Semi-Automated Heavy-Mineral Analysis by Raman Spectroscopy

Abstract

A significant amount of information on sedimentary provenance is encoded in the heavy minerals of a sediment or sedimentary rock. This information is commonly assessed by optically determining the heavy-mineral assemblage, potentially followed by geochemical and/or geochronological analysis of specific heavy minerals. The proposed method of semi-automated heavy-mineral analysis by Raman spectroscopy (Raman-HMA) aims to combine the objective mineral identification capabilities of Raman spectroscopy with high-resolution geochemical techniques applied to single grains. The Raman-HMA method is an efficient and precise tool that significantly improves the comparability of heavy-mineral data with respect to both overall assemblages and individual compositions within solid solution series. Furthermore, the efficiency of subsequent analysis is increased due to identification and spatial referencing of the heavy minerals in the sample slide. The method is tested on modern sediments of the Fulda river (central Germany) draining two Miocene volcanic sources (Vogelsberg, Rhön) resting on top of Lower Triassic siliciclastic sediments. The downstream evolution of the volcanic detritus is documented and the capability to analyze silt-sized grains has revealed an additional eolian source. This capability also poses the possibility of systematically assessing the heavy-mineral assemblages of shales, which are often disregarded in sedimentary provenance studies.

Provenance of Bengal Shelf Sediments: 1. Mineralogy and Geochemistry of Silt

“…In the mineralogical study of the >63-µm (sand), 32-63-µm (very coarse silt), 15-32-µm (coarse silt), and 10-15-µm (medium silt) classes of each sample, we routinely coupled optical microscopy and Raman spectroscopy, whereas both LM and HM fractions of the 5-10-µm class (fine silt) were studied with Raman spectroscopy only. On the HM fraction of each class, at least 200 transparent-heavy-mineral grains were counted on grain mounts under the microscope by the area method [63], and all grains of uncertain identification were checked systematically with Raman spectroscopy [64,65]. The definition of heavy minerals and the followed methodological protocol were according to Andò [66] and Garzanti and Andò [67].…”

Composition of Amphiboles in the Tremolite–Ferro–Actinolite Series by Raman Spectroscopy

“…In the tremolite-ferro-actinolite series, Ca 2 2 (0.5 < X < 0.9), and ferro-actinolite extends from Ca 2 Mg <2.5 Fe 2+ >2.5 Si 8 O 22 (OH) 2 to Ca 2 Fe 2+ 5 Si 8 O 22 (OH) 2 X < 0.5 [4]. A great variety of techniques can be applied for the identification of single crystal in sediments: optical microscope studies [5], microprobe analysis [6], Raman spectroscopy [7], Fourier transform infrared spectroscopy and X-ray diffraction [8]. In provenance studies and heavy-mineral analysis of sediments, Raman discrimination of amphiboles is not yet widely applied [9].…”

Which particles to select, and if yes, how many?

“…The measurement time and the computational resources to process the resulting amount of data [7,27] will increase substantially, due to the fact that smaller particles demand the use of higher magnification objectives (such as 50×). For the optical imaging of the complete filter, this leads to a very high number of images that have to be stitched together (our RM would need around 12100 images for a filter with 22 mm diameter, not including image overlap for stitching).…”