Composite crystals and twins

Most crystalline natural minerals, and many crystals produced industrially, exhibit some form of aggregation or intergrowth, and prevention of the formation of these composite crystals is one of the problems of large-scale crystallization. The presence of aggregates in a crystalline mass spoils the appearance of the product and interferes with its free-flowing nature. More important, however, aggregation is often indicative of impurity because crystal clusters readily retain impure mother liquor and resist efficient washing (section 9.7.2). 

The formation of crystal clusters, aggregates or conglomerates which possess no symmetrical properties is probably more frequently encountered in large-scale crystallization than the formation of twins. Relatively little is still known 

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