Crystal chemistry of Belgian ardennites

<p>The mineral ardennite-(As), which belongs to the <i>ardennite</i> group, was originally described in the locality of Salmchâteau, Stavelot Massif, Belgium. In the past 10 years, several new samples of ardennites have been found at seven localities of this region, motivating us to reinvestigate the crystal chemistry of the ardennite group. Under the polarizing microscope, most ardennites form lamellae or needles included in quartz veins or constituting the matrix of red Ordovician schists. Electron-microprobe analyses, as well as single-crystal structure refinements, show a homovalent substitution of As<span class="inline-formula">⁵⁺</span> by V<span class="inline-formula">⁵⁺</span> at the <span class="inline-formula"><i>T</i></span>4 tetrahedral site, leading to a complete solid solution between ardennite-(As) and ardennite-(V). Minor substitutions on that site allow the incorporation of a maximum of 0.28 P<span class="inline-formula">⁵⁺</span> atoms per formula unit (apfu) and of less than 0.1 Si<span class="inline-formula">⁴⁺</span> pfu, except in a sample from Arbrefontaine, where Si reaches 0.74 apfu. The main substitution mechanism, affecting both the <span class="inline-formula"><i>T</i></span>4 and <span class="inline-formula"><i>M</i></span>3 sites, is <span class="inline-formula">^<i>T</i>4</span>Si<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi/><mrow><mn mathvariant="normal">4</mn><mo>+</mo></mrow></msup><mo>+</mo></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="20pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="1a1a45fb1386d06f6300b4069c658080"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-687-2024-ie00001.svg" width="20pt" height="13pt" src="ejm-36-687-2024-ie00001.png"/></svg:svg></span></span> <span class="inline-formula">^<i>M</i>3</span>(Al,Fe)<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M11" display="inline" overflow="scroll" dspmath="mathml"><mrow><msup><mi/><mrow><mn mathvariant="normal">3</mn><mo>+</mo></mrow></msup><msup><mo>↔</mo><mrow><mi>T</mi><mn mathvariant="normal">4</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="34pt" height="13pt" class="svg-formula" dspmath="mathimg" md5hash="5c69b206a94fef5312f4e7c551aaacfb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-687-2024-ie00002.svg" width="34pt" height="13pt" src="ejm-36-687-2024-ie00002.png"/></svg:svg></span></span>(As,V,P)<span class="inline-formula">⁵⁺</span> <span class="inline-formula">+^<i>M</i>3</span>Mg<span class="inline-formula">²⁺</span>. Crystal-chemistry calculations indicate a positive correlation between the unit-cell parameters and the (Ca <span class="inline-formula">+</span> Mg <span class="inline-formula">+</span> Fe) contents of ardennites, as well as a negative correlation between the bond length distortion coefficients at the <span class="inline-formula"><i>T</i></span>4 and <span class="inline-formula"><i>M</i></span>3 sites. The existence of possible Si-rich and P-rich end-members, as well as the nomenclature of the ardennite group in which <i>dewalquite</i> could be revalidated, is discussed.</p>