Pilanesbergite: a new rock-forming mineral occurring in nepheline syenite from the Pilanesberg Alkaline Complex, South Africa
<p>The new mineral pilanesbergite, with the ideal formula Na<span class="inline-formula"><sub>2</sub></span>Ca<span class="inline-formula"><sub>2</sub></span>Fe<span class="inline-formula"><sub>2</sub&...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2024-01-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/36/73/2024/ejm-36-73-2024.pdf |
| Summary: | <p>The new mineral pilanesbergite, with the ideal formula Na<span class="inline-formula"><sub>2</sub></span>Ca<span class="inline-formula"><sub>2</sub></span>Fe<span class="inline-formula"><sub>2</sub></span>Ti<span class="inline-formula"><sub>2</sub></span>(Si<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>7</sub></span>)<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>2</sub></span>F<span class="inline-formula"><sub>2</sub></span>, was found in a nepheline syenite, locally known as green foyaite, from the Pilanesberg Complex located in the North West Province of South Africa. Pilanesbergite occurs in green foyaite in association, and partly intergrown, with aegirine. The two minerals share an assemblage of inclusions, comprising euhedral nepheline, titanite and minor sodalite. Pilanesbergite belongs to the wöhlerite group and is isomorphic with låvenite, normandite and madeiraite. It is related to these species through the homovalent chemical substitutions <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi mathvariant="normal">Mn</mi><mrow><mn mathvariant="normal">2</mn><mo>+</mo></mrow></msup><mo>↔</mo><msup><mi mathvariant="normal">Fe</mi><mrow><mn mathvariant="normal">2</mn><mo>+</mo></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="91863247f31023252986c8b37f6003cb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-73-2024-ie00001.svg" width="67pt" height="13pt" src="ejm-36-73-2024-ie00001.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi mathvariant="normal">Zr</mi><mrow><mn mathvariant="normal">4</mn><mo>+</mo></mrow></msup><mo>↔</mo><msup><mi mathvariant="normal">Ti</mi><mrow><mn mathvariant="normal">4</mn><mo>+</mo></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="59pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="13dfe66edc46bd2aa44807383b2f488a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-73-2024-ie00002.svg" width="59pt" height="13pt" src="ejm-36-73-2024-ie00002.png"/></svg:svg></span></span>. The empirical formula calculated on the basis of 18 anions is Na<span class="inline-formula"><sub>2.00</sub></span>(Ca<span class="inline-formula"><sub>1.74</sub></span>Na<span class="inline-formula"><sub>0.26</sub></span>)<span class="inline-formula"><sub>Σ2.00</sub></span>(Fe<span class="inline-formula"><sub>1.00</sub></span>Mn<span class="inline-formula"><sub>0.52</sub></span>Ca<span class="inline-formula"><sub>0.49</sub></span>Zr<span class="inline-formula"><sub>0.05</sub></span>)<span class="inline-formula"><sub>Σ2.06</sub></span>(Ti<span class="inline-formula"><sub>1.69</sub></span>Zr<span class="inline-formula"><sub>0.14</sub></span>Mg<span class="inline-formula"><sub>0.09</sub></span>Nb<span class="inline-formula"><sub>0.08</sub></span>)<span class="inline-formula"><sub>Σ2.00</sub></span>(Si<span class="inline-formula"><sub>2</sub></span>O<span class="inline-formula"><sub>7</sub></span>)<span class="inline-formula"><sub>2.00</sub></span>O<span class="inline-formula"><sub>1.84</sub></span>F<span class="inline-formula"><sub>2.16</sub></span> (<span class="inline-formula"><i>Z</i>=2</span>). The new mineral is translucent with a brown orange colour and a brownish streak. The Mohs hardness is estimated between 5 and 6 by comparison with låvenite, and no cleavage is observed. Measured and calculated densities are <span class="inline-formula"><i>D</i><sub>meas</sub>=3.47</span> g cm<span class="inline-formula"><sup>−3</sup></span> and <span class="inline-formula"><i>D</i><sub>calc</sub>=3.40</span> g cm<span class="inline-formula"><sup>−3</sup></span>. In the thin section the pleochroism is strong, between straw yellow and orange red, while in immersion the strong pleochroism is observed between light yellow (<span class="inline-formula"><i>α</i></span>) and yellowish orange (<span class="inline-formula"><i>γ</i></span>). The crystals are optically biaxial (<span class="inline-formula">+</span>) with <span class="inline-formula"><i>α</i>=1.743(3)</span>, <span class="inline-formula"><i>β</i>=1.768(3)</span>, <span class="inline-formula"><i>γ</i>=1.795(5)</span> and a 2 <span class="inline-formula"><i>V</i></span> angle close to 90<span class="inline-formula"><sup>∘</sup></span>. The crystal structure is monoclinic (<span class="inline-formula"><i>P</i></span>2<span class="inline-formula"><sub>1</sub></span>/<span class="inline-formula"><i>a</i></span>), with the unit-cell parameters <span class="inline-formula"><i>a</i>=10.7811(2)</span>, <span class="inline-formula"><i>b</i>=9.7836(1)</span>, <span class="inline-formula"><i>c</i>=7.0348(1)</span> Å, <span class="inline-formula"><i>β</i>=108.072(2)</span><span class="inline-formula"><sup>∘</sup></span> and <span class="inline-formula"><i>V</i>=705.41(2)</span> Å<span class="inline-formula"><sup>3</sup></span>, and has been refined to <span class="inline-formula"><i>R</i><sub>1</sub>=2.06</span> %. The strongest lines of the powder X-ray diffraction pattern [<span class="inline-formula"><i>d</i></span>, Å (<span class="inline-formula"><i>I</i></span>, %) (<i>h k l</i>)] are 3.219 (60) (310), 2.851 (100) (12-2), 2.802 (51) (320), 2.743 (27) (22-2), 2.423 (19) (40-2) and 1.723 (19) (44-2). Pilanesbergite formed under relatively reducing conditions from an agpaitic nepheline syenite magma that had evolved by fractional crystallization mainly of aegirine. Further crystallization of arfvedsonite caused an increase in oxygen fugacity and a change towards higher <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M57" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">Mn</mi><mo>/</mo><mi mathvariant="normal">Mn</mi><mo>+</mo><mi mathvariant="normal">Fe</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="62pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="5546fbdbeb12496a10cc0f543ca8d7df"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-73-2024-ie00003.svg" width="62pt" height="14pt" src="ejm-36-73-2024-ie00003.png"/></svg:svg></span></span> of the magma, causing a change of mineral composition from pilanesbergite towards normandite.</p> |
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| ISSN: | 0935-1221 1617-4011 |