IDENTIFICATION OF UNKNOWN MINERALS

Given a sample of an unknown mineral:

What steps and tests would you undertake to completely describe and identify the unknown?

 

In most cases a hand sample of the unknown will be available and from this you can prepare samples for examination using a petrographic microscope by:

1.     grinding some sample to a powder for use in grain mounts, or

2.     cutting a thin section

 

1) Hand Sample

The first question to ask is with respect to the origin of the hand sample. Whether the hand sample is igneous, metamorphic or sedimentary? Identify the known minerals in the hand sample and determine the physical properties of the unknown mineral of interest. Record the following properties:

Colour

Luster

Streak

Hardness

etc.

From these provide a tentative identification of the unknown.

 

2) Thin Section

As a first step in the examination of the thin section, scan the whole thin section to identify and examine grains of the unknown that exhibit different optical orientations (remember that different orientations will exhibit different optical properties), looking for:

 

colour              crystal shape

relief               textures

twinning          alteration products

 

These provide the best basis for distinguishing different minerals.

Remember to cross and uncross the polars and rotate the stage as needed.

Different grains of the same mineral may exhibit different pleochroic colours depending on the orientation.

 

From the thin section examination record:

1) Colour and pleochroism

2) Relief relative to cement or oil

3) Habit, textures and alteration

4) Isotropic or anisotropic

5) Nature of twinning, if present

6) Nature of cleavage or fracture

7) Any other property

 

To serve as a guide to assist you in the systematic examination of a thin section to aid in the identification of an unknown the following flow chart is provided.

 

Part I

 

 

 

Part II

 

 

 

Identifying the Unknown

•        Once the above steps have been completed you will have a list of optical and physical properties  - What next?

•        By comparing the determined optical properties with the properties listed in Appendix B of Nesse, several possible matches may present themselves

•        This list of potential matches can be further refined by looking at the individual mineral descriptions in Nesse

•        Ultimately you want to narrow the possibilities down to one mineral which may involve going back to the sample and comparing the unknown to the mineral description

 

NOTES OF CAUTION – THINGS TO WATCH FOR!!!!

•        Thin sections may contain a variety of ‘foreign’ material that may be mistaken for minerals – you will find these!

•        Common materials are:

–       Bubbles

–       Grinding powder

–       Textile fibres, hair

 

 

 

 

REALITY

•        Identification of minerals is subject to ambiguity and uncertainty

–       Different minerals have similar properties

–       Some minerals have a wide range of properties

–       Properties may be incorrectly measured

–       Samples may be too small or in unusable orientations

•        These problems, while real, become manageable with experience – the more you do the easier identification becomes