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In certain conditions of mineral formation, especially in metamorphic deposits, a developing crystal does not have free space to grow such as in hydrothermal veins, but has to dissolve and/or push previously formed minerals to make a space for its growth. In this scenario, particles of the host rock minerals can be easily trapped by the growing crystal as inclusions. Such inclusions are called protogenetic, from Greek “proto-“ first, earliest and “genesis” – origin, formation. These minerals, now found as inclusions, could have been formed hundreds of millions of years ago with regards to the host crystal; they correspond to totally different geological events and conditions.
Protogenetic inclusions are usually partly dissolved and have corroded, relictic shapes. Sometimes, original texture of the host rock can be conserved in the pattern of inclusions that maintain their position and orientation from the host rock.
Partly dissolved mica flakes in emeralds from metamorphic deposits are a good example of protogenetic inclusions. In the photo below, mica particles conserve their orientation even when trapped inside emerald crystal. Also, different relictic parts of the same mica flake have the same crystallographic orientation, unambiguously showing that they belonged to the same mica crystal, partly dissolved during emerald formation but trapped without changes in their original orientation.
Protogenetic inclusions of mica particles in emeralds form the Urals, Russia (see explanation above).
Petrographic thin section, crossed polars. Field of view 0.15 mm.
Another clear example of protogenetic inclusions – parallel needles (probably amphibole) in emerald from the Urals, Russia (doubly polished section, transmitted light). Note that in some stages of crystal growth these needles are completely dissolved, while in others they are trapped by growing emerald crystal, conserving the same orientation in all areas, corresponding to their position in the host rock.
Protogenetic inclusions of mica and chromite in emerald form Brazil.
Note how black chromite grains where intensively dissolved during emerald growth. Field of view 4 mm.
Brecciated and partly dissolved chromite inclusion in Brazilian emerald, SEM image in backscattered electrons. Note that in larger relicts brighter rims can be observed, corresponding to higher concentration of Cr and Fe and less Al and Mg chromite, as a result of dissolution process (see image below), indicating undoubtedly the protogenetic character of chromite inclusions in this case.
Microprobe line scanning profiles across one of the chromite particles shown in the image above. Note that during chromite dissolution, first elements to be lixiviated are Al and Mg, causing the enrichment in Cr and Fe on the borders, seen as brighter rims in the image above.
It has been also shown by Pignatelli et al. that in Colombian trapiche emeralds, black sectors are formed due to numerous inclusions of carbonous material of the host rock (black shales), giving another example of protogenetic inclusions in gems. In this case, crystallographic orientation of different growing faces makes a difference with regards to the capacity of the crystal to push or entrap particles of the host rock, providing the phenomenon of textural sector zoning observed in trapiche emeralds. Another good example of the same phenomenon can be a well known chiastolite, the variety of andalusite coming from metamorphic rocks, with marked textural zoning in a form of a cross due to numerous black inclusions of the host rock in particular growth sectors.
Trapiche emerald (Colombia), 3.41 ct.
Polished chiastolite, variety of andalusite with textural sector zoning in a form of cross (Asturias, Spain), 8.32 ct.