The Application of X-ray Powder Diffraction to Marble Definition and Classification

Marble primarily consists of three types: calcite, dolomite and magnesite. Marble classification is based mainly on the structures and mineral compositions seen under the microscope. It is very difficult to determine calcite, dolomite and magnesite under the microscope because each type has the same crystal optical characteristics, such as flash protrusions, senior white interference color, uniaxial optical negative and rhomboid cleavage, which belong to the triclinic system. In order to identify the mineral types and the relative content of the carbonate in marble accurately, 32 marble samples were tested with microscope and X-ray Powder Diffraction using semi-quantitative analysis technology. The identification of thin section results showed that: Marble rock-forming minerals are calcite, dolomite, magnesite, quartz, plagioclase, muscovite, biotite, chlorite, clays minerals and metal ores. According to the characteristics of the rock structure and the mineral components, 32 marbles can be divided into calcite marble, felsic calcite marble, quartz chlorite dolomite marble, dolomite marble, mica-felsic dolomite marble, magnesite marble and other 15 types. The X-ray Powder Diffraction semi-quantitative analysis results showed that: the marble rock-forming minerals are calcite, dolomite, magnesite, quartz, plagioclase, K-feldspar, mica, chlorite, talc and montmorillonite. Comprehensive analysis showed: it's difficult to distinguish calcite, dolomite and magnesite and other carbonate minerals, and tiny quartz, K-feldspar and plagioclase, muscovite and other scaly talc silicate minerals by thin section with microscope identification technology. X-ray Powder Diffraction semi-quantitative analysis technique can not only accurately detect the type and relative content of calcite, dolomite, magnesite, and other carbonate minerals, (D values of dolomite, calcite and magnesite, the major X-ray Diffraction peak, were 0.303 nm, 0.288 nm and 0.274 nm), but also identify effectively the rock silt plagioclase, feldspar and quartz with D values of respectively 0.319 nm 0.324 nm and 0.334 nm; and identify montmorillonite, chlorite, mica, talc and other layered silicate minerals with D values of respectively 1.400 nm, 0.705 nm, 0.989 nm and 0.938 nm. The application of X-ray Powder Diffraction semi-quantitative analysis techniques is a breakthrough on marble mineral identification technology. According to comprehensive results of thin section under polarizing microscope identification and X-ray Powder Diffraction analysis, 32 marble samples are divided into 22 kinds of rock types eventually. The study showed that using microscope or X-ray Powder Diffraction identification technology separately is insufficient for accurately identifying marble rocks. Both techniques should be combined with field observation, to identify accurately the marble type.