*Article* **Gravimetric Separation of Heavy Minerals in Sediments and Rocks**

#### **Sergio Andò**

Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, University of Milano-Bicocca, Piazza della Scienza 4, 20126 Milan, Italy; sergio.ando@unimib.it

Received: 15 February 2020; Accepted: 17 March 2020; Published: 18 March 2020

**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.

**Keywords:** handbook for laboratory procedures; nontoxic heavy liquids; wet sieving of silt; size-window for analysis; zircon separation; heavy-mineral mounts; provenance analysis

*"Day by day, what you choose, what you think, and what you do is who you become"*

Heraclitus

#### **1. Introduction**

A detailed and efficient protocol for heavy-mineral separation from sediments and rocks represents a handy practical support tool designed to be consulted routinely in the laboratory. Continuing a tradition started in Italy more than a century ago by De Filippi (1839) [1], who performed a pioneering quantitative provenance study of heavy minerals in sediments of the Ticino River and Artini (1891) [2], who investigated mineralogy of the Po plain sediments, this little handbook will enable Master and PhD students to use it as a substantial help to carry out autonomously a quick and effective heavy-mineral separation, but also, more expert researchers will discover simple practical solutions to speed the separation procedure.

As in Garzanti and Andò (2019) [3], the definition of heavy minerals used in this article includes only minerals of certain extrabasinal terrigenous origin (i.e., ultimately eroded from bedrock exposed in source areas), denser than 2.90 g/cm3, and occurring either as single detrital grains or in rock fragments. Grains of suspect intrabasinal (e.g., carbonates, bioclasts, and glaucony); pedogenic or diagenetic

(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., [7–10]), 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 and sands is concerned. Only a limited attention would be dedicated instead to lithified sandstones, and chemical methods for disaggregation will not be illustrated here. We strongly suggest to minimize the use of chemicals in the lab to a very minimum and especially to avoid the use of carcinogenic organic compounds such as bromoform, which has represented for a century the standard dense liquid used in mineral separation and which is still unfortunately in use in several laboratories worldwide. Other standard procedures include chemical attack with oxalic, acetic, or even chloridric acids, but such attacks are in most cases not only scarcely helpful but also counterproductive. Besides incremental time and cost, every additional passage involves material loss during cleaning, precipitation of insoluble salts, and even loss of key provenance indicators such as olivine or apatite. Another critical aspect that we emphasize here is first and foremost the necessary care taken in the field to collect pristine samples following criteria apt to guarantee both consistency and representativeness.

#### **2. Sampling**

No study can be better than the samples collected in the field. Great attention should thus be dedicated to the sampling plan, concerning locations, sampling spacing, and representativity of the targeted sedimentary system [3]. In the case of modern sediments, it is vital to avoid the detrimental practice of panning, which concentrates the densest species and consequently modifies irreparably the original proportions among different detrital minerals.

Once the sample is collected it is important to adopt a good systematic practice in sample labelling, in order to simplify laboratory procedures and avoid bad mistakes. For instance, in the case of modern sediment samples, we simply label them in progressive numerical order specifying the name of a nearby site and, in case of fluvial sediments, the name of the river.

During the successive steps of laboratory protocol, it is wise to add a series of simple coded symbols to readily identify the separated grain-size or density fraction (e.g., 15–500 μm, L = light, <2.90 g/cm3 and H = heavy, >2.90 g/cm3). Color-coded dots can be used to designate sediment fractions separated for specific petrographic, mineralogical, or geochemical analyses.

#### **3. Safety Rules and Pre-treatments**

This section illustrates the practical duties that need to be carried out before starting to process a series of sediment samples. Specific care should be dedicated to health and safety issues following good practices in the lab and especially avoiding wherever possible the use of toxic chemicals. The laboratory for mineral separation should be clean, safe, and well-organized. The use of blotting paper before starting any procedure is advisable. Reading carefully safety data sheets for each chemical product used (e.g., acids, dense liquids, etc.) is essential.

*NB: Never ever use bromoform. This organic liquid is toxic and carcinogenic!*

#### *3.1. Preparation of Na-Polytungstate*

Sodium-polytungstate (SPT; salt formula Na6 (H2W12O40)·H2O) is a more suitable heavy liquid, perfectly soluble in water and widely used for heavy-mineral separations. The use of SPT [11] combined with the centrifuge [12,13] has replaced the traditional but very dangerous use of bromoform, also using a much smaller amount of dense liquid and thus saving both costs and time during the procedure. Solutions with density up to 3.15 g/cm<sup>3</sup> can be prepared with SPT, which may be helpful to concentrate mineral of interest for geochronological and thermochronological analysis (see Section 6.2 below). Lower densities can be simply achieved by adding deionized or distilled water and higher densities by evaporation in a fume hood. This versatility allows us to separate isodensimetric fractions to concentrate specific minerals (e.g., quartz or K-feldspar for cosmonuclide or optically stimulated luminescence analysis; [14]).

