They become an essential method for the study of minerals too small to be studied under the optical microscope, like most of the main minerals in sedimentary rock (clay minerals).
(They; become; an essential method for the study of minerals too small to be studied under the optical microscope, like most of the main minerals in sedimentary rock)
They
become
an essential method for the study of minerals too small to be studied under the optical microscope, like most of the main minerals in sedimentary rock