Gravimetric Separation of Heavy Minerals in Sediments and Rocks

Abstract

The potential of heavy minerals studies in provenance analysis can be enhanced conspicuously by using a state-of-the-art protocol for sample preparation in the laboratory, which represents the first fundamental step of any geological research. The classical method of gravimetric separation is based on the properties of detrital minerals, principally their grain size and density, and its efficiency depends on the procedure followed and on the technical skills of the operator. Heavy-mineral studies in the past have been traditionally focused on the sand fraction, generally choosing a narrow grain-size window for analysis, an approach that is bound to introduce a serious bias by neglecting a large, and sometimes very large, part of the heavy-mineral spectrum present in the sample. In order to minimize bias, not only the largest possible size range in each sample should be considered, but also, the same quantitative analytical methods should be applied to the largest possible grain-size range occurring in the sediment system down to 5 µm or less, thus including suspended load in rivers, loess deposits, and shallow to deep-marine muds. Wherever the bulk sample cannot be used for practical reasons, we need to routinely analyze the medium silt to medium sand range (15-500 µm) for sand and the fine silt to sand range (5-63 or > 63 µm) for silt. This article is conceived as a practical handbook dedicated specifically to Master and PhD students at the beginning of their heavy-mineral apprenticeship, as to more expert operators from the industry and academy to help improving the quality of heavy-mineral separation for any possible field of application.Minerals 2020, 10, 273 2 of 15 (e.g., aggregates of iron or titanium oxides); and anthropogenic origin (e.g., barite in core samples and moissanite) are thus neglected. Phyllosilicates are neglected as well. Transparent heavy minerals identified under the microscope are considered separately from opaque and altered heavy minerals.The study of heavy minerals, which was quite popular in the past-for a historical overview, the reader is referred to Carver, 1971 [4] and to the monumental book edited by Mange and Wright, 2007 [5])-has recently seen a renovated and increasing interest also in source-to-sink studies aimed at the search for hydrocarbon reservoirs (e.g., Morton and McGill, 2018 [6]). Laboratory practice for sample treatment and heavy-mineral separation was thoroughly investigated and illustrated a half century ago (e.g., [7][8][9][10]), but, later on, the methodology has seen a sort of standardization, and not only for the best, and the stimulus for the betterment of laboratory procedures has seen little progress since then. An updated recent publication in which the state-of-the art techniques for heavy-mineral separation are thoroughly explained through the entire process, from sampling to mineralogical analysis, thus does not exist to the best of my knowledge. The purpose of this article is to fill this gap, especially as the study of recent unconsolidated silts a...

Neogene hyperaridity in Arabia drove the directions of mammalian dispersal between Africa and Eurasia

“…For heavy mineral separation, we selected four fluvial sandstone samples (Gachsaran Fm, IZ 165 SS, 13.1 Ma; lower Agha Jari Fm, IZ 158 SS, 9.47 Ma; Lahbari Mb, IC 26c SS, 7.0 Ma and IC 67 SS, 4.27 Ma) in addition to four silty samples (lower Agha Jari Fm, IC 35, 6.25 Ma; Lahbari Mb, IC 45, 5.50 Ma, IC 50c, 4.93 Ma and IC 71, 3.98 Ma). Heavy minerals were separated from the crushed and sieved samples using sodiumheteropolytungstate in water at a density of 2.85 g cm −3 , following the general separation scheme of Andò 82 . To avoid any biasing of ferromagnetic heavy minerals, no metal tools were used during the entire sample preparation process.…”

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

“…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]. For each sample and size class, heavy-mineral and transparent-heavy-mineral concentrations (HMC and tHMC indices) were calculated according to Garzanti and Andò [68].…”

Provenance of Neogene sandstones in western Taiwan traced with garnet geochemistry and zircon geochronology

“…A split aliquot of 34 selected sand and sandstone samples was wet‐sieved. From a 3.5 to 5 ϕ‐wide size window (from 15 or 32 µm to 350 or 500 µm) thus obtained, dense grains were separated by centrifuging in sodium polytungstate (2.90 g/cm 3 ), recovered by partial freezing with liquid nitrogen, micro‐quartered, and mounted on a glass slide using Canada Balsam as bounding resin (method described in detail in Andò, 2020). To efficiently locate each garnet for Raman analysis, all garnet grains in each slide were identified under a Leica DM750 microscope and labelled with a progressive number on a map of the slide obtained with a camera attached to the Raman spectroscope.…”