Re-definition, nomenclature and crystal-chemistry of the hellandite group.

Detailed X-ray single-crystal refinements and complete (SIMS + EMPA) microprobe chemical analyses of a series of non-metamict samples of hellandite recently found in Latium province (Italy), together with a critical re-evaluation of the existing data on hellandite, allow a better understanding of the crystal-chemistry of the hellandite group. Relative to the previously determined crystal-structure arrangement, a new tetrahedrally coordinated site has been detected; this may be fully, or in part, occupied by Li and Be. These cations occur at the center of the tetrahedral cavity where the H atom, which is bonded to the O5 oxygen, protrudes; thus the H content in hellandite is constrained to values equal to or lower than 2 - (Li + Be + F). A new general formula for hellandite-group minerals could therefore be proposed: X4 Y2 Z T2 [B4 Si4 O22] W2, where X = Na, Ca, Y, LREE3+ at the [8]-coordinated M3 and M4 sites; Y = Ca, Y, HREE3+, Th4+, U4+ at the [8]-coordinated M2 site; Z = Al, Mn3+, Fe3+, Ti4+ at the octahedral M1 site; T = Vacancy, Li, Be at the new tetrahedrally coordinated site; W = OH, F, O2- at the O5 site. Eight root end-member compositions are identified: four of these correspond to known compositions: hellandite-(REE), tadzhikite-(REE), and two correspond to new minerals, mottanaite-(Ce) and ciprianiite. A sequence of incorporation based on crystal-chemical arguments is provided and allows evaluation of the site populations for nomenclature purposes.