VARIATIONS OF MINERALS OBSERVED IN OCHRE AT ABANDONED MINE DRAINAGE SITES IN MAHANOY CREEK WATERSHED, PA
This study documents the supergene environmental minerals resulting from the abandoned mine drainage (AMD) conditions in MCW. Ochre samples were collected along AMD sites in 2010, 2011 and 2012. Predictions of possible precipitates in the MCW were made from the physical parameters and major ions present in the water samples using PHREEQC. These predictions were then compared against mineralogical data obtained from X-ray powder diffraction (XRD) analysis of the ochre.
PHREEQC analysis shows presence of Hematite (Fe2O3), Goethite (FeOOH), Ferrihydrite (Fe(OH)3), Jarosite-K (KFe3(SO4)2(OH)6), Rhodochrosite (MnCO3) and Siderite (FeCO3). However, XRD analysis shows the presence of Goethite and Ferrihydrite and also revealed the existence of soluble metal salts- mostly sulfates such as Epsomite (MgSO4.7H2O), Hexahydrite (MgSO4.6H2O) and Starkeyite (MgSO4.5H2O) in ochre samples. While Ferrihydrite was found at sites with near neutral pH, Goethite was observed nearly throughout the entire pH range (3.7 to 6.9) pointing to its significant stability. Both these observations agree with existing literature on the mineralogy of precipitates associated with acid mine drainage. Several secondary effects of these minerals can also be postulated; for example, high concentrations of dissolved Mg were observed in mine drainages, possibly due to dissolution of SO4 minerals such as Starkeyite. Additionally, iron-oxyhydroxide minerals may adsorb or desorb significant quantities of trace elements depending on the environmental parameters.