These are so large that optical properties should not be your concern. You'll find great robustness to any changes in such properties (and robustness should be used to justify any selection of optical properties). The point of biggest concern in reproducibility is the taking of a representative sample. Are you measuring wet or dry?

Thank you Alan, I am measuring wet, and I have measured already but since I do not know the refractive index of MnCoSi, I think the particle sizes obtained are not correct. During the measurement since I did not know the refrative-index of MnCoSi, I used 5 as the refrative index, but for the absorption, I measuresed with UV-VIS to be around 0.246. I am of the view that my real particle size would be within 200 -400nm for the smallest size up to 300 microns for the largest particles, judging from the sieve sizes used for the separation. I am considering using Malvern Mastersizer that can make measurements on dry samples instead of Zetasizer that measures on solution but I still need refrative index. I only need intensity versus particle size information

@Kenneth Chibueze Ugochukwu

In your question, you talk about sizes of 300 - 800 microns. This size is not suitable for DLS (Zetasizer) as the top limit is around 1 micron or less for samples as dense as your metal powders. Laser diffraction (Mastersizer) is the only option - this provides a volume distribution not an intensity one. Again, I state that optical properties are practically irrelevant for samples this size (ISO 13320:2009 will state that optical properties will be important below 50 microns or so). Obviously if you micronized such material, then this would not be true. Your biggest issue is representative sampling and getting the minimum mass of sample in the system. Screen results will be different for reasons that should be obvious to you. For assistance see:

Sampling for particle size analysis - estimation of standard error

https://www.brainshark.com/malvern/vu?pi=zH8zgQtOvz5chz0&tx=preview

https://www.brainshark.com/malvern/vu?pi=zGBzlGgVGz5chz0&tx=preview

A comparison of sieves and laser diffraction for particle size analysis

https://www.brainshark.com/malvern/vu?pi=721845422&b=1&tx=120486&c1=22

If you are insistent in getting the optical properties of your alloy then the Gladstone-Dale (GD) route offers an approximation (again you'd test this with a robustness test). If you don't know what robustness is see (opens in IE ONLY):

Determination of the optical properties of CeO2 and CaCO3 by the volume concentration experiment - the 3 S's, Beer-Lambert and Mie

https://www.brainshark.com/malvern/OpticalPropertiesCeO2

Back to G-D, then it's easy to obtain the optical constants (and density) for each of the metals in your alloy (632.8 nm):

Mn: 2.53 - i.3.6 (https://refractiveindex.info/?shelf=main&book=Mn&page=Johnson) Density = 7.43 g/cm3

Co: 2.21 - i.4.17 (https://refractiveindex.info/?shelf=main&book=Co&page=Johnson) Density = 8.86 g/cm3

Si: 4.21 - i.0.42 (https://refractiveindex.info/?shelf=main&book=Si&page=Pierce) Density = 2.32 g/cm3

As a first guesstimate then you could simply average the above:

n k 2.53 3.6 2.21 4.17 4.21 0.42 Average 2.98 - i.2.73

But as light passes through a volume of space it would be better to get the relative volumes in the alloy - I'll leave that with you - I've provided the densities for you so you have an easy calculation

You can also read my chapter on 'Refractive index measurement' in the 3rd Encyclopedia of Spectroscopy and Spectrometry 2017, Pages 936-945 You can buy the chapter alone for about $30.

Kenneth Chibueze Ugochukwu

What is your reason for stating: 'Since I do not know the refractive index of MnCoSi, I think the particle sizes obtained are not correct?' Please provide literature justification for your comment.

BTW, an RI of 2.98 - i.2.73 means an effectively opaque material.

Chapter reference: https://tinyurl.com/yyofp8qe

Alan F Rawle , Thank you so much for providing a wonderful clarity on this subject of estimating the refractive index of my sample. Your explanation is so apt that it has cleared the knotty areas surrounding my measurements. I am definitely going to tell my supervisor to get me the book: Encyclopedia of Spectroscopy and Spectrometry 2017, I think it is going to make a nice read. Also thanks for suggesting that I use Mastersizer, I think that would give a better estimate of the particle size of the samples. I am grateful. Thank you so much.

Thank you for your kind comments. You must calculate the minimum mass of sample for a specified standard error as your material is extremely large from a sampling perspective (>> 100 microns). For example, if the density of your material is 6.81 g/cm3 (Google Search) and your x99 point is at 800 microns then you'll need about 1830 g of material to hit 1% standard error on the x99 point. Obviously, you'd never get this amount of material in a wet accessory. You'll see how the calculations are carried out in the url I indicated above. You can reverse the calculations to calculate an estimated standard error based on the mass of sample you use/take in your experiment. This gives you the best standard error based solely on the heterogeneity of the material (FSE = Fundamental Sampling Error). If you require the x90 to 5% standard error then the minimum mass I gave you before can be reduced by a factor of 250 (to a more reasonable 7.33 g).

I'm sure your supervisor or library will not buy this book/encyclopedia as it's over $2000 for the entire thing. I provided a link where you can get the relevant chapter for $30 or so.