Dear Mr. Udagedara,

is your chemical calculation following an assessment of the mineralogical composition or is it a stand-alone study solely based on chemical composition.

How did you perform your chemical analyses?

It is a prerequisite to check whether the technical approaches have been taken in a proper way.

First and foremost what is your study on weathered rocks aimed at?

Best regards

Harald G. Dill

Perhaps check whether you are supposed to use weight% or that the calculation is on a molar basis. Take someone else's published values and do the calculation, to see whether you can reproduce their results. If yes, your calculation is OK; if no, it's wrong.

Of course, if your rock is not weathered, the value will be high. If you have (mistakenly) analysed a limestone, then, again, the number will be rather high.

Dear Professor Harald,

Thank you for your response.

Its based on chemical composition which was obtained by an XRF analysis of garnet sillimanite gneiss. I think I am confident the way that XRF was done.

Overall I am seeking for answers on a case of rapid rock weathering and at this stage, I want to establish different weathering grades on the basis of chemical composition.

Dear Professor Merlina,

Thank you for your response. its not limestone, a garnet sillimanite gneiss.

In Parker index, i used the weight percentage of oxides.

Well. I am not quite sure whether it needs molar concentration.

Sure, I'll recalutate published data and see, if you know something with Parker's please let me know

Dear Tharanga, you obviously have internet access, and if you google something like 'Parker Index of Weathering', you will undoubtedly get some results for papers that have been published using the PIW.

If your rock is still recognisable as a garnet-sillimanite gneiss, it may not be very weathered anyway.

Dear Mr. Udagedara,

the question is how the sampling has been carried out. Was it done on a more random basis or have taken different samples from the fresh rock/unaltered rock through the saprock, saprolite to the topsoil ? Too often grap sampling is not helpful and a directed sampling campaign the only way to get precise results. I am also a very strict person in terms of taking an approach. I follow up this way: (1) geology (2) mineralogy (3) chemistry. Did you follow this way or did jump the line of study? Is your study for a "paper" or for "giving a piece of advice" to an authority? You have two end members your fresh rock garnet-sillimanite gneiss and a weathered material which I do not know where it has taken from. This should already give you an idea. I sampled a lot of section in various morpho-climatic zoned across the globe. It is only a directed sampling which may help you minimize the field of speculation.

Best regards

Harald G. Dill

I used Parker index to estimate 'weatherability' of volcanic glass of different compositions (peralkaline, andesitic, rhyolitic) in tephras, and it was very effective. Method is given in Table 3 of  a paper Lowe 1988 NZ Jl Geology & Geophysics 32, 125-165.

Best,

David Lowe

Dear colleagues,

I have been carefully monitoring the discussion on the weatherability of certain rocks, focusing on chemical analyses only.

Why not conducting basic mineralogical work prior to the use of XRF whole-rock analyses? I do chemical work, as mentioned previously, second in the row. A heavy and light mineral analysis, including phyllosilicates can provide you with a lot of information about the mineral assemblage by quality and quantity. Each grain may tell you its own story. And there is ample room for the application of more advanced methods such as Rietveld-based mineral quantification, single grain analysis by means of EMPA etc.. The chemical analysis is fine but too often it only provides us with numbers. Some questions raised here, would not have been raised, if we had fulfilled our mineralogical preparatory work.

I would agree that  regionwide or statewise surveys devoted to the weathering or soil-formation can be done sometimes more smoothly and cost-efficiently by whole-rock analysis, but even there I would not go through the project without some reference mineral-logs either at a regular spacing or at pronounced litho-facies changes.

Best regards

Harald G. Dill

Dear Professor Harald,

Thanks and answering your 2nd comment. This is a part of my PhD study and these rocks are from Sri Lanka. There are no previous studies on these rocks.

I collected samples based on their weathering grades that was visually assessed. Thin sections were observed already and going for the chemistry now. I am having problems with calculating some of weathering indices since many authors had done it many different ways, thus I've got a sort of stuck, especially with this Parker index.

Best regards

Tharanga

Thanks Professor David!