One liter of solution with density 2.90 g/cm<sup>3</sup> is obtained from 2420 g of SPT and 478 g of deionized or distilled water poured in a 5-L beaker. Add the SPT in progressive steps, a few grams at a time. Put the beaker on a magnetic stirrer with an anchor for an hour until a homogeneous solution, transparent and pale yellow in color, is obtained. Stop the stirrer and check the density with a lead densimeter with density range 2.50–3.00 g/cm3, in a cylinder with a 4-cm diameter, adding 250 mL of SPT solution.

#### *3.2. Preparation of Na–Dithionite–Citrate–Bicarbonate*

The presence of iron oxides and hydroxides (e.g., hematite, lepidocrocite, and goethite) as coatings on single minerals or rock fragments may modify their density and hamper their proper identification under the microscope. This problem needs to be faced while studying surficial textures of weathered minerals in deeply weathered tropical and equatorial soils and paleosols. As a most effective way for iron-oxide removal from clays, the use of *Na–Dithionite–Citrate–Bicarbonate* (DCB) was proposed by Mehra and Jackson (1958) [15]. This mildly acid solution does not corrode apatite, monazite, or olivine and can thus be used for treatment of sedimentary samples.

The following protocol, derived from procedures used at the Natural History Museum of Milano, is recommended. Take a 5-L beaker with 2 L of deionized water and put the beaker on a magnetic stirrer at maximum speed. Add 120 g of Na-citrate, 40 g of Na-ditionite, and 16 g of Na-bicarbonate. After ca 15 min, when salts are all in solution, the DCB is ready. The DCB is poured in a 500-mL labeled beaker containing the sediment until the sediment is submerged; the rest can be stored for future use. Place the beaker under a fume hood and let the reaction to go on for 12 h at least at room temperature; after which, the sampled is cleaned with abundant (1–2 L) tap water to eliminate acid residues and finally wash with deionized water. This procedure can be used also on rock samples and large crystals and is most effective if followed by energic cleaning with universal degreaser and a brush.

#### *3.3. Preparation of Nylon Sieves with 5* μ*m and 15* μ*m Mesh*

Steel sieves, which can be cleaned in ultrasonic bath, are commercially available in the market down to 32 (or 20) μm. Since in sediments deposited by tractive currents the finest tail of the size distribution is markedly enriched in ultra-dense minerals (e.g., monazite, magnetite, and zircon; [16]), in very-coarse silt to very-fine sand samples, it is crucial to consider the finest size classes of the sample as well in order not to obtain a biased heavy-mineral suite [17,18]. This holds true also for poorly sorted sediments and especially for cohesive muds and mudrocks, for which including even classes as fine as 5–15 μm is compulsory. For this purpose, specific tools must be prepared.

Currently, in our lab, we use handmade tissue sieves with 15 μm, 10 μm, and 5 μm mesh. Nylon mesh rolls that are commercially available are cut in 10-cm-wide strips and then cut in turn in 10 × 10-cm-square pieces that are stored in a clean plastic bag on which the mesh size is clearly indicated. A one-square-piece is mounted on a PVC or plexiglass ring with a diameter of 8 cm obtained

by cutting a PVC or plexiglass gutter pipe in 10-cm-high pieces. Both basal and top surfaces of the ring must be made flat and smooth by using sandpaper and then carefully washed.

The tissue sieve is glued well-centered on the surface of the ring using a nontoxic and nonrapid glue spread thinly and evenly all around the rim of the ring. Next, to seal the tissue sieve, a few drops of glue are spread all around the rim with a toothpick, and the handmade tool is dried in a clean oven at ca. 40 ◦C for several hours. Finally, the tissue in excess is cut with scissors (Figure 1).

**Figure 1.** Tissue sieve made with nylon mesh. From bottom to top: toothpick and 100-cm2-piece of nylon mesh; PVC ring and tissue sieve ready for use.

