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<p>Several geological processes such as crustal and mantle metasomatism, high-grade metamorphism, and the formation of ore deposits involve the exchange of halogens, especially chlorine, between silicate minerals and fluids. It is well established that the presence of octahedral iron is vital to the process of Cl incorporation into amphiboles. However, other compositional controls on Cl incorporation, such as the effect of <span class="inline-formula">^<i>T</i></span>Al, <span class="inline-formula">^<i>A</i></span>Na, <span class="inline-formula">^<i>A</i></span>K, and <span class="inline-formula"><i>A</i></span>-site occupancy are not as well constrained. This study reports on the compositional and structural relationships amongst a suite of 25 synthetic calcium amphiboles with a diverse range of Cl contents (0–1.68 atoms per formula unit (apfu)). Most amphiboles were synthesized along the potassic-hastingsite–potassic-chloro-hastingsite join (KCa<span class="inline-formula">₂</span>(Fe<span class="inline-formula">₄</span>Fe<span class="inline-formula">³⁺</span>)(Al<span class="inline-formula">₂</span>Si<span class="inline-formula">₆</span>)O<span class="inline-formula">₂₂</span>(OH,Cl)<span class="inline-formula">₂</span>). Additional work involved substitution of Na for K, variable Al content, and substitution of Mg for Fe<span class="inline-formula">²⁺</span> in the bulk composition to explore these compositional effects on Cl incorporation. The relationship between pressure and the Cl content of amphiboles was also explored over the pressure range 0.3–2.0 GPa. Otherwise, synthesis conditions were 700 °C and 0.3 GPa with <span class="inline-formula"><i>f</i></span>O<span class="inline-formula">₂</span> near Ni–NiO. Iron (II) chloride was used as the source of Cl. Some experiments used variably concentrated FeCl<span class="inline-formula">₂</span> brines (0–100 molal), while others were nominally anhydrous, containing only the water absorbed by the hygroscopic FeCl<span class="inline-formula">₂</span>. All amphiboles were characterized by Rietveld refinements of powder X-ray diffraction patterns, electron microprobe analysis, and Mössbauer spectroscopy, the latter to determine Fe<span class="inline-formula">³⁺</span> fraction. This study finds a positive relationship between pressure and the Cl content of the amphibole. Both Fe<span class="inline-formula">²⁺</span> and Fe<span class="inline-formula">³⁺</span> are positively correlated with Cl among amphiboles with variable Fe#, but the correlation is poor to nonexistent for Mg-free amphiboles. Results indicate that the substitution of <span class="inline-formula">^<i>C</i></span>Fe<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><msup><mi/><mrow><mn mathvariant="normal">2</mn><mo>+</mo><mo>,</mo><mn mathvariant="normal">3</mn><mo>+</mo></mrow></msup></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="9pt" class="svg-formula" dspmath="mathimg" md5hash="2c4c625910065992dd77b516a2560dc7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-247-2024-ie00001.svg" width="25pt" height="9pt" src="ejm-36-247-2024-ie00001.png"/></svg:svg></span></span> for <span class="inline-formula">^<i>C</i></span>Mg and <span class="inline-formula">^<i>C</i></span>Fe<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M24" display="inline" overflow="scroll" dspmath="mathml"><msup><mi/><mrow><mn mathvariant="normal">2</mn><mo>+</mo><mo>,</mo><mn mathvariant="normal">3</mn><mo>+</mo></mrow></msup></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="9pt" class="svg-formula" dspmath="mathimg" md5hash="7dc78a47fe956ddc84880d2619786922"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-36-247-2024-ie00002.svg" width="25pt" height="9pt" src="ejm-36-247-2024-ie00002.png"/></svg:svg></span></span> for <span class="inline-formula">^<i>C</i></span>Al facilitate Cl incorporation, with the former having a larger effect. The <span class="inline-formula"><i>A</i></span>-site occupancy is positively correlated with Cl in all amphiboles except those synthesized in dilute FeCl<span class="inline-formula">₂</span> brines (<span class="inline-formula">≤</span> 6 <span class="inline-formula"><i>m</i></span>), which are negatively correlated. No correlation is observed between the Cl content of the amphibole and either the species of A cation (K vs. Na) or <span class="inline-formula">^<i>T</i></span>Al. The composition vs. unit cell parameter data were analyzed using multivariate linear regression to better understand the process of Cl incorporation and to predict the molar volume of endmember chloro-amphiboles. The regression indicates an expansion of 0.181 Å (0.99 %) and 0.048 Å (0.90 %) along the <span class="inline-formula"><i>b</i></span> and <span class="inline-formula"><i>c</i></span> edges respectively, a reduction in <span class="inline-formula"><i>β</i></span> by 0.76° (0.72 %), and a contraction of 0.060 Å (0.59 %) along <span class="inline-formula"><i>a</i></span> when fully substituting Cl for OH in potassic-hastingsite. The multivariate linear regression equations allow the prediction of molar volumes for select endmember chloro-amphiboles, such as potassic-chloro-hastingsite <span class="inline-formula">=</span> 964.63 <span class="inline-formula">±</span> 1.29 Å<span class="inline-formula">³</span> or 290.5 <span class="inline-formula">±</span> 0.4 cm<span class="inline-formula">³</span> mol<span class="inline-formula">⁻¹</span>.</p>