<p>I would suggest that there is a combination of effects here: </p>

<p>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.</p>

<p>The clumping in less weathered samples could be purely an artifact of the nature of the parent rock <a href="https://en.wikipedia.org/wiki/Granodiorite" rel="nofollow noreferrer">Granodiorite</a> is relatively enriched in Calcium over Potassium so Calcium shows up as large concentrations while Potassium is more thinly spread through the samples.</p>

CC BY-SA 4.0
2018-06-04T13:34:34.317Z
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
CC BY-SA 4.0
2018-06-04T14:24:41.270Z
@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
CC BY-SA 4.0
2018-06-05T09:13:18.717Z
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
CC BY-SA 4.0
2018-06-05T11:27:35.117Z
@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
CC BY-SA 4.0
2018-06-05T14:03:15.607Z
@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
CC BY-SA 4.0
2018-06-04T13:00:23.727Z
2018-06-04T13:00:23.727Z
Ash
<p>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. </p>

<p><strong>Data</strong> </p>

<p><a href="https://i.sstatic.net/lfH1a.png" rel="noreferrer"><img src="https://i.sstatic.net/lfH1a.png" alt="enter image description here"></a></p>

CC BY-SA 4.0
2018-06-04T11:22:35.633Z
Can you show your data?
Gimelist
CC BY-SA 4.0
2018-06-04T11:33:03.083Z
@Gimelist Added.
Yaboy33
CC BY-SA 4.0
2018-06-04T13:02:20.260Z
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
CC BY-SA 4.0
2018-06-04T13:22:39.133Z
@Ash Oh. Sorry I mean't finer spread to translate to clumpy.
Yaboy33
CC BY-SA 4.0
2018-06-04T13:25:16.580Z
@Yaboy33 Have a look at my answer and see if it bears any relation to what you're seeing and get back to me in the comments there.
Ash
CC BY-SA 4.0
2018-09-03T11:41:38.133Z
Looks like you have four outliers. Does excluding them change the correlation coefficients any?
Spencer
CC BY-SA 4.0
2018-12-02T00:14:28.933Z
Voting to close an unclear because it is actually unclear, and keeps getting bumped
Gimelist
CC BY-SA 4.0
2020-04-25T10:29:06.593Z
One thing you need to be aware of is that the lower the slope, the less predictive value the correlation coefficient. Also, those 4 outlier samples might be contaminating your results.
Spencer
<geology><rocks><mineralogy>
Why would CaO be more finely spread than K2O in Granodiorite?
Yaboy33
# Why would CaO be more finely spread than K2O in Granodiorite?

(Asked by: username_0 on 2018-06-04)

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**

[![enter image description here](https://i.sstatic.net/lfH1a.png)](https://i.sstatic.net/lfH1a.png)

**Tags:** geology, rocks, mineralogy

> **Comments:**
> **username_1**: Can you show your data?  
> **username_0**: @username_1 Added.  
> **username_2**: 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".  
> **username_0**: @username_2 Oh. Sorry I mean't finer spread to translate to clumpy.  
> **username_2**: @username_0 Have a look at my answer and see if it bears any relation to what you're seeing and get back to me in the comments there.  
> **username_3**: Looks like you have four outliers. Does excluding them change the correlation coefficients any?  
> **username_1**: Voting to close an unclear because it is actually unclear, and keeps getting bumped  
> **username_3**: One thing you need to be aware of is that the lower the slope, the less predictive value the correlation coefficient. Also, those 4 outlier samples might be contaminating your results.  

---

## 1 answers

### Answer by username_2 on 2018-06-04. Score: 1

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](https://en.wikipedia.org/wiki/Granodiorite) is relatively enriched in Calcium over Potassium so Calcium shows up as large concentrations while Potassium is more thinly spread through the samples.

> **Comments:**
> **username_0**: 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?  
> **username_2**: @username_0 Yeah that makes sense, even highly impermeable materials will eventually "soak through" if weathering near the water table allowing soluble minerals to migrate.  
> **username_4**: 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.  
> **username_2**: @username_4 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.  
> **username_0**: @username_4 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?  

---

