Microstructure and mechanical properties of massive and porous of lavas from S. Miguel Island (Azores, Portugal)

The role of elongated pores and crystals of lavas influences their mechanical and physical behaviour, providing a first microstructural clue. In the context of a doctoral project, two samples – a trachyte and a basalt/andesite (s.l.) - representative of Fogo Volcano (S. Miguel Island, Azores, Portugal) were collected as part of an intact rock study. They were microstructurally assessed in the INGV-OV (Naples) using the ZEISS Xradia Versa 410 X-ray computed microtomography to obtain high-resolution 3D images, as well as to perform real-time in-situ mechanical tests (uniaxial – 7 mm diameter cylinders - and Brazilian – 13 mm diameter discs) to assess how elongated pores/crystals control strength. In addition to 3D images of the samples, which allow segmentation of the pore space and crystals, mechanical tests show that trachytes are more competent than vesicular basalts/andesites. Both pores and crystals control the development of the crack pattern.

X-ray computed microtomography (X-μCT) : ZEISS Xradia Versa 410 (INGV-OV) This micro-CT scan is equipped with a microfocus X-ray source capable of energies from 40 to 150 kV and detectors with magnifications ranging from 0.4X to 20X (down to 0.9 μm resolution). Subsequent image analysis using the PerGeos Software for mineral and void space segmentation, as well as greyscale image optimization.

High-resolution scanning: - X-ray imaging was performed in absorption mode, acquiring 4001 2D radiographs (projections) over air and a 360° rotation with 100 to 110 kV voltage, 10 W power, and 1.5 s exposure time. - An optical magnification of 10× was used, resulting in a pixel size of ~2 µm. Mechanical tests: The tests were conducted at a loading rate of 0.03 mm/min using a Deben in-situ load cell stage (CT5000H250), with a maximum load capacity of 5210 N, incorporated into the Zeiss equipment at the INGV-OV. X-ray imaging of the cylinders was performed, collecting 801 projections over a total angular scan of 180◦ (plus fan angle) with a scan time of 5-5.5 s per projection at 90 kV and 10 W, and an optical magnification of 0.4× to obtain 3D images with a voxel size of 15 μm. The LE2 filter was used.