Monozygotic twins are contained within the same chorionic membrane and three-quarters of MZ twins share the same placenta. This also means they have the same blood type. Incomplete or late division of the zygote and subsequent cell masses can result in conjoined or Siamese twins . Monozygotic twins make up about a third of all twin births but their occurence has nothing to do with heredity, unlike dizygotic twinning. MZ twinning occurs randomly in all racial groups and follows no discernable hereditary pattern.
Mineralogical twins are analogous to monozygotic, Siamese twins.
The twinned crystals are, by definition, compositionally and structurally identical. Twinning occurs for many different minerals, and across many of the chemical and structural groups of minerals. Most mineral twins, at least those visable to the eye, form during crystal nucleation and growth. Mineral twinning during crystal growth may be random, but the particular symmetry operations that join mineral twins are constrained by the crystal structure. Certain twin laws are more common in particular crystal systems, and they are also diagnostic features for many minerals.