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In many cases, different minerals are formed during the same mineralization process, time and P-T-X conditions. Minerals that form together can be frequently found as inclusions in each other. Such inclusions are called syngenetic, from Greek “syn–“ together, “genetic” formation.
Syngenetic euhedral pyrite inclusions in emerald from Colombia, typical inclusion well known to gemologists (left, field of view 4 mm), together with a curious sample of just the contrary case of pyrite crystal overgrowing emerald prism, from Colombia (right, sample length 4 cm).
Sometimes syngenetic inclusions can mark a certain stage of the host crystal growth, producing an entire zone of inclusions and even forming phantoms. In other cases they can be randomly distributed in the volume of the host crystal.
It is typical for syngenetic inclusions to have well formed crystal shapes (euhedral or idiomorphous crystals) in comparison to protogenetic inclusions that usually present rather irregular and partly dissolved forms. That’s because minerals that form together are stable in the same conditions, they develop well formed crystals and do not suffer posterior dissolution process as in the case of protogenetic inclusions.
Syngenetic inclusions of pyrite twinned crystals in quartz from Brazil. Pyrite crystals have pentagon dodecahedron forms and are twinned in “iron cross” form typical for pyrite. Field of view 7 mm.
Another typical characteristic that allows distinguishing unambiguously syngenetic inclusions is the formation of so called “induction faces” or “induction striation” between two different minerals growing in contact. In such cases, crystals of two different minerals growing together develop faces corresponding to each one of them alternatively and subsequently on their contact surface, as a product of alternation of growth episodes corresponding to each one of them.
Induction faces (induction striation) formation scheme. When two such crystals are separated, they have not only their proper crystal faces corresponding to the symmetry of their crystal structure, but also those induced by adjacent crystal, presenting clear evidence of simultaneous growth.
Image: Syngenetic fluorite inclusions in quartz from Madagascar. Note that several fluorite octahedrons start their growth from the same plane within quartz crystal. At the beginning, they stat as very small crystals, so their final octahedrons miss one of the sides, corresponding to the beginning of growth. In contrasts, precisely in this starting side, induction faces are developed between the host quartz and fluorite inclusions, confirming their simultaneous growth.