Mineralogical model of the mantle of Mercury
Authors: Cioria Camilla , Mitri Giuseppe 
Published: September 2022

&lt;p&gt;Mercury possesses the second, highest density (5.23 g/cm&lt;sup&gt;3&lt;/sup&gt;) in the Solar System after Earth. This high density is likely the result of the presence of a large inner core, composed of iron-light elemental alloys, overlayed by a relatively thin silicate shell, comprising the crust and the mantle [1].&amp;#160; The mercurian crust has been analyzed by the Messenger spectroscopic suite of instruments, which included, among others, the XRS (X-ray Spectrometer) and GRS (Gamma-Ray Spectrometer) spectrometers, capable of detecting the elements present on Mercury&amp;#8217;s surface. The surface mineralogy of Mercury is dominated by enstatite and plagioclase, with small amounts of sulfides (oldhamite, CaS), the presence of which is a strong clue of the extremely reducing conditions which have led to Mercury&amp;#8217;s accretion and differentiation [2]. The mercurian crust has been found to be very thin with estimates ranging between 26&amp;#160; &amp;#177; 11 km and 35 &amp;#177; 18&amp;#160; km [3,4]. &amp;#160;Moreover, the mercurian mantle is also thin, thinner than other terrestrial planets' mantles, with an estimated thickness between 300 km &amp;#8211; 500 km [5]. In addition,&amp;#160; the mantle shows a great lateral heterogeneity in mineral compositions, as indicated by the local, abrupt chemical changes in crustal chemistry [6].&lt;/p&gt;
&lt;p&gt;Mercury&amp;#8217;s large metallic core, likely partially molten and making up to 42% of its volume, combined with surficial observations (which have revealed a very small FeO concentration), and the peculiar position occupied by Mercury in the solar nebula, lead us to hypothesize a very reduced geochemical environment as its birthplace [7]. In literature, chondrites belonging to CB and enstatite chondrites (EN) have been considered the best precursor materials for Mercury&amp;#8217;s composition [6, 8, 9, 10], sharing many analogies both in geochemistry and thermal evolution.&lt;/p&gt;
&lt;p&gt;In light of the above, we chose a CB-like bulk composition to model the mineral assemblage of the mercurian mantle.&lt;/p&gt;
&lt;p&gt;We reconstruct the evolution of the mercurian mantle starting from a CB chondrite-like bulk silicate composition, at thermodynamic equilibrium, as a function of temperatures and pressures estimated for Mercury&amp;#8217;s mantle employing the Perple_X algorithm (6.9.1 version) [11]. We describe a dry scenario because the water abundance estimated for the bulk composition of Mercury silicate shells is quite low (0.3wt%, [12]) and due to the high-temperature ranges included in the model.&lt;/p&gt;
&lt;p&gt;We predict that the peculiar geochemical environment where Mercury may have originated is characterized by a very low oxygen fugacity, which would result in a very reduced mineral assemblage for the mantle, dominated by pyroxenes and silica polymorphs, as shown in [9]. We expect that significant mantle phase transitions are unlikely due to the relative thinness of the mantle and the consequent low-pressure ranges (always &lt;10 GPa) [13].&lt;/p&gt;
&lt;p&gt;In conclusion, contrary to the terrestrial mantle, olivine is not predicted to be stable in our model. In effect, the low &lt;em&gt;f&lt;/em&gt;O&lt;sub&gt;2&lt;/sub&gt; results in stabilizing pyroxenes relative to olivine [9], producing mineral assemblages quite different from terrestrial peridotites.&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;Acknowledgments&lt;/strong&gt;&lt;/p&gt;
&lt;p&gt;G.M. and C.C. acknowledge support from the Italian Space Agency (2017-40-H.1-2020).&lt;/p&gt;
&lt;p&gt;&lt;strong&gt;References &lt;/strong&gt;&lt;/p&gt;
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