Dear Dr. Xian, though Chemical Index of Alteration (CIA) is used to study the intensity of chemical weathering that the source rocks of any siliciclastic rock might have undergone during weathering, it is also affected (other than climate) by rock type (lithology), tectonics and recycling processes. Further, K-metasomatism also affects CIA of siliciclastic rocks. All other factors remaining constant, any plagioclase-rich source rock is likely to show higher CIA than K-feldspar-rich source rock.

In your case, the CIA of both green and red siltstones are ideally likely to be similar. If you find any difference, their REE pattern can give you idea, whether the difference is due to source rock or abrupt climatic change.    

Dear Dr. Xian,

Dr. Basu is right. But why only restricting your investigation to the chemical part of weathering ?  At first glance, chemical investigation used as a stand-alone procedure is tempting because you might pass a lot of samples through your analytical devices, and obtain results which you can be plotted in a x-y plot or any other sort of diagrams. In many cases it is still a matter of conjecture that can only be disentangled by integrating geology and mineralogy ,e.g., heavy minerals, clay mineralogy... I am very suspicious of such approaches and recommend a broadening of your angle to look at this topic.

Best regards

H.G.Dill

the critic question is that why does the sedimentary rocks has different color? Of course CIA values could be different in the same deposition base. red color indicates Fe rich mineral. Due to effect of redox conditions, depending on the sea depth also color can be changed. For example, Although Vanadium (V) in oxic zone has a diffrent color, the form of it (II) has got different color in disoxic zone. So, CIA values can be different for a sediment due to effect of climate, redox condition and distance of source rocks to deposition base. Of course these are my opinion.

Murad Cilsal and Harald G. Dill  are very right. Lithology has positive impact on CIA value. Because grain size can make the composition different, CIA value depends on composition, climate, intensity of weathering, radox condition etc. Green and red colours of sediments/siltstone are not the product of same chemical condition. red colour is the indication of intense oxidation but although it is not always but green colour is indicating an anoxic condition.

The green color indicates anoxic condition and red color shows oxic zone as indicated by Mr Sultan, so in the same deposition basen we can see different redox conditions. If we look at your question, it is normal that CIA values could be different in the same lithology.

I think so Mr Murad. Same lithology shows different CIA value if these suffered different weathering condition. At the same time clastic rocks are named on the basis of texture as sandstone, siltstone or clay. Their composition may be different.