*NB: Carefully wash the PVC ring with tap water to eliminate residues of abrasive grains used in sandpaper (e.g., moissanite but sometimes also garnet) that may contaminate the heavy-mineral fraction.*

#### *3.4. Sample Drying*

The sample may reach the lab in diverse conditions and packages. In case of loose sediment, in order to reduce loss or contamination risks the sample is best laid on a large sheet of clean paper. If the sample is wet, then it may be placed in an aluminum tray and left overnight in the oven at a temperature not exceeding 40–60 ◦C.

#### **4. Let Us Start!**

#### *4.1. Rock and Consolidated Samples*

The initial laboratory procedures depend on sample type. In the case of hard rocks, the first step is to mechanically disaggregate the sample minimizing pulverization. Rocks are split with a manual or hydraulic press in small centimetric chips (Figure 2). Between 20 and 40 g are generally sufficient to obtain the desired amount of heavy minerals, and the rest of the chips can be archived. The part of the sample collected in a tray is weighed and placed in an agate mortar with 10–20 mL of deionized water to prevent loss of material and the production of powder, which is harmful to breathe. A pressure is exerted repetitively with the agate pestle, avoiding rotational movements that may lead to grain grinding. Gentle percussions have been experimentally demonstrated to cause negligible breakage of heavy minerals (Henningsen, 1967 [19] and Mange and Maurer, 1992 [20] (p. 11)).

In the case of indurated silt or sand, it may suffice to place the weighed sample in a glass beaker, to add 100 mL of deionized water and to stir for several minutes with a metal spatula until complete disaggregation. The obtained sediment suspension can be poured onto the 500-μm mesh sieve as illustrated in Section 5.1 below.

**Figure 2.** Manual press designed to split sedimentary rocks using protective eye goggles.

#### *4.2. Micro-Sampling of Loose Sediment*

The representativity of the sample is granted not only by careful collection in the field but in the laboratory as well, by following suitable procedures that largely depend on sample grain size. In the simplest case of clean sand, the sample can be split repeatedly in two parts by means of a riffle box (Figure 3). The operation is repeated until the desired amount is obtained, weighed, and placed in a suitable labeled plastic vial. The discarded quantity is archived in the original container. More complex is the case of muddy samples, which are dried, placed on a clean sheet of paper, and well-homogenized horizontally by rolling the paper until a cone of sediment is obtained. Following Parfenoff, 1970 [21] (pp. 45–47), the cone is divided into four parts; a quarter of it is taken, and the rest is stored. The procedure is repeated until an appropriate quantity is obtained and transferred with a suitable tool (Even a bus ticket would do, but do not use electrostatic plastic!) on another clean sheet of paper and weighed. The same Parfenoff method is used in the case of gravelly sand.

**Figure 3.** Riffle box designed to split sediments (length 10 cm, width 8 cm, and height 8 cm).

#### *4.3. How Much Do We Need?*

The amount of sample needed for heavy-mineral separation depends on several factors, including expected heavy-mineral concentration, grain size, sorting, and type of study. Heavy-mineral concentration ranges widely from typically << 1% in case of ancient sandstone or quartzose modern sand heavily weathered in equatorial environments or recycled from ancient quartzarenites (e.g., [22]) to 5%–10% in modern orogenic sediments, reaching up to >50% in placer sands (e.g., [23]).

*NB: For modern sands, heavy-mineral concentration can be expeditiously assessed by weighing a well-filled plastic vial of known volume.*

As a general standard, we use up to 50–60 g of very coarse sand and 30–40 g of coarse sand, whereas 5–15 g are sufficient for medium sand, 2–3 g for fine to very fine sand, and even only 1 g for silt. For clayey silt and silty clay, it is advisable to take about 5 g.

A semi-quantitative operational approach can be assisted by a simple formula that takes into account not only grain size but also the age of the sample (a rough proxy of the intensity of selective diagenetic dissolution), the average density of expected source rocks, and the degree of heavy-mineral enrichment (as judged empirically by sample color and weight). The required amount of sample *X* is thus given by: **X** = **t** · ρ**R** · **S** · **H**, where **t** ranges between 1 for Holocene samples to 10 for Mesozoic or older samples; ρ**R** ranges between 1 for medium/high-grade source rocks to 10 for sedimentary source rocks; **S** is 0.5 for silt; 1 is for fine sand, 2 for medium sand, 3 for coarse sand, 10 for very coarse sand, and 30 for pebbly sand; and **H** ranges between 0.5 for blackish-reddish placer lags to 5 for white antiplacer sand.
