It would be helpful if you could tell what type of microscopy you're using and are the samples loose grains or cemented together. If the grains are cemented together you could consider making thin sections and use a petrographic microscope to identify individual grains. Alternatively if you have loose grains, think about embedding them in an appropriate resin (like Canada Balsam) and create a thin section that way. the quartz will stand out under the petrographic microscope under crossed polarized light because it will not show any lamellar twinning and it is uniaxial, while the feldspars are biaxial. Potassium feldspar vs plagioclase is a bit more difficult. If they show twinning, which is relatively common, then you can distinguish them by the difference in twin laws. Plagioclase will show distinct lamellar twins that are much wider than in K-feldspars, while potassium feldspar shows what could be described as Scotch plaid twinning. See for examples images in Dexter Perkins and Kevin R. Henke - Minerals in Thin Section, Prentice Hall, 2000. Hope this helps. Theo

Well very good respond from Theo. it would be really helpful to tell whether your samples are consolidated or unconsolidated. There are many kinds of resins which are applied on unconsolidated sands to give them cohesion, and this include epoxy, polymerized and others. It is very important to note that most of these resins are hydrophobic so your samples wud have to be free of water and this may be achieved even through air drying. Talking of hard rocks you simply prepare thin sections and carry out petrographic studies using petrographic microscope of course. Quartz grains differs from feldspar grains as they shows no cleavage planes and they tend to have wavy extinction even though feldspar grains sometimes do resemble such property as well. other characteristic to look at is their interference properties where quartz grains are uniaxially positive. Most of time the presence of feldspar grains is distinguished by distinct lamellae and when feldspars are subjected under weathering or alteration shows a bright sperk appearance which also helps to identify from quartz grains. Quarts grains normally shows a diagenetic features of overgrowths

I surely agree with Theo and Kinshasa, I also generally use the optical method. But if you need to work on many samples and many grains, I would not rule out the staining method. The preparation surely takes time; but once samples are prepared, I suppose you can work on image analysis. 

Dear Dr.Xuejun Jiang,

browsing the answers given previously to mine I can conclude everything has been told as far a the use of petrographic microscopy is concerned. In your case you have to deal with an unknown but large number of samples composed of a rather uniform mineral assemblage (quartz, feldspar, carboante..) where qualitative approaches will not help you gain much information, therefore I encourage you to shift from the petrographic microscope to XRD and try and widen your database by using the rietveld refinement to obtain quantitative results. It will be a great step forward in your case.

With kind regards

H.G.Dill

For microscopy, we should not forget that determination of refractive indices (RI) can be quite useful and fairly quick. For example, if you use a liquid of RI 1.527, Or grains would have lower RI in any orientation, whereas Qtz and Pl grains would have highern RIs. Carbontes should not pose any problem because of their high interference colour. But the procedure is tedious and gone unpopular.

Hello

If you get thin sections of your samples you can also use SEM with an EDX system to discriminate very easily potassium and plagioclase feldpars. Recent SEM generally provides user friendly software wich allow to quantify the amount of the different detrital minerals in a thin section.