Why make thin-sections?
Example #1: "Aluminous stone" in-glass, PLM 100X
Stone is too dense and opaque to see anything of value to help determine crystalline phase(s) composition and its source material.
Example #1: "Aluminous stone" after thin-sectioning, PLM 225X
At higher magnifications the Tabular Alumina, secondary a-Al2O3, B-Al2O3 and Nepheline phases are visible. The stone is from an alumina fortified Mullite or Sillimanite class brick.
Example #2: "Silica type stone" in-glass, PLM 100X
Stone is really too thick to be properly identified in-glass even though you can discern some Tridymite laths at the thinner edges or ends of the stone.
Example #1: "Aluminous stone" after thin-sectioning, PLM 100X
Crystalline morphology and phase(s) can begin to be identified at low magnifications.
Example #2: "Silica type stone" in-glass, Stereo-zoom microscope (SMZ) 50X
Stone is translucent white, "lumpy" and includes small outgassing bubbles which serves to indicate it most likely has a siliceous composition
Example #2: "Silica type stone" after thin-sectioning, PLM 100X
It is now easy to identify the stone is composed entirely of secondary Tridymite laths and thus originated from silica refractory brick in the colder, front end of the refiner at temperatures < 1470C inversion to Cristobalite.