Mineral Process Chemistry
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2 Mineral deposits and how they were formed: study guide
Chapter 2 Geochemistry of mineral deposits
This chapter explains how the Earth’s mineral deposits were formed, recalling first what you probably studied at GCSE. Remember the rock cycle and the three principal categories of rocks: igneous, sedimentary and metamorphic. Page 9 reviews plate tectonics. This is only relevant to Mineral process chemistry because of mentions in the preceding sections.
The information box at the bottom of page 6 is very important. It introduces and defines a number of terms used in the remainder of the Study. Make sure you know these and can explain each of them.
You are not expected to know the average crustal abundances and cut-off grades given in figure 2.2, but some of them might come as surprises. Look at the high position of potassium, for example, and the very low position of gold.
Sections 2.2 and 2.3 describe the principal ways in which mineral deposits were formed. You should expect to know the differences between these ways and be able to describe each in outline.
In Experiments 2.1 and 2.2 in section 2.4, give most attention to ores containing copper, zinc and tungsten. These are the ores you will be working with in later chapters. You could research these ores on the internet but, of course, there is nothing like first-hand contact to bring the subject to life. Bear in mind, though, that mineralogy is a big subject and you cannot be expected to know a wide range of examples.
As a very broad generalisation, the ores of unreactive metals are frequently sulphides, oxides or carbonates, and are not soluble in water. The sources of more reactive metals are likely to be chlorides which are water-soluble, or carbonates which are generally much less so. You can gauge the extent of the truth of this generalisation by reference to the tables on page 14.
Another generalisation coming out of the experiments is that, if you heat a sulphide ore in air, it is converted into the oxide of the metal and sulphur dioxide is given off. This process is called ‘roasting’, and later experiments depend on it.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Experiment 2.1 Identifying minerals
The idea here is to relate your observations to the list of properties in the table on p14. This could be a very time-consuming business but many students find this fascinating. Despite this, it is important to set limits of time, perhaps by restricting yourself to a small number of examples.
1 Visual examination
As the paragraph just above the table on page 14 makes clear, visual inspection can be misleading – iron compounds, reddish brown in colour, tend to confuse the issue. Look out for clearer more obvious clues such as:
• Green: malachite, copper carbonate
• Yellow: pyrite (fool’s gold), iron sulphide
• Black, cubic: galena, lead sulphide
2 & 3 Physical properties
Density, hardness, electrical and magnetic properties – compare these with the table but do not try to remember numerical data.
4 Chemical tests
This is where there are some links with the main course, but it remains an area of uncertainty whether the tests have to be learned. Clearly where a test is specifically mentioned in the main course, it must be considered ‘fair game’ for examiners. The following is a minimum:
• Chloride ions: white precipitate with silver nitrate solution
• Sulphate ions: white precipitate with barium chloride solution
• Groups 1 and 2: flame test colours
• Carbonates: effervescence with acids
• Sulphides: hydrogen sulphide on heating with acids
• Transition elements: characteristic colours of ions in solution e.g. copper, blue, iron(III), yellow.
• Sulphur dioxide: acidified potassium dichromate(VI) , orange -> green
• Hydrogen sulphide: lead ethanoate paper -> black.
Experiment 2.2 Examining ores
The most important part of Experiment 2.2 is the picking out of the ‘black bits’ from sample B.
The black material is wolframite, an ore of tungsten, and you are going to need this in Experiments 4.1 and 5.1.
In fact, it’s quite a good idea to consider Experiment 4.1 straight away. By coarsely crushing the ore, you get some more wolframite from it.
Further reading
‘Geoscience – Understanding geological processes’ by Dee Edwards and Chris King. Published by Hodder & Stoughton 1999. This book contains a chapter on mineral deposit formation and exploitation written by Alastair Fleming, the author of this Special Study.
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updated: 01 March 2006
