Mullite-2<i>c</i> – a natural polytype of mullite
<p>A single crystal (<span class="inline-formula">∼</span> 20 <span class="inline-formula">µ</span>m <span class="inline-formula">×</span> 20 <span class="inline-for...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-03-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/32/235/2020/ejm-32-235-2020.pdf |
| Summary: | <p>A single crystal (<span class="inline-formula">∼</span> 20 <span class="inline-formula">µ</span>m <span class="inline-formula">×</span> 20 <span class="inline-formula">µ</span>m <span class="inline-formula">×</span> 330 <span class="inline-formula">µ</span>m) of mullite-2<span class="inline-formula"><i>c</i></span>, a natural
polytype of mullite, was separated from a radially grown cluster of acicular
crystals from Ettringer Bellerberg (Quarternary Eifel volcanic fields,
Germany). The chemical composition determined from electron microprobe
analysis (EMPA) is
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msub><mi mathvariant="normal">Na</mi><mn mathvariant="normal">0.01</mn></msub><msub><mi mathvariant="normal">Mg</mi><mn mathvariant="normal">0.05</mn></msub><msub><mi mathvariant="normal">Al</mi><mn mathvariant="normal">8.52</mn></msub><msubsup><mi mathvariant="normal">Fe</mi><mn mathvariant="normal">0.29</mn><mrow><mn mathvariant="normal">3</mn><mo>+</mo></mrow></msubsup><msub><mi mathvariant="normal">Si</mi><mn mathvariant="normal">3.13</mn></msub><msub><mi mathvariant="normal">Ti</mi><mn mathvariant="normal">0.02</mn></msub><msub><mi mathvariant="normal">O</mi><mn mathvariant="normal">19.55</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="197pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="c50cca7629249c7ad5e395c36083603f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-235-2020-ie00001.svg" width="197pt" height="17pt" src="ejm-32-235-2020-ie00001.png"/></svg:svg></span></span>,
corresponding to <span class="inline-formula"><i>x</i>=0.22(8)</span> in the generalised mineral formula
<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M12" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">M</mi><mi>y</mi><mo>+</mo></msubsup><msubsup><mi mathvariant="normal">Mg</mi><mi>z</mi><mrow><mn mathvariant="normal">2</mn><mo>+</mo></mrow></msubsup><msubsup><mi mathvariant="normal">M</mi><mrow><mn mathvariant="normal">8</mn><mo>+</mo><mn mathvariant="normal">4</mn><mi>x</mi><mo>+</mo><mi>y</mi><mo>-</mo><mn mathvariant="normal">2</mn><mi>z</mi></mrow><mrow><mn mathvariant="normal">3</mn><mo>+</mo></mrow></msubsup><msubsup><mi mathvariant="normal">M</mi><mrow><mn mathvariant="normal">4</mn><mo>-</mo><mn mathvariant="normal">4</mn><mi>x</mi><mo>-</mo><mi>y</mi><mo>+</mo><mi>z</mi></mrow><mrow><mn mathvariant="normal">4</mn><mo>+</mo></mrow></msubsup><msub><mi mathvariant="normal">O</mi><mrow><mn mathvariant="normal">20</mn><mo>-</mo><mn mathvariant="normal">2</mn><mi>x</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="178pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="519e8c7792d94a260628f24967e14818"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-235-2020-ie00002.svg" width="178pt" height="18pt" src="ejm-32-235-2020-ie00002.png"/></svg:svg></span></span>.
Only <span class="inline-formula">Fe<sup>3+</sup></span> as foreign cation was considered in the refined structure
model, partially replacing <span class="inline-formula">Al<sup>3+</sup></span> in the octahedral chains. A crystal of
a similar type, though exhibiting a significantly different composition with
<span class="inline-formula"><i>x</i>=0.02</span>, was first described in 2015, tentatively named “sillimullite”
by Fischer et al. (2015). This crystal and our new
sample have similar structural properties, now classified as a polytype of
mullite, designated mullite-2<span class="inline-formula"><i>c</i></span>. Single-crystal X-ray diffraction showed that the mullite-2<span class="inline-formula"><i>c</i></span> crystal investigated here exhibits partial Si <span class="inline-formula">∕</span> Al ordering in
the double chains of <span class="inline-formula">(Si,Al)O<sub>4</sub></span> tetrahedra in contrast to the sample
described in 2015 as being completely ordered. The ordering in mullite-2<span class="inline-formula"><i>c</i></span>
results in a doubled <span class="inline-formula"><i>c</i></span> lattice parameter with respect to mullite. It
crystallises in space group <i>Pnam</i>, with cell parameters for the new sample of
<span class="inline-formula"><i>a</i>=7.5432(5)</span> Å, <span class="inline-formula"><i>b</i>=7.7048(5)</span> Å, <span class="inline-formula"><i>c</i>=5.7965(3)</span> Å,
<span class="inline-formula"><i>V</i>=336.89(6)</span> Å<span class="inline-formula"><sup>3</sup></span> and <span class="inline-formula"><i>Z</i>=1</span>. X-ray powder diffraction data are
presented with a detailed discussion of the differences between the
diffraction patterns of sillimanite, mullite and mullite-2<span class="inline-formula"><i>c</i></span>. Crystals of
mullite-2<span class="inline-formula"><i>c</i></span> are translucent to lightly violet, they possess a vitreous lustre
and the calculated density is 3.199 g cm<span class="inline-formula"><sup>−3</sup></span>. The optical
character is biaxial (<span class="inline-formula">+</span>), with refractive indices determined by
spindle-stage microscopy of <span class="inline-formula"><i>n</i><sub><i>x</i></sub>=1.6673</span>, <span class="inline-formula"><i>n</i><sub><i>y</i></sub>=1.6687</span> and
<span class="inline-formula"><i>n</i><sub><i>z</i></sub>=1.680(4)</span> (adjusted to conform to <span class="inline-formula">2<i>V</i><sub><i>Z</i></sub>=39(4)</span><span class="inline-formula"><sup>∘</sup></span>).
Applying the Gladstone–Dale approach, the compatibility index is 0.007,
representing superior compatibility. In terms of chemical composition and
structural features mullite-2<span class="inline-formula"><i>c</i></span> is an outstanding example of mullite-type
compounds falling into the postulated miscibility gap between sillimanite
and mullite. Its crystal structure combines characteristics from both
mullite (oxygen vacancies, triclusters of tetrahedral building units) and
sillimanite (high degree of Si <span class="inline-formula">∕</span> Al ordering in the tetrahedral building units, causing the doubled <span class="inline-formula"><i>c</i></span> parameter). The lattice parameters (normalised to 1<span class="inline-formula"><i>c</i></span>) of the new sample lie between those of sillimanite and 3 / 2 mullite; the chemical composition is close to 3 / 2 mullite and thus differs significantly from the silica-rich composition of the species previously
determined by Fischer et al. (2015), indicating a relatively large
compositional variation.</p> |
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| ISSN: | 0935-1221 1617-4011 |