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The process of mineral identification

Mineral Classification

Hardness

Colour in flame

Crystal system

Cleavage

Colour of light-reflection minerals

Colour of powder

Lustre

Solubility

Gem Cutting

    Minerals can often been identified by following these general procedural steps:

1. Observe the minerals's morphological characteristics, including:
   • Crystal shape
     A mineral species may show many different forms
   • Crystal habit
     The manner in which minerals typically in homogeneous group
     i.e. Cassiterite and Sphalerite look similar, but the former associates which mica and quartz, the latter with other sulphides
2. Physical testing for properties:
   • Cleavage
   • Fracture
   • Colour
   • Lustre
   • Diaphaneity
3. Further physical testing :
   • Hardness
   • Tenacity
   • Specific Gravity
   • Magnetism
   • Streak
4. Chemical and laboratory testing :
   • Solubility
   • Taste (be careful here !)
   • Odour
   • Heating
   • Electrical testing
   • Radioactivity, using a Geiger counter
   • X-ray diffraction crystallographic analysis

Over 2,000 minerals are presently known, with a further dozen or discovered each year.

Using the Hugo Strunz system these are divided into 9 classes, these being sorted then into sub groups and series.

The 9 mineral classes are :

1. Elements (together with compounds, carbides, nitrides, phosphides)
   ~ 50 varieties
2. Sulphides (with selenides, tellurides, arsenides, antimonides and bismuthides
   ~ 300 varieties
3. Halides
   ~ 100 varieties
4. Oxides and hydroxides
   ~ 250 varieties
5. Nitrates, carbonates, borates
   ~ 200 varieties
6. Sulphates (with chromates, molydates, wolframates)
   ~ 200 varieties
7. Phosphates, arsenates, vanadates
   ~ 350 varieties
8. Silicates
   ~ 500 varieties
9. Organic substances
   ~ 20 varieties

Hardness defined as the degree of resistance a crystal has to be scratching and is reckoned using Mohs empirical scale :

Simple tests will tell us whether it is, for example, gypsum, fluorite or quartz. The hardness depends on the minerals cohesion, its hardness varying with structural direction. Though difficult to assess exactly, this hardness can be measured on various types of sclerometer.

Using Seebecks Sclermoeter the specimen is held on a moving platform with a balanced needle above, the degree of hardness being calculated by the weight on the counter arm used to produce a minimal scratch. Variations of hardness between the ten graded minerals are irregular, but each mineral will scratch all those above it. If the sample will scratch one of the standard minerals, but not the one below, its hardness is reckoned to me halfway between the two grades.

Test scratches should be done on a fresh, unweathered, flat surface of a sample and the edges and corners are sometimes harder than its face or cleavage planes. Be sure to wipe away the ensuing dust to is it the specimen being scratched at not the scale sample. Of course, if they neither - or both - scratch each other the mineral samples with have the sample hardness. On that note, don't mistake hardness for strength and minerals are still brittle. A diamond may easily engraved tempered steel but even a light blow of a hammer with turn the diamond into so much dust...

A field hardness kit can be as simple as selection of known items to compare. i.e. :

Watch for things that cause false results...

• Weathered surfaces may be softer than unscathed surfaces
• Impure sample of the mineral giving false readings
• Edges instead of planes tested.
  
  A minerals planar (flat) surfaces tend to be softer and give a truer indication of the correct figure. You may want test scratches at different parts of the sample to be sure.
• Hardness is different in different direction, tending to be much softer when scratched against the grain than when scratched with the grain.
• Crystal clusters tend to be softer than single crystals of a given mineral
• Very soft, porous and delicate specimens may be difficult to test properly due to their fragile nature.

The colour of a flame will change when elements are held in them and this is used to indicate what elements are present in a given mineral

The accepted way to perform this test is to dip a piece of platinum wire in hydrochloric acid to clean it, take up a pinch of the powdered mineral on the wire and put it in the flame.

To come...

Some minerals break cleanly when hit, leaving pieces with smooth surfaces. These clean breaks are called cleavages and each mineral tends to break or "cleave" more easily in some directions than in others. You can often identify minerals by a combination of their crystal shape and cleavage pattern.

Some, like Mica, cleave in only one direction, the layers pealing off.

Others, like Feldspar, cleave in two directions. A same like this could have a smooth top and sides but would have jagged ends.

Galena and halite are just two minerals that cleave in three direction, allowing you, for example, to carve a stepped shape. If the cleavage is not at right angles it with result in six sided rhomboids. Calcite can me cut so.

Finally are minerals that can cleave in four directions giving eight sided octahedrons.
Diamonds are the best know gem that split this way.

Cleavage can also be Perfect, Imperfect or Nonexistent, depend how well, or how bad it splits, or doesn't.

To come...

Most minerals can some is a variety of colours and shades. Quartz runs the gamete of clear through red and into darker colours. However, once ground down minerals of the same kinds have a constant colour which rarely varies.

The commonest way to perform this Streak test is to draw the mineral firmly over a white unglazed tile and check the resulting line of powder. By way of example, Malachite leaves a pale green streak.

These colours can be:

White or pale
Grey
Black
Yellow
Green
Rusty brown
Blue
Red
Orange, pink
Violet, purple

To come...

Almost all minerals will dissolve in one acid or another, depending on the elements present in them. This too is used when identifying new or unknown samples. A weak solution of Hydrochloric acid is most commonly used, though others like Hydrofluoric acid may be used in the laboratory. Some, like Calcite, will even dissolve in water, yet others will even begin to dissolve in air, absorbing water from the atmosphere.

Test solutions are :

Water
Hydrochloric acid
Nitric acid
Sulphuric acid
Hydrofluoric acid
Insoluble in acids

To come...