From Online Dictionary of Crystallography
Cristal à mosaique (Fr). Mosaikkristall, Mosaikstruktur (Ge). Cristallo a mosaico (It). モザイク結晶 (Ja). Estructura de mosaico (Sp).
The mosaic crystal is a simplified model of real crystals proposed by C. G. Darwin in 1922 (The reflection of X-rays from imperfect crystals. Philos. Mag. 43, 809–829; but the term 'mosaic’ was actually introduced by P. P. Ewald). In this model, a real crystal is described as a conglomerate of minute crystalline blocks, tilted to each other by fractions of a minute of arc. Each block is separated from the surrounding blocks by faults and cracks.
In a diffraction experiment, interference between waves only occurs inside a block, whose dimensions satisfy the theoretical conditions of applicability of the kinematical theory. Because of the loss of coherence between the waves diffracted from different blocks, the diffracted intensity from the whole crystal is equal to the sum of the intensities diffracted by every block. If the blocks are not small enough, dynamic effects take place within each block, reducing the diffracted intensity. This is primary extinction. If the blocks are not sufficiently misoriented with respect to each other, reduction of the diffracted intensity also occurs. This is secondary extinction.