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Version vom 24. Juli 2018, 14:05 Uhr

Probenaufbereitung2.png Probenaufbereitung > Probenaufbereitung: Methoden Methoden > Alle Methoden Alle Methoden > Mineral separation

Mineral separation


Wozu werden Minerale aus Gestein abgeschieden?

Die Separierung von Mineralen aus Gestein wird typischerweise aus einem der folgenden Gründen vorgenommen:

Industrie Wissenschaft
- Apatit zur Düngemittelgewinnung - Chemische Analyse
- Feldspat für Zement und Keramik - Spektroskopie
- Metalle - Stable isotope analysis
- Gold - Radiometrische Datierung
- ... - ...

Vorbereitung der Feldarbeit

Geologischer Hintergrund

Um ein gutes Verständnis der Geologie des Untersuchungsgebietes zu erhalten, muss eine eingehende Auseinandersetzung mit der entsprechenden Literatur, vor allem in Form aktueller Paper, stattfinden. Nur so kann eine effektive und wissenschaftliche Feldarbeit gewährleistet werden.

Sammlung von Kartenmaterial, Konzeptkarte und Fernerkundung

Während der Forschungsphase werden alle verfügbaren Daten und alles Kartenmaterial des Untersuchungsgebiets gesammelt. Die Nutzung von Fernerkundungsdaten vor dem Beginn der Feldarbeit hilft bei der Organisation und von Feldkampagnen und ermöglicht einen Überblick über die geologischen Eigenschaften und Merkmale des Untersuchungsgebietes.

Kleidung

Packing appropriate clothing for the anticipated weather conditions of the mapping area is essential for safety.

Rucksack im Feld

Necessary tools for a field geologist to collect data in the field.

Reference materials

* Barnes, J.W. and Lisle, R.J., 2004. Basic Geological Mapping (4th edition). John Wiley & Sons Ltd. 184p. 
Coe, A.L., 2010. Geological Field Techniques. Wiley-Blackwell/The Open University. 323p. 
Compton, R.R., 1985. Geology in the Field. John Wiley and Sons, 416pp. 


* Fry, N., 1984. Field Description of Metamorphic Rocks. Geological Society of London Handbook Series. John Wiley and Sons, 110pp. 


* McClay, K.R., 1987. The Mapping of Geological Structures. Geological Society of London Handbook Series. Open University Press, 161pp. 


* Thorpe, R.S. and Brown, G.C., 1985. Field Description of Igneous Rocks. Geological Society of London Handbook Series. John Wiley and Sons, 154pp. 


* Tucker, M.E., 2003. Sedimentary Rocks in the Field (3rd ed). Geological Field Guide Series. John Wiley and Sons, 244pp. 


* Pocket-sized reference books designed to be taken into the field.

Probennahme

The sampled rock has to be in-situ and fresh. The rock should not have alteration rims (can be removed in the lab with saw), weathering nor veins. The sample should be transported and stored in a dry environment. The labelling is usually a maximum of seven characters with two letters and five numbers (e.g. ID 12345). More information on the sampling methods is discussed here.

Sample size

To separate minerals from a rock, you must have some prior knowledge about the relative abundance of that mineral in the rock, which would then serve as a guide as to how much of the rock sample you should process. Prior to beginning your work, you should also know (at least in a rough sense) the grain size of the minerals you wish to separate. A third piece of information is whether the mineral of interest contains any inclusions (as these are definitely not desired as they will interfere with most of your analyses). If the mineral is coarse grained, and is fairly abundant within the sample (modal content of > 10%), then a grapefruit sized sample is probably enough. On the other hand, if you are interested in separating an accessory mineral like sphene, zircon, or apatite, then your sample should be several kg in weight (minimum size that of a human head). Depending on the abundance of the available datable mineral the sample size will vary from 2-3 kg to 20-40 kg. Several points have to be taken into account such as:

- The yield of analysable material from a given sample is often unknown

- The bigger the sample the more options are given in selecting mineral grains for analysis

- Cost

Probenzerkleinerung

There are several techniques that can be used to crush rocks down to a desired size:

- hammer/chisel

- diamond saw

- hydraulic press

- Jaw crushers

- mills

Sieve

Sieving should be done with several sieves of different mesh sizes with the larger mesh at the top and the smallest mesh at the bottom of the tower. More information about the sieving steps can be found here.

Panning

Panning is a quick and efficient method to separate the heavy fraction from the rest of the sample.

Schütteltisch

The Wilfley table is a type of shape/density separator and is used extensively to separate zircons for U-Pb dating, or heavy minerals (e.g native gold) for commercial use.

Nach der Justierung des Schütteltischs im benötigten Winkel und der Einstellung der Bewegungsgeschwindigkeit (Hubgeschwindigkeit, rpm), wird die Probe in den Vibrationsförderer gegeben der den Schütteltisch durch den Einspeisungskasten mit einer konstanten Rate speist (siehe Abbildung). Die Probe wird damit in drei Fraktionen geteilt:

Schwere Fraktion

The heavy mineral fraction is concentrated in minerals such as zircon, garnet, apatite, sphene, monazite...

Before proceeding to the picking of the desired mineral a magnetic separation is required to facilitate the picking.

If the heavy mineral fraction is still encumbered by lighter minerals such as quartz it is preferable to add a heavy liquid separation or a panning step to make the picking easier.

Mittlere Fraktion

The middle fraction is usually concentrated in minerals such as quartz, feldspars...

Leichte Fraktion

The light fraction is usually concentrated with minerals such as micas and clays...

Isodynamischer Magnetabscheider

General Comments

The Frantz Isodynamic Separator (FIS) separates minerals according to their magnetic susceptibility, and how the individual grains respond to the electromagnetic field versus gravity. It is a high precision, but very slow piece of equipment that is useful for processing and handling small quantities of your sample. It can only work with a grain size between 28 and 200 mesh. Samples larger than 28 mesh will clog the separator.

Principle of operation

Magnetic properties of minerals

Minerals are classified into four groups based on their attraction to a magnetic field (~magnetic susceptibility), namely:

1. diagmagnetic: repels a magnet, zero dipole moment

2. paramagnetic: random arrangement of magnetic dipoles in a crystal structure; drawn weakly to a magnet (e.g. olivine).

3. ferromagnetic: similar to paramagnetic but has domains that align easily in the presence of an external magnetic field; therefore strongly attracted to a magnet (e.g. Fe).

4. ferrimagnetic: antiparallel ionic spin moments; permanent magnetic domains are there (e.g. magnetite).

The FIS has a chute between the poles of an electromagnet. This chute is partitioned along its length. It can be tilted in two direction: forward (herein called slope), and sideways (herein called tilt). Forward slope controls the speed of movement of the grains down the chute, whereas tilt controls how well the minerals are separated according to their susceptibility. The FIS is typically set up with a slope of 15° and a tilt of 10 – 15° away from the operator. The electric current is then varied to separate the different minerals as they move down the chute.

Schweretrennungsflüssigkeit

Picking

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