Olivine Before Desktop Weathering Experiment
The Erlenmeyer flasks were put on a table top rotary shaker, so that the grains kept rotating along the bottom. The Erlenmeyers were open to the air, permitting CO2 exchange. In all cases the tap water had an initial pH of 8.22, and the olivine grains had been washed to remove any attached dust.
Olivine After Weathering 10 Days
The abraded fine material caused the water in the bottles with fine-grained olivine to be cloudy. In the bottles with coarse-grained olivine the water was a milky white, opaque suspension. After 6 hours the pH had risen to 8.82 in one bottle with coarse-grained olivine. After 1, 4, and 10 days the pH in one of each of the two series of bottles was measured. In the bottle with fine-grained olivine, the pH had risen to 8.91 after 24h, and in a bottle with coarse grains to 9.02
“Table 1. Analytical data of samples after 1, 4 and 10 days, and 12 days for the 50/50 experiment. Fine olivine sand is 710–1400µm, coarse olivine 2–5mm. Samples were shaken continuously unless indicated otherwise. 50/50 refers to the experiment with a mixture of 15g of the fine, and 15g of the coarse fraction. The difference in Mg/Si ratios must be due to the precipitation of Mg- and Si-containing minerals. Indeed, XRD analysis showed a saponite-like (Mg 3 Si 4 O 10 (OH) 2 ). 4H 2 O) phase and others that could not be readily identified. This is irrelevant in an open marine environment, where the sea water is continuously refreshed and such saturation is never reached. The decrease of C inorg in the water must be due to the precipitation of mineral phases insufficiently developed to be recognized with XRD.”
This implies that the coarse grains had produced more
slivers than the finer grains, likely due to their greater mass and consequently heavier mutual impacts. In this experiment, the olivine reacted with a limited, fixed amount of water. Thus, logically the accumulation of reaction products slowed down and eventually stopped the reaction (Table 1), contrary to shallow marine settings where water refreshment is continuous and the tiny slivers, together with the dissolved magnesium and silica, are carried away.
Figure 3 shows the grain size distribution of the material that was scraped off the grains in the test with 50/50 coarse and fine grains. The amount of scraped off material was 9g (30%), considerably more than the amounts in the single grain size experiments which were slightly over 1g in the separate fine and coarse fractions. The pH in the 50/50 mixture rose to 9.42 in 12 days. About 50wt% of the abraded particles is 5µm or less (Figs. 3 and 4). Such fine grains of olivine weather fast, as is evident from the rapid rise of pH during the experiments. The rate of solution may have been further increased by the deformation that grains undergo when they are strained. The phenomenon of enhanced dissolution of fine particles by repeated particle impact and
rubbing is well known in the field of mineral processing, where it is called mechanical activation (Tromans and Meech, 2001).
Contrary to the common opinion that olivine weathering in nature is a slow process (Lal,2008; Hangx and Spiers, 2009; Köhler et al., 2010; Pronost et al., 2011), our experiments show that olivine grains when kept in motion weather fast because continuous
mutual impacts remove reaction-inhibiting silica from the surface and tiny µm-size slivers are produced allowing a fast chemical reaction. The application of olivine and other (ultra)mafic minerals like serpentine in high-energy shallow marine environments can
make a significant contribution in the fight against climate change. The counteracting effect on ocean acidification is immediate.