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<p>Laurentthomasite, ideally <span class="inline-formula">Mg₂</span>K(<span class="inline-formula">Be₂</span>Al)<span class="inline-formula">Si₁₂</span><span class="inline-formula">O₃₀</span>, is a new milarite-group member found within quartz-syenite pegmatites from the Ihorombe region, Fianarantsoa Province, Madagascar. It occurs as euhedral {0001} hexagonal crystals, maximum 15&thinsp;mm large and 5&thinsp;mm thick. The crystals show a very strong dichroism with cobalt blue and green-yellow colours when observed along [0001] and [1000], respectively. The mineral is transparent, uniaxial (<span class="inline-formula">+</span>) and its lustre is vitreous. The hardness is about 6 (Mohs scale), showing a poor {0001} cleavage, irregular to conchoidal fracture, and a measured density of 2.67(8)&thinsp;g&thinsp;cm<span class="inline-formula">⁻³</span>. Laurentthomasite is hexagonal, space group <span class="inline-formula"><i>P</i></span>6/<i>mcc</i> (no. 192), with <span class="inline-formula"><i>a</i>=9.95343(6)</span>&thinsp;Å, <span class="inline-formula"><i>c</i>=14.15583(8)</span>&thinsp;Å, <span class="inline-formula"><i>V</i>=1214.54(1)</span>&thinsp;Å<span class="inline-formula">³</span> and <span class="inline-formula"><i>Z</i>=2</span>. The strongest nine lines in the X-ray powder diffraction pattern [<span class="inline-formula"><i>d</i></span> in Å – (I) – <i>hkl</i>] are 3.171 – (10) – 211, 4.064 – (8) – 112, 2.732 – (8) – 204, 4.965 – (6) – 110, 2.732 – (4) – 204, 3.533 – (3) – 004, 7.055 – (2) – 002, 4.302 – (2) – 200 and 3.675 – (2) – 202. Chemical analyses by electron microprobe and several spectroscopies (inductively coupled plasma, ICP; optical emission, OES; mass, MS; and Mössbauer) give the following empirical formula based on 30 anions per formula unit: (<span class="inline-formula">Mg_0.86</span> <span class="inline-formula">Sc_0.54</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">Fe</mi><mn mathvariant="normal">0.35</mn><mrow><mn mathvariant="normal">2</mn><mo>+</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="145cda4999dea87b487f108d9132fef9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-355-2020-ie00001.svg" width="30pt" height="17pt" src="ejm-32-355-2020-ie00001.png"/></svg:svg></span></span>&thinsp;Mn<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mn mathvariant="normal">0.26</mn></msub><msub><mo>)</mo><mrow><mo>∑</mo><mo>=</mo><mn mathvariant="normal">2.01</mn></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="bfb15aaeafb8ead61c20010fbd16e8d0"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-355-2020-ie00002.svg" width="50pt" height="15pt" src="ejm-32-355-2020-ie00002.png"/></svg:svg></span></span>(<span class="inline-formula">K_0.89</span> <span class="inline-formula">Na_0.05</span> <span class="inline-formula">Y_0.02</span> <span class="inline-formula">Ca_0.01</span> Ba<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi/><mn mathvariant="normal">0.01</mn></msub><msub><mo>)</mo><mrow><mo>∑</mo><mo>=</mo><mn mathvariant="normal">0.98</mn></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="f241fb4212c2e68fd93548b4bdd3e263"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-355-2020-ie00003.svg" width="50pt" height="15pt" src="ejm-32-355-2020-ie00003.png"/></svg:svg></span></span>[(<span class="inline-formula">Be_2.35</span> <span class="inline-formula">Al_0.50</span> <span class="inline-formula">Mg_0.11</span> Fe<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M34" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">0.03</mn><mrow><mn mathvariant="normal">3</mn><mo>+</mo></mrow></msubsup><msub><mo>)</mo><mrow><mo>∑</mo><mo>=</mo><mn mathvariant="normal">2.99</mn></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="18pt" class="svg-formula" dspmath="mathimg" md5hash="d4f52d6e952c3eb02e345673b38d81b6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-355-2020-ie00004.svg" width="50pt" height="18pt" src="ejm-32-355-2020-ie00004.png"/></svg:svg></span></span>(<span class="inline-formula">Si_11.90</span> <span class="inline-formula">Al_0.10</span>)<span class="inline-formula">O₃₀</span>]; the simplified formula is (Mg, Sc)<span class="inline-formula">₂</span>(K, Na)[(Be, Al, Mg)<span class="inline-formula">₃</span>(Si, Al)<span class="inline-formula">₁₂</span><span class="inline-formula">O₃₀</span>]. The crystal structure was refined to an <span class="inline-formula"><i>R</i></span> index of 1.89&thinsp;% based on 430 reflections with <span class="inline-formula"><i>I</i></span>o&thinsp;<span class="inline-formula">&gt;</span>&thinsp;2<span class="inline-formula"><i>σ</i>(<i>I</i>)</span> collected on a four-circle diffractometer with Cu<span class="inline-formula"><i>K</i><i>α</i></span> radiation. By comparison with the general formula of the milarite group, <span class="inline-formula"><i>A</i>₂<i>B</i>₂<i>C</i></span>[<span class="inline-formula"><i>T</i></span>(2)<span class="inline-formula">₃<i>T</i></span>(1)<span class="inline-formula">₁₂</span><span class="inline-formula">O₃₀</span>](<span class="inline-formula">H₂O</span>) <span class="inline-formula"><i>x</i></span> (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M54" display="inline" overflow="scroll" dspmath="mathml"><mrow><mn mathvariant="normal">0</mn><mo>&lt;</mo><mi>x</mi><mo>&lt;</mo><mi>n</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="46pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="ef44310ef0a77d9a56fc910a51b62dd2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-355-2020-ie00005.svg" width="46pt" height="10pt" src="ejm-32-355-2020-ie00005.png"/></svg:svg></span></span>, with <span class="inline-formula"><i>n</i>&lt;2</span>&thinsp;pfu, per formula unit), the laurentthomasite structure consists of a beryllo-alumino-silicate framework in which the <span class="inline-formula"><i>T</i></span>(1) site is occupied by Si and minor Al and forms [<span class="inline-formula">Si₁₂</span><span class="inline-formula">O₃₀</span>] cages linked by the <span class="inline-formula"><i>T</i></span>(2) site mainly occupied by (Be&thinsp;<span class="inline-formula">+</span>&thinsp;Al). The <span class="inline-formula"><i>A</i></span> and <span class="inline-formula"><i>C</i></span> sites occur in the interstices of the framework while the <span class="inline-formula"><i>B</i></span> site is vacant. The origin of the strong dichroism is related to a charge transfer process between <span class="inline-formula">Fe²⁺</span> and <span class="inline-formula">Fe³⁺</span> in octahedral <span class="inline-formula"><i>A</i></span> sites and tetrahedral <span class="inline-formula"><i>T</i></span>(2) sites, respectively.</p>