Experimental Study of Basalt Alteration At 150 °C: An Approach To Understand Smectite Nucleation And Crystallization Pathways

The global dominance of basaltic rock in the crust plays a dominant role in the global elemental geochemical cycle. Smectite clay is a ubiquitous alteration product of basalt. Despite having a complex silicate structure, a phyllosilicate dominance of smectite raises a question on the kinetics aspect of its formation and underlying nucleation and growth mechanism during basalt alteration. In the experimental formation of smectite, Zhang et el., 2019 have shown that coherent scattering domain size (CSDS) remains constant for starting few days during the growth of the sheet, and then stacking of fundamental particles takes place, resulting in the growth along C*. A later study by Kuliviesz et al., 2018 has shown variation in the layer charge with clay size. This implies chemical changes in the clay with its growth, which complicates the problem. The current study aims to investigate the mechanism and pathway of smectite crystallization. We have studied the role of magnesium in the growth of smectite clay and its mechanism.

Experimental alteration of basaltic glass has been performed at 150 degree Celsius in two different fluid compositions. One is containing 0.3M MgCl₂ and the other pure water. Products have been investigated under XRD, FTIR, TEM, and SEM; solution has been investigated under ICP AES.

We found that both the experimental setup resulted globular flower-like feature along with a honeycomb structure on the glass surface. There is no XRD-detectable crystalline product, but electron microscopy has confirmed smectite, the globular flower-like structure (similar to Fiore et al. 2001). There is a discernible difference in the size of globules. Mg bearing condition has leather size flakes and globule than without Mg. Magnesium bearing experiment has shown higher Si concentration in the solution than without Mg, similar to Al's behaviour.

Smectite formed in the Mg bearing experiment has twice the amount of Mg in clay structure than smectite formed in an experiment without Mg. Based on 11 oxygen, the chemical composition of smectite has been used to calculate the abundance of cation pairs in the octahedral sheet if there is a random distribution of all cations. At the same time, observation based on FTIR spectra has shown a dominance of Mg-Mg pairs. This has been observed in both experiments. Our study confirms a preferential ordering of Mg-Mg cation pairs in the octahedral sheet and better growth in the presence of Mg.

Further TEM investigation is being performed to observe the fundamental particle size and chemical relation.