Draft:Pilanesbergite
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| Pilanesbergite | |
|---|---|
| General | |
| Category | Sorosilicate |
| Formula | Na2Ca2Fe2Ti2(Si2O7)2O2F2 |
| Crystal system | Monoclinic |
| Space group | P21/a |
| Identification | |
| Color | Brown-orange |
| Cleavage | None observed |
| Tenacity | Brittle |
| Luster | Vitreous |
| Streak | Brownish |
| Diaphaneity | Translucent |
| Density | 3.47 g/cm3 (measured); 3.40 g/cm3 (calculated) |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.743; nβ = 1.768; nγ = 1.795 |
| Pleochroism | Strong; straw yellow to orange-red in thin section |
Pilanesbergite is a sodium–calcium iron titanium sorosilicate mineral with the ideal chemical formula Na2Ca2Fe2Ti2(Si2O7)2O2F2. It is a member of the wöhlerite group and was discovered in nepheline syenite from the Pilanesberg Complex in the North West Province of South Africa.[1][2]
Naming and approval
[edit]The mineral is named after its type locality, the Pilanesberg Complex. The mineral and its name were approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification under proposal number IMA2023-007.[1][3]
Occurrence
[edit]Pilanesbergite was identified in a medium-grained variety of nepheline syenite locally called green foyaite. The type material came from a natural exposure near the southern margin of the Pilanesberg Complex. In the host rock, pilanesbergite occurs interstitially between feldspar grains and is generally associated or intergrown with aegirine.[1][4]
The mineral formed during a late stage of magmatic crystallization, after alkali feldspar, nepheline and sodalite. Associated minerals include arfvedsonite, titanite, apatite, lamprophyllite and, less commonly, aenigmatite. Textural relationships indicate that pilanesbergite can replace titanite and aenigmatite and can itself be replaced by titanium-rich aegirine.[1]
Crystal chemistry and structure
[edit]Pilanesbergite belongs to the wöhlerite group. It is isomorphic with låvenite, normandite and madeiraite.[5] These minerals are related by substitutions involving manganese for iron and zirconium for titanium. The empirical composition of the type material includes appreciable manganese substituting for iron and minor zirconium substituting for titanium, producing compositions that trend toward normandite and madeiraite.[1]
It crystallizes in the monoclinic crystal system with space group P21/a and two formula units per unit cell. Reported unit-cell parameters are a = 10.7811 Å, b = 9.7836 Å, c = 7.0348 Å and β = 108.072°, with a unit-cell volume of 705.41 Å3.[1]
Physical and optical properties
[edit]Pilanesbergite is translucent, brown-orange and has a brownish streak and vitreous lustre.[2] It is brittle, has no observed cleavage and has an estimated Mohs hardness of 5–6. Its measured density is 3.47 g/cm3, compared with a calculated value of 3.40 g/cm3.[1]
The mineral is optically biaxial positive. Its measured refractive indices are α = 1.743, β = 1.768 and γ = 1.795, with a calculated 2V angle of approximately 88°. It displays strong pleochroism, ranging from straw yellow to orange-red in thin section.[1]
Geological significance
[edit]A 2026 review of the Pilanesberg Complex describes pilanesbergite as a rock-forming mineral in the Green and Ledig foyaite units. In the Green Foyaite, it formed relatively early, replacing titanite and sometimes aenigmatite while coexisting with aegirine and lamprophyllite; it was later replaced by titanium-rich aegirine. The review also reports a continuous compositional transition toward normandite as magmatic fractionation and oxygen fugacity increased.[4]
Because wöhlerite-group disilicates can respond to changes in magma composition and crystallization conditions, the occurrence of pilanesbergite and related minerals can provide information about the evolution of peralkaline magmas.[4][5]
Type material
[edit]Cotype material used for electron-microprobe analysis and crystal-structure refinement is held by the Natural History Museum at the University of Oslo under catalogue number KNR 44406. Additional cotype material used for powder X-ray diffraction, optical measurements and density determination is held in the mineralogical collection of the University of Liège under catalogue number 21980.[1]
See also
[edit]References
[edit]- 1 2 3 4 5 6 7 8 9 Dal Bo, Fabrice; Friis, Henrik; Elburg, Marlina A.; Hatert, Frédéric; Andersen, Tom (16 January 2024). "Pilanesbergite: a new rock-forming mineral occurring in nepheline syenite from the Pilanesberg Alkaline Complex, South Africa". European Journal of Mineralogy. 36 (1): 73–85. doi:10.5194/ejm-36-73-2024.
- 1 2 Ybarra Grande, Joan Manuel; Garrido Rufaste, José Luis (November 2024). "Nuevas especies minerales: de enero a junio de 2024" (PDF). Paragénesis (in Spanish). 4 (4): 88.
- ↑ "The New IMA List of Minerals" (PDF). Commission on New Minerals, Nomenclature and Classification, International Mineralogical Association. November 2024. Retrieved 29 August 2026.
- 1 2 3 Elburg, Marlina A.; Andersen, Tom (2026). "The Pilanesberg complex and related igneous rocks". South African Journal of Geology. 129 (1): 207–232. doi:10.25131/sajg.129.2739.
- 1 2 Qu, Kai; Dong, Guochen; Li, Ting; Fan, Guang; Gu, Xiangping; Wang, Yufei; Wang, Yanjuan (2025). "Moxuanxueite, NaCa6Zr(Si2O7)2OF3, a new wöhlerite-group mineral from Gejiu alkaline complex, Yunnan Province, China". American Mineralogist. 110 (10): 1527–1537. doi:10.2138/am-2024-9397.
Category:Sorosilicates Category:Sodium minerals Category:Calcium minerals Category:Iron minerals Category:Titanium minerals Category:Monoclinic minerals Category:Minerals described in 2024
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