Definition: Bentonite is defined as a naturally occurring material that is composed predominantly of the clay mineral smectite.  Most bentonites are formed by the alteration of volcanic ash in marine environments and occur as layers sandwiched between other types of rocks.  The smectite in most bentonites is the mineral montmorillonite, which is a dioctahedral smectite but occasionally other types of smectite may be present.   It is the presence of smectite which imparts the desirable properties to bentonites, although associated factors such as the nature of the exchangeable cations in the interlayer also affect properties.  For example naturally occurring bentonites with Na+ as the interlayer cation can have very different properties to bentonites where the interlayer cation is Ca++.  Most commercial bentonites contain more that 80% smectite, however, a wide variety of other minerals may occur as impurities.  The image on this page shows a geosynthetic liner with bentonite granules as fill.

 Geosynthetic clay liner, with fill of bentonite granulesBentonite powder
Analyses:  We have wide experience in the analysis of bentonites for their mineralogical composition.   This is done by full pattern fitting of experimental X-ray powder diffraction patterns.  Typical impurities in bentonites include quartz, feldspars, zeolites and carbonates. Additionally some bentonites can contain the silica mineral cristobalite.  In addition to quartz and cristobalite, some bentonites particularly 'white bentonites' can contain large amounts of opal, which may occur in a variety of paracrystalline forms.  The form known as opal-C is easily confused with cristobalite and we have developed (Hillier and Lumsden, 2008) a routine test based on solubility in NaOH, combined with XRPD analysis, that aids the distinction of the opaline forms of silica from crystalline cristobalite. This method can also be used to analyse for TiO2 polymorphs in bentonites, the two most common forms being anatase and rutile.
The smectite in bentonite often requires more detailed study and this is achieved by analysis of oriented clay fractions which allow the analysis of the positions and relative intensities of basal peaks.  Such analyses may be used to determine if the smectite is pure smectite or mixed-layered with other minerals.  Analysis of basal peak positions and responses to various ancillary treatments can be used to determine other characteristics of the smectite. 
Hillier, S. and Lumsden, D., 2008, Distinguishing opaline silica from cristobalite in bentonites: a practical procedure and perspective based on NaOH dissolution, Clay Minerals, 43, 477-486.