Dear Mr. Erkan,

what is your study aimed at ? Do want to determine the refractive index of phyllosilicates in kaolin or those of accessory minerals like anatase, brookite, andalusite etc. ? In both cases the grain size has a limiting effect on the determination of these optical properties using the petrographic microscope and/ or the refractometer. A grain size classification and separation is wise to be done prior to the optical investigation.

Best regards

H.G.Dill

Dear Mr. Dill,  I only want to determine the refracitve index of kaolin or may be if it will necessary the refractive index of calcined kaolin. I need a simple and usefull tool for this purpose. Can you please specify the refractometer you mentioned.

Dear Mr. Erkan,

By definition, kaolin is a rock and as such mineral properties can only be determined of the phyllosilicates, prevalently kaolinite-group minerals and minor constituents which may be opaque (e.g.magnetite), semiopaque (e.g. rutile) or transparent (e.g. zircon). Only the last two mineral groups are amenable to optical methods using the petrographic microscope or refractometer. Phyllosilicates should be investigated by means of IR spectrocopy, XRD etc. (see comment of Dr. Towe). Optical methods are not the method of choice. The situation becomes more delicate as kaolin is calcined.

Between 550 to 600°C kaolinite [Al2Si2O5(OH)4] dehydrates and results in the formation of metakaolin [Al2Si2O7], which is amorphous. This process may be continued to up to approx. 900°C achieving a sort of "semi-crystalline mineral" . Rising the temperature to as much as 950°C leads to what is called γ-aluminum oxide. A further increase in temperature to 1050°C leads to a decomposition of the afore-mentioned spinel into mullite [Si2Al6O13] and cristobalite.

I am not sure whether the methods considered in your questions will really broaden significantly your knowledge. The different "minerals" in calcined kaolin are described in textbooks on technical mineralogy. The combination of artificial products of different crystallinities are certainly a hard nut to crack. I assume thermo-analytical methods such TG and DTA are much better suited.

I wish you all the best.

H.G.Dill

Dear Dr. Towe and Dr Dill, I like to thank you for your wide knowledge about the subject. It will help a lot. Thank you very much again.