Sorption of atmospheric carbon dioxide and structural changes of Ca and Mgsilicate minerals during grinding - I. Diopside

Considerable sorption of atmospheric carbon dioxide by the ground mineral, alongside hydration due to atmospheric moisture, occurs in the course of pr olonged dry grinding of natural and synthetic diopside in laboratory condit ions. Grinding of natural diopside for 36 h results in about 10 wt.% of CaC O3 in the ground sample. A unique double peak in the 1430-1515 cm(-1) regio n in the FT-IR spectrum, attributable to the CO32- group, shows that carbon dioxide is present in the ground diopside, in the same form as in syntheti c and natural silicate glasses after dissolution of CO2 at high temperature s and pressures relevant to the magma stare. This conclusion is supported b y the C-13 CP/H-1-decoupling MAS-NMR spectrum of ground diopside, which has a strong resonance signal at 167.4 ppm. Carbonate groups are present not o nly on the surface, but also in the bulk of mineral grains. The appearance of the new peak at approximately - 108 ppm in Si-29 MAS-NMR spectrum of the ground diopside after 36 h of grinding shows that the Q(2) structure of cr ystalline diopside is partially transformed into the Q(4) structure, Togeth er with XRD data, this result indicates the formation of quartz, which may occur through a re-polymerisation of an amorphous phase. (C) 2001 Elsevier Science B.V. All rights reserved.