Dissolution‐repackaging of hellandite‐(Ce), mottanaite‐(ce)/ferri‐mottanaite‐(ce)

We investigated hellandite‐group mineral phases from the Roman Region, alkali syenite ejecta, by multimethod analyses. They show a complex crystallisation history including co-precipitation of hellandite‐(Ce) with brockite, resorption, sub‐solidus substitution with mottanaite‐ (Ce), exsolution of perthite‐like ferri‐mottanaite‐(Ce), overgrowth of an oscillatory‐zoned euhedral shell of ferri‐mottanaite‐(Ce) and late, secondary precipitation of pyrochlore in the cribrose hellandite‐(Ce) core. LREE/HREE crossover and a negative Eu anomaly in hellandite‐group minerals follows fO2 increase during magma cooling. The distinction among the hellandite‐group minerals is based on the element distribution in the M1, M2, M3, M4 and T sites. Additional information on miscibility relationship among the hellandite sensu strictu, tadzhikite, mottanaite, ferri‐mottanaite and ciprianiite endmembers derives from molar fraction calculation. We observed that change in composition of hellandite‐group minerals mimic the ligands activity in carbothermal-hydrothermal fluids related to carbonatitic magmatism.