Mass-specific light absorption coefficients of mineral particles in aqueous suspension for the ultraviolet to near-infrared radiation spectral region (200–2500 nm)

Light absorption by in-water suspended natural particles in the near-infrared radiation (NIR; 780–3000 nm) region has received little attention. Minerogenic matter is thought to be one source for NIR light absorption in aquatic environments. Here, mass-specific particulate light absorption coefficients of several particulate single minerals and mineral samples for the spectral range of 200–2500 nm are presented. The current methodology allows very sensitive measurements of particle suspension with a detection limit of about <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>2</mml:mn> </mml:mrow> <mml:mo>×<!-- × --></mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−<!-- − --></mml:mo> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> <mml:mspace width="thickmathspace" /> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> <mml:mspace width="thinmathspace" /> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−<!-- − --></mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> for the mass-specific absorption coefficient. Except for one, all mineral materials examined possessed significant light absorption throughout the full spectral range considered. The spectra revealed absorption features of specific elements (like iron) and from water structures ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">H</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msub> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">O</mml:mi> </mml:mrow> </mml:math> , O-H bonds) in the mineral or crystal structure that have been known from reflectance measurements of minerals. The specific absorption in the NIR was as high as <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0.02</mml:mn> </mml:mrow> <mml:mspace width="thickmathspace" /> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mn>2</mml:mn> </mml:msup> </mml:mrow> <mml:mspace width="thinmathspace" /> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:msup> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">g</mml:mi> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>−<!-- − --></mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> for laterite earths samples, but also below the detection limit for a steatite sample in a narrow spectral region (1600–1800 nm). The specific absorption by mineral particles in the NIR was, hence, highly variable from strong absorbing black minerals (magnetite) to low absorbing white clays. The information in the absorption coefficient spectrum can be used not only to identify specific mineral in natural particle assemblages but also to quantify their contribution to total particulate absorption in the NIR.