<?xml version='1.0' encoding='utf-8'?>
<oai_dc:dc xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:oai_dc="http://www.openarchives.org/OAI/2.0/oai_dc/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/oai_dc/ http://www.openarchives.org/OAI/2.0/oai_dc.xsd">
  <dc:contributor>J.J. Hemley</dc:contributor>
  <dc:contributor>W. M. d’Angelo</dc:contributor>
  <dc:creator>D.A. Sverjensky</dc:creator>
  <dc:date>1991</dc:date>
  <dc:description>&lt;p&gt;The thermodynamic properties of minerals retrieved from consideration of solid-solid and dehydration equilibria with calorimetric reference values, and those of aqueous species derived from studies of electrolytes, are not consistent with experimentally measured high-temperature solubilities in the systems K&lt;sub&gt;2&lt;/sub&gt;O- and Na&lt;sub&gt;2&lt;/sub&gt;O-Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt;-SiO&lt;sub&gt;2&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&gt;O-HCl (e.g., K-fs — Ms — Qtz — K&lt;sup&gt;+&lt;/sup&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;— H&lt;sup&gt;+&lt;/sup&gt;). This introduces major inaccuracies into the computation of ionic activity ratios and the acidities of diagenetic, metamorphic, and magmatic hydrothermal fluids buffered by alkali silicate-bearing assemblages. We report a thermodynamic analysis of revised solubility equilibria in these systems that integrates the thermodynamic properties of minerals obtained from phase equilibria studies (&lt;span class="small-caps"&gt;Berman&lt;/span&gt;, 1988) with the properties of aqueous species calculated from a calibrated equation of state (&lt;span class="small-caps"&gt;Shock&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="small-caps"&gt;Helgeson&lt;/span&gt;, 1988). This was achieved in two separate steps.&lt;/p&gt;&lt;p&gt;First, new values of the free energies and enthalpies of formation at 25°C and 1 bar for the alkali silicates muscovite and albite were retrieved from the experimental solubility equilibria at 300°C and P&lt;sub&gt;sat&lt;/sub&gt;. Because the latter have stoichiometric reaction coefficients different from those for solid-solid and dehydration equilibria, our procedure preserves exactly the relative thermodynamic properties of the alkali-bearing silicates (&lt;span class="small-caps"&gt;Berman&lt;/span&gt;, 1988). Only simple arithmetic adjustments of −1,600 and −1,626 (&lt;span class="math"&gt;&lt;span id="MathJax-Element-1-Frame" class="MathJax_SVG" data-mathml="&lt;math xmlns=&amp;quot;http://www.w3.org/1998/Math/MathML&amp;quot;&gt;&lt;mtext&gt;&amp;amp;#xB1;500&lt;/mtext&gt;&lt;/math&gt;"&gt;&lt;span class="MJX_Assistive_MathML"&gt;±500&lt;/span&gt;&lt;/span&gt;&lt;/span&gt;) cal/mol to all the K- and Na-bearing silicates, respectively, in&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="small-caps"&gt;Berman&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;(1988) are required. In all cases, the revised values are within&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="math"&gt;&lt;span id="MathJax-Element-2-Frame" class="MathJax_SVG" data-mathml="&lt;math xmlns=&amp;quot;http://www.w3.org/1998/Math/MathML&amp;quot;&gt;&lt;mtext&gt;&amp;amp;#xB1;0.2%&lt;/mtext&gt;&lt;/math&gt;"&gt;&lt;span class="MJX_Assistive_MathML"&gt;±0.2%&lt;/span&gt;&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;of calorimetric values. Similar adjustments were derived for the properties of minerals from&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="small-caps"&gt;Helgeson&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;et al. (1978).&lt;/p&gt;&lt;p&gt;Second, new values of the dissociation constant of HCl were retrieved from the solubility equilibria at temperatures and pressures from 300–600°C and 0.5–2.0 kbars using a simple model for aqueous speciation. The results agree well with the conductance-derived dissociation constants from&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="small-caps"&gt;Franck&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;(1956a,b) for temperatures from 300–550°C. Compared to the conductance-derived results of&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="small-caps"&gt;Frantz&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="small-caps"&gt;Marshall&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;(1984), our dissociation constants agree well at the highest densities, but are greater at lower densities. At the lowest density, at 600°C and 1 kbar, the discrepancy of 0.9 log units is within the overall uncertainties associated with our experimental results and those associated with deriving dissociation constants from conductance measurements in highly associated solutions (&lt;span class="small-caps"&gt;Oelkers&lt;/span&gt;&lt;span&gt;&amp;nbsp;&lt;/span&gt;and&lt;span&gt;&amp;nbsp;&lt;/span&gt;&lt;span class="small-caps"&gt;Helgeson&lt;/span&gt;, 1988). Finally, we also report an equation of state fit to the standard thermodynamic properties of the aqueous HCl molecule that is consistent with a wide array of independently determined dissociation constants of HCl and permits interpolation and extrapolation of the dissociation constant of HCl to 1000°C and 5.0 kbars.&lt;/p&gt;</dc:description>
  <dc:format>application/pdf</dc:format>
  <dc:identifier>10.1016/0016-7037(91)90157-Z</dc:identifier>
  <dc:language>en</dc:language>
  <dc:publisher>Elsevier</dc:publisher>
  <dc:title>Thermodynamic assessment of hydrothermal alkali feldspar-mica-aluminosilicate equilibria</dc:title>
  <dc:type>article</dc:type>
</oai_dc:dc>