Minasgeraisite-(Y)

Minasgeraisite-(Y) is typically purplish-lavender in color. It is transparent with a sub-vitreous, resinous, or dull luster When viewed with polarized light under a petrographic microscope, minasgeraisite-(Y) appears colorless, pale grayish yellow, and lavender purple and moderately exhibits pleochroism. It is biaxial positive and has an optic angle (2V) of 68°. When measured along different crystallographic directions, its refractive indices are nα = 1.740, nβ = 1.754, and nγ = 1.786. This gives it a birefringence of 0.046. == Origins ==

Minasgeraisite-(Y) was described as a new member of the gadolinite group in a paper by Eugene E. Foord et al. published in 1986, with its type locality designated as the Jaguaraçu granitic pegmatite in Minas Gerais, Brazil. The mineral was discovered at the Mr. José Pinto quarry, Subsequent re-investigations of material from the type locality, including crystallographic and chemical studies, led to the discreditation of minasgeraisite-(Y) as a distinct species in 2023. == Occurrence ==

Minasgeraisite-(Y) occurs as a late-stage accessory mineral that formed during the late stages of pegmatite crystallization. Within the zoned, complex pegmatite, it appears in small druses. The mineral is found as minute sheaf-like crystals less than 3–5 μm across, forming rosettes that range from 0.2 to 1.0 mm in diameter, which may occur singly or in aggregates. and Rigó Hill in Hungary. However, these specimens did not match the ideal end-member composition, and the researcher Oleg S. Vereshchagin considered the Brazilian locality to be the only confirmed source of minasgeraisite-(Y) as originally described. == Structure ==

Minasgeraisite-(Y) was originally described as a member of the gadolinite supergroup, a group of minerals characterized by the general formula , where A, M, Q, T represent distinct crystallographic sites occupied by different cations, and φ denotes anions such as hydroxyl or oxygen. This reduction in symmetry is attributed to significant cation ordering at the A sites. In this triclinic structure, the single A site splits into four distinct and non-equivalent sites (denoted W1 through W4), occupied by bismuth, calcium, and rare-earth elements (primarily yttrium and other heavy lanthanides). Bismuth predominates at W1, calcium at W2, and yttrium and other rare-earth elements occupy W3 and W4, demonstrating nearly complete cation ordering. The currently accepted formula for minasgeraisite-(Y) is therefore represented as . == References ==