"ca. 1857" Elisabeth de La Croix de Castries, la maréchale de Mac Mahon by Sergei Lvovich Levitsky (Musée d'Orsay - Paris France)
Her article in thepeerage is here. Her husband's Wikipedia article is here.
From Musée d'Orsay; spots removed with Photoshop and image enlarged by half and sepia tone removed by gogm.