Background
Shale and slate are sometimes used interchangeably, but the materials are not the same. Many of the items sold today for landscaping (flagstones, retaining walls) and construction (chalkboards, roofing tiles, pool tables, etc.) uses as "slate " is actually the much more inferior form—shale. Shale is sedimentary rock, lightly compacted into thin, crumbly layers. It may contain high concentrations of quartz, feldspars, pyrites amorphous silica, and clay minerals: illite, kaolinite, chlorite and smectite (montmorillionite). This smectite clay is a highly absorbent component that can swell to several times it's normal size, making the shale loose strength. Slate, a metamorphic rock, on the other hand is much stronger and does not absorb water (remember how fast those school blackboards would dry when you had to stay after class?) Generally, it has much greater strength. However, certain types of slate still may be less than desirable. There also is a high calcite/dolomite form of slate called Marl Slate, from Durham, UK, which can be softer and easily attacked by acids. Shales, a much cheaper material, may be sold as slates at higher prices with a substantially reduced strength and product life.
Investigation
The MiniFlex benchtop XRD system can measure both the bulk piece of the slate or a ground powder form of the shale and identify each material.
The use of X-ray fluorescence elemental mapping analysis is also helpful in identifying the concentration variations across the surfaces and verify the X-ray diffraction phase analysis. Please see the application byte Mapping and semi-quantitative analysis of shale-slate for details.