# Why would CaO be more finely spread than K2O in Granodiorite?

I recently interpreted some XRF data on samples of a granodiorite which were classified under three different types of physical characteristics: very little, mild and significant levels of weathering. I plotted ratios between $Al_2O_3$ for $CaO$ and $K_2O$ and noticed $CaO$ had a much lower correlation coefficient, i.e., it was more clumped. I know it's about which of these elements go into clay compounds but I need a bit more help understanding.

Data

• Can you show your data? – Gimelist Jun 4 '18 at 11:22
• @Gimelist Added. – Yaboy33 Jun 4 '18 at 11:33
• Sorry I just realised, after posting my answer that the title and the question don't seem to match which is the more finely divided compound? The title say Calcium is finer the question days it's "clumpy". – Ash Jun 4 '18 at 13:02
• Note ionic potential AtomRadious/charge. K has +1 and is bigger than Ca (+2), so movilized earlier. – user12525 Jun 6 '18 at 9:32
• Looks like you have four outliers. Does excluding them change the correlation coefficients any? – Spencer Sep 3 '18 at 11:41

## 1 Answer

I would suggest that there is a combination of effects here:

Potassium(K) compounds are usually more soluble than their Calcium(Ca) equivalents, and often at much lower temperatures. For example $CaCO_3$ is in considered insoluble in even boiling water, while $K_2CO_3$ is fully soluble at room temperature. Potassium can therefore migrate more easily during weathering involving water infiltration than can Calcium leading to a finer distribution. The "clumps" of $CaO$ represent more closely it's original crystal positions while the finely divided $K_2O$ has migrated farther dispersing it through the samples.

The clumping in less weathered samples could be purely an artifact of the nature of the parent rock Granodiorite is relatively enriched in Calcium over Potassium so Calcium shows up as large concentrations while Potassium is more thinly spread through the samples.

• Indeed. The results are far more indicative of the reasoning in your last paragraph since these samples were relatively impermeable. Perhaps considering the composition of the clay minerals in the granodiorite and relating that to solubilities may be the answer? Maybe its simply that since $K_2CO_3$ is more soluble when water has manage to permeate throughout the mineral then their has been a slow "shift" in these compositions over a long period of time? What do you think? – Yaboy33 Jun 4 '18 at 13:34
• @Yaboy33 Yeah that makes sense, even highly impermeable materials will eventually "soak through" if weathering near the water table allowing soluble minerals to migrate. – Ash Jun 4 '18 at 14:24
• I don't see how the data supports K being more thinly spread through the sample. What the data shows is that the variation in potassium concentrations in higher than that in calcium concentrations. – bon Jun 5 '18 at 9:13
• @bon I can't make heads-nor-tails of the data set as presented, I have simply answered the question presented to me to the best of my knowledge of geochemistry. You need to comment on the question if you think it proceeds from a false premise. – Ash Jun 5 '18 at 11:27
• @bon Thinly spread as in the data points not K itself. That's what I meant. Anyways... what would your reasoning be for the disparity amongst the two sets? – Yaboy33 Jun 5 '18 at 14:03