Minasgeraisite-(Y)
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| Minasgeraisite-(Y) | |
|---|---|
-109493.jpg) Red microcrystals of minasgeraisite-(Y). Field of view is three millimeters (0.12 in). | |
| General | |
| Category | Nesosilicate minerals, gadolinite supergroup, group, and subgroup |
| Formula | (Ca2Y2)□2(Be2B2)Si4O16(OH)4 |
| IMA symbol | Mgr-Y |
| Strunz classification | 9.AJ.20 |
| Crystal system | Triclinic |
| Space group | P1 |
| Identification | |
| Color | Purple to lavender purple |
| Tenacity | Brittle |
| Mohs scale hardness | 6–7 |
| Luster | Sub-vitreous, resinous, dull |
| Streak | Very pale purple |
| Diaphaneity | Transparent |
| Specific gravity | 4.29 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.740 nβ = 1.754 nγ = 1.786 |
| Birefringence | 0.046 |
| Pleochroism | Moderate |
| 2V angle | 68° (average) |
| References | [1][2][3] |
Minasgeraisite-(Y) is a discredited mineral species in the gadolinite supergroup with the chemical formula (Ca2Y2)□2(Be2B2)Si4O16(OH)4. Typically appearing as minute, purplish-lavender rosettes in granitic pegmatites, it was first identified at the José Pinto quarry in 1986 in Jaguaraçu, Minas Gerais, Brazil. The mineral was named after its type locality and approved by the International Mineralogical Association (IMA). Subsequent crystallographic and chemical analyses of the type material, as well as of comparable specimens from Norway and Central Europe, led to the mineral's discreditation in 2023. It is now regarded as a bismuth- and manganese-rich variety of hingganite-(Y).
Minasgeraisite-(Y) is typically purplish-lavender in color. It is transparent with a sub-vitreous, resinous, or dull luster[2] and a pale purple streak. The mineral is brittle and has a specific gravity of 4.29. Minasgeraisite-(Y) has a hardness of 6–7 on the Mohs scale.[1][2][3]
When viewed with polarized light under a petrographic microscope, minasgeraisite-(Y) appears colorless, pale grayish yellow, and lavender purple and moderately exhibits pleochroism.[1] 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.[1][2][3]
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.[4][3] The mineral was discovered at the Mr. José Pinto quarry,[4] a site known for its complex, zoned pegmatitic formations, in which different minerals occur in distinct internal layers.[3] The mineral was initially named minasgeraisite and approved by the International Mineralogical Association (IMA) under proposal 83-90. The name was later changed to minasgeraisite-(Y) to conform to IMA nomenclature standards for rare-earth element minerals, with the suffix "-(Y)" indicating yttrium as the dominant rare-earth element.[3][4] The initial discovery of minasgeraisite-(Y) stemmed from investigations led by Foord following the collection of fine milarite crystals at the site in 1980. During this work, small amounts of an unidentified lilac-colored mineral were found in the drusy cavities.[3] In 2021, minasgeraisite-(Y) was assigned the IMA symbol Mgr-Y.[5]
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.[4] The discreditation was made based on IMA-CNMNC Proposal 23-F.[4] Modern analyses have demonstrated that the material originally described as minasgeraisite-(Y) is structurally and compositionally consistent with bismuth- and manganese-rich varieties of hingganite-(Y).[6]