Our thin section maker deliberately varied the thickness to which he polished our Aguas Zarcas CM2 thin sections, from the standard 30 microns to thinner, sometimes much thinner. At the ‘proper’ thickness there are few crystalline features large enough to effectively span the thickness of the opaque, fine grain matrix. In transmitted light there is a lot of black. Thinner samples mean smaller grains get polished on their tops and bottoms and pass light. In transmitted light we see more features, sometimes many more. Thinner mineral grains have ‘incorrect’ birefringent colors in cross-polarized light – sometimes no color at all, simply white.
Incidentally, Aguas Zarcas contains abundant phyllosilicates. We heard of an individual specimen that was recovered after a rain that had absorbed water, swollen and cracked. Our TS maker used no water on these sections. He mentioned that it is difficult material, similar to Tagish Lake.
Dust jacketed components are apparent under the ‘scope in incident, not transmitted, light. The thin section’s thickness does not matter. The dust mantles accreted within the solar nebula before incorporation in the parent body. Seemingly, most coarse components have dust rims – chondrules, mineral aggregates, mineral grains, CAI etc.