Dear Mr. Sunkari,

It is one of the numerous examples where you put the cart before the horse. I know that the number of young colleagues who want to swiftly have figures and for whatever reason jump the line geology-mineralogy-chemistry increases. If you have no precise information about the geology, to be more precise to avoid mapping, if you feel lacking experience in mineralogy and therefore cast this discipline aside rather than to try and upgrade yourself in this way and use the old CIA index from the 1980s to solve your problem, it will be wasted time and wasted money to spend on the topic.

What is a labile mineral (e.g. oxide)? Olivine s.s.s. and pyroxene s.s.s. are labile minerals. On the opposite side of the line zircon, rutile and tourmaline are ultrastable minerals in terms of weathering.

I could speculate on the reasons held accountable for that concentration of minerals which I do not know and about the geology which I do not know about, either.

But it would be wasted time.

Sorry to be so outspoken, but it makes no sense to lead young colleagues astray.

With kind regards H.G.Dill

Dear Dr Dill,

Thanks for your thought-provoking response.

Dear Sunkari,

Welldone. No question is useless.

Let us try to resolve the challenge. What are the values of your CIA ? The CIA value of PAAS is reported to be 70-75 which is considered to represent low to moderate degree of weathering. Be sure you computed your CIA values accurately using molar proportions and have made all necessary corrections that may affect your interpretation.

If u are sure, compare your results with other indices . There are many other proxies that will guide you to either agree or disagree with your CIA values. You can also check if there is evidence of sediment recycling in your sediments.

Also use Index of compositional variability ( ICV) to look at the compositional maturity of your sediments since ICV values are usually < 0.84 for minerals such as kaolinite, illite and muscovite and >0.84 in rock forming minerals such as plagioclase, K-feldspar, amphiboles and pyroxenes .

For the labile minerals , which oxides precisely are you referring to ? This will help me constrain my answer.

Dear Dr Overare,

I really appreciate your enlightening response to my question. First of all, the beach sands are located around highland rocks that are composed of pyroclastics, phyllites, metamorphosed lavas, schists and granitic rocks. The SiO2 concentrations range from 56 to 96 wt.%. The CIA values range from 65 to 80.

I compared the results with PIA and ICV. In doing so, the computed PIA values range from 20 to 80 pointing to low-moderate weathering in the source areas. The ICV values also vary from 3.2 to 17.1, > 0.84 which may imply the dominance of rock-forming minerals in their source rocks.

On the bivariate plot of Th/Sc versus Zr/Sc (Taylor and McLennan, 1985), the samples straddle the boundary between the UCC and mafic source but largely closer to the UCC with implication of sediment recycling/zircon concentration. For the labile minerals, I am particularly referring to Na2O (0.2 - 0.9 wt.%) and CaO (0.8 - 18.4). The high concentrations are observed in a couple of the samples.

Ok, we are progressing.

First, what tool did you use for your geochemical analysis ?

Secondly, your weathering indices are suggesting low to moderately intensed weathering and the ICV values are >1 indicating immature sediments which are not typical for alteration products such as Kaolinite, illite and muscovite . So all your sediments may not be as weathered as you may have inferred.

I suggest you plot an AI2O3-(CaO*+Na2O)-K2O (A-CN-K) trilinear diagram which is very useful in evaluating chemical weathering trends instead of relying only on simple comparison of numerical value .The diagram may also be used to evaluate bulk source rock composition, diagenetic effects, and tectonism in the source area. You can easily monitor the weathering trend using this plot.

Thirdly, If your CIA values range from 65-80, what is the average ? How many of these samples have CIA values close to 80 ? Completely altered material (e.g kaolinite, gibbsite) has PIA value of 100 while the value for unweathered plagioclase is 50. The range of your PIA suggests some of your samples are completely unweathered. If the weathering has not proceeded to a stage at which significant amounts of the alkali and alkali earth elements are removed then you are likely to still have high amount of the ''labile minerals'' you are talking about.

Fourthly, check to see whether samples with low CIA , ICV and PIA correspond to the ones that have high percentage of the labile minerals. what did you observe ?

The shocking high values of CaO in your sediments may lead to delusive computation of weathering indices except you did your corrections for CaO as noted by literatures such as Mc Lennan et al. (1993), Bock et al. 1998 etc. Should the need arise, just note that in quantifying source weathering of carbonate-bearing siliciclastic rocks, one approach is using a modified version of CIW (Cullers, 2000) .

I hope my suggestions are helpful. Sorry, I may not be able to reply very fast . I am quite busy.

I really appreciate your time and contribution so far. I have already got a clue to my question from your submission. This is from the line " If the weathering has not proceeded to a stage at which significant amounts of the alkali and alkali earth elements are removed then you are likely to still have high amount of the ''labile minerals'' you are talking about". The major and trace elements were analysed using ICP-ES.

Yes, I agree that not all my sediments may have weathered based on the ICV values. I plotted the ACKN diagram before and majority of the samples plot along the A-CN boundary close to the plagioclase zone. A few samples (4 fine-grained, 1 medium-grained and 1 coarse-grained) have indication of weak weathering (CIA = 50-60) and a sample each of fine and medium-grained suites fall in the intermediate weathering field (CIA = 60-80) whereas only one fine-grained sample plot in the strongly weathered field (CIA = 90-100).The general plotting of the samples along the A-CN line is an indication of clastic materials belonging to an intermediate/mafic source rock with dominant plagioclase and anorthite minerals.

The average CIA value is 23 and only 4 samples have CIA values close to 80. The samples with low CIA , ICV and PIA have varying percentages of the labile minerals. Some appear to be high and others are moderate compared to the samples with high CIA , ICV and PIA. Indeed by your proposition, the weathering may not have proceeded to a stage at which significant amounts of the alkali and alkali earth elements were removed. That might have accounted for the high amount of the labile minerals in some of the sediments that have low CIA values.

I sincerely appreciate your time Sir. This is an ongoing project and I just needed to be cautious of what I am obtaining. Moreover, we are doing mineralogical analysis which could solve this constraint.

You are welcomed. I am glad I was able to assist you in resolving the challenge.

It is also fufilling to know that you are integrating your results with minerological studies. Petrographic studies will also be useful, if you can incorporate it, this will make your studies more robust.

You did not say anything about the high values of CaO in your sediments that may lead to erroneous computation of weathering indices.

Wishing you the best in your research

Dear Sunkari

CaO% in your analysed sediments was more than Na2O, You have carbonate minerals . first please make correction.

please check heavy minerals.

thanks

Dear Dr Qasim,

Thank you so much for such an observation. We are currently working on heavy mineral chemistry and your suggestion is really apt.