Study of Olivine Morphology, Spinifex-Textured Komatiite, by Atomic Force Microscopy

Extract

Atomic Force Microscopy (AFM) imaging of a komatiite specimen from the Morro do Ferro Greenstone Belt, M.G, Brazil, has revealed thin (0.2 - 2.0 micron in width), randomly oriented olivine crystals (fig. 1a,b), of prismatic shape. The length of such crystals is highly variable, ranging from less than 1 μm to more than 10 μm. The randomly oriented crystals typically occur in three or more distinct directions, indicating that the spinifex texture is reproduced at the micron-scale.

In addition, typical skeletal crystals were observed (figure 1c,d) with width of the order of one micron. Figure 1d is the sketch representation of the skeletal crystal shown on figure 1c Figures 1c,d also show that skeletal crystals sometimes branch out thinner crystals, indicating that nucleation also occurs at the surface of previously crystallized minerals, besides generating isolated new crystal seeds.

It is worth noting that the olivine crystals are characterized as low topographic regions on the AFM images. What distinguishes them from polishing scratches is the fact that scratches typically do not mimic crystal shapes, especially skeletal, or fabrics which are readily seen on different scales, i.e., petrographic sections or hand-specimens.