Nanoscale mineralogy of arsenic in a region of New Hampshire with elevated As-concentrations in the groundwater
Journal
American Mineralogist
Journal Volume
88
Journal Issue
11-12
Start Page
1844
End Page
1852
ISSN
0003-004X
Date Issued
2003-11
Author(s)
Abstract
Bedrock samples were examined from a region in south-central New Hampshire in order to understand the factors responsible for elevated As-concentrations (up to 180 ppb) in the groundwater. Although the As is predominantly from natural sources, the precise mineralogy of the As-bearing phases was unknown. As-bearing samples were examined in detail using advanced electron microscopy techniques, including high-angle annular dark field scanning electron microscopy (HAADF-STEM), STEM elemental mapping, and high resolution TEM. Numerous As-bearing minerals were observed, predominantly arsenopyrite (FeAsS) with some westerveldite (FeAs). The arsenopyrite was partially altered to nanocrystalline (∼20 nm) magnetite and westerveldite, most likely during hydrothermal circulation of fluids following the emplacement of a nearby granitoid pluton. A reaction rim surrounds the pristine cores of arsenopyrite, with compositionally variable amorphous zones (containing Fe, As, K, and O), a Cu-sulfide, and a uranium-bearing phase, suggesting at least three distinct fluid compositions during alteration. Because the K-Fe-As-oxide is amorphous and the nanocrystalline FeAs has a high surface area per unit mass, the dissolution rate of As-bearing phases during recent low-temperature weathering is increased. The rapid dissolution of the reduced As phase (FeAs), which is unstable under oxidizing conditions, may be an important factor in the elevated As-concentrations in the groundwaters of this region.
Subjects
New Hampshire
North America
United States
Copper compounds
Dissolution
Iron oxides
Magnetite
Minerals
Nanocrystals
Scanning electron microscopy
Sulfur compounds
Temperature
Dissolution rates
Elemental mapping
High surface area
High-angle annular dark field scanning electron microscopies
High-resolution TEM
Hydrothermal circulation
Microscopy technique
Oxidizing conditions
arsenic
arsenopyrite
dissolution
groundwater pollution
mineralogy
redox conditions
Groundwater
Publisher
Mineralogical Society of America
Type
journal article