysis. However, the laser-assisted APT analysis of highly anisotropic and chemically diverse minerals, such as phyllosilicates, may prove especially challenging due to the complex interaction between the crystal structure and the laser pulse upon applying a high electric field. Micas are a representative group of nonswelling clay minerals of relevance to a number of scientific and technological fields. In this study, a Mg-rich biotite was analyzed by APT to generate preliminary data on nonisotropic minerals and to investigate the effect of the crystallographic orientation on mica chemical composition and structure estimation. The difference in results obtained for specimens extracted from the (001) and (hk0) mica surfaces indicate the importance of both experimental parameters and the crystallography. Anisotropy of mica has a strong influence on the physicochemical properties of the mineral during field evaporation and the interpretation of APT data. The promising results obtained in the present study open the way to future innovative APT applications on mica and clay minerals and contribute to the general discussion on the challenges for the analysis of geomaterials by atom probe tomography.

Effect of crystallographic orientation on atom probe tomography geochemical data?

The recent application of atom probe tomography (APT) to minerals is becoming a powerful tool to unravel geological information from the nanoscale perspective. Yet, there are still unknown fundamental aspects of this microscopy technique for geological applications and the potential crystallographic orientation effect is a significant one. Here, the influence of the crystallographic orientation on the quality of atom probe tomography geochemical data is investigated for two minerals with the same crystal system and different morphology: spinel (isometric, hexoctahedral, octahedron morphology) and galena (isometric, hexoctahedral, cube morphology). Two separate crystals of barite (orthorhombic, dipyramidal, prism morphology) were also analyzed to test the reproducibility of APT data. Despite the general absence of expected stoichiometry, overall bulk and isotopic chemical composition are not affected by crystallographic orientation. 3D data reconstructions of the specimens showed similar spatial distribution of the ion species for each mineral and 2D density maps showed identical (barite, galena) or specular (spinel) patterns for each pair of planes analyzed. Our findings indicate a negligible effect of the crystallographic orientation in APT geochemical data for standard highly symmetric minerals but also suggest the possible influence of the crystallographic structure and composition on the mineral stoichiometry and elements spatial distribution density.

Analysis of Natural Rutile (TiO2) by Laser-assisted Atom Probe Tomography

Since the introduction of laser-assisted atom probe, analysis of nonconductive materials by atom probe tomography (APT) has become more routine. To obtain high-quality data, a number of acquisition variables needs to be optimized for the material of interest, and for the specific question being addressed. Here, the rutile (TiO2) reference material 'Windmill Hill Quartzite,' used for secondary ion mass spectrometry U-Pb dating and laser-ablation inductively coupled plasma mass spectrometry, was analyzed by laser-assisted APT to constrain optimal running conditions. Changes in acquisition parameters such as laser energy and detection rate are evaluated in terms of their effect on background noise, ionization state, hit-multiplicity, and thermal tails. Higher laser energy results in the formation of more complex molecular ions and affects the ionization charge state. At lower energies, background noise and hit-multiplicity increase, but thermal tails shorten. There are also correlations between the acquisition voltage and several of these metrics, which remain to be fully understood. The results observed when varying the acquisition parameters will be discussed in detail in the context of utilizing APT analysis of rutile within geology.