engleski

Turning mineral grains into voxels: destructive tomographic techniques reveal the true nature of a peculiar granitic rock texture in 3D

A 3D study of various samples of granitic rocks from the northern slopes of MG containing nodules has been conducted in order to investigate morphology, occurrence and internal structure of this "peculiar" feature together with their distribution inside the host rock and spatial relations among individual nodules. Their general characteristics are observable even in 2D: elliptical to circular, sometimes even slightly ameboidal contours and a complex texture comprising two clearly distinct units. The core, a spherical aggregate of tourmaline, quartz, albite and K-feldspar is enveloped by a leucocratic halo, consisting of quartz, feldspar ± muscovite. The outer boundary between nodule's halo and granitoid host can be visually clearly defined by the absence or presence of biotite: host rock contains up to 15 vol. % of biotite in contrast to almost completely biotite-free leucocratic halo. This characteristic is evident even on an outcrop scale and has been used as the main criteria for the separation of nodules as autonomous bodies inside the host rock. Rock samples were analysed using destructive tomographic techniques: serial slicing (sectioning) and serial lapping (grinding), depending on the desired resolution of the final 3D model. Namely, the ability of analyzing larger samples in "low-resolution" gave information on their morphology and relations inside the host rock, while the serial grinding method, performed on smaller samples, provided detailed information about nodule's internal structure. A 3D computer reconstruction of studied material by destructive techniques involved following steps: 1) sample preparation, 2) data acquisition 3) data classification (segmentation), 4) separation (3D reconstruction and 5) 3D model visualization. Obtained 3D reconstructions of rock volumes containing tourmaline nodules showed that they are indeed isolated spherical bodies dispersed inside the granitic host and not vein formations. Also, the two structural units of a nodule, core and halo, are clearly distinguishable in 3D and show sharp contacts to each other but also to the granitic host. Another feature that distinguishes nodules from pegmatites/aplites is the crystal habit of tourmaline: nodule cores contain anhedral tourmaline in form of a tourmaline-quartz "network" whilst veins contain subhedral to euhedral tourmaline.

3D texture; granite; tourmaline nodule; morphology; internal structure; voxel; serial sectioning tomography

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