Industrial activities such as plating/metal industries and tanneries can lead to Cr(VI) pollution; however, far more wide-spread hazard results from indigenous Cr residing within geologic strata.  Chromium(III) within minerals common to ultramafic rocks, their metamorphic derivatives, and weathering products can be oxidized to Cr(VI) via reaction with Mn-oxides.  Limiting the reaction, however, is the physical transport providing a window of reactivity between two solubility limited phases.  Here we examine the coupled processes leading to the oxidation of mixed Cr(III),Fe(III)-hydroxides, common weathering products of ultramafic minerals, by the Mn-oxide birnessite.  In the case of the Cr(III)-hydroxide precipitates used in this study, solubility decreases with increasing iron substitution. Batch reactors were used to investigate the oxidative capacity of a series of synthesized mixed Cr-Fe hydroxides. Rates of Cr(III) oxidation upon reaction with birnessite were experimentally determined  as a function of Cr:Fe ratio and pH.  Batch reactors were run in triplicate at ≈ 25°C using a mixed Cr-Fe hydroxides (Fe_1-xCr_x(OH)₃) with x ranging from 1 to .25. Reactions were conducted at pH 5, 6.7, and 8. A 10mM acetate buffer was used for vials at pH 5, and 10 mM 3-(N-morpholino)propanesulfonic acid (MOPS) was used for reactions at pH 6.7 and 8. Fe_0.5Cr_0.5(OH)₃ and birnessite were also run individually at pH 5. Spectrophotometric methods were used to measure soluble Cr(VI) and Fe(II). Total dissolved Cr, Fe, and Mn were measured by inductively coupled plasma mass spectrometry (ICP-MS). Pure Cr-hydroxide yielded the highest Cr(VI) production as predicted with solubility. The information provided in this study provides key information on the environmental conditions that control the formation of Cr(VI) from native Cr(III) phases.