Explanation of Properties of Minerals

I. Introduction

 

A. Review the definition of minerals and of their most useful diagnostic properties. Also remind yourself of how they are classified and which are the most common and economically important.

 

B. The most useful properties for identifying minerals are:

1. Cleavage

2. Crystal habit

3. Hardness

4. Luster

5. Streak

 

C. Other properties you will use to distinguish some important minerals are:

1. Color

2. Reaction to dilute acid

3. Magnetism

4. Presence of striations

5. Density

6. Odor

7. Feel

 

II. Cleavage and fracture. - These are the most difficult physical properties to recognize and describe so be patient until you learn to see the pattern.

A. Fracture

1. Minerals which have fracture are broken in an irregular way, and lack flat, smooth, light-reflecting surfaces. Two examples of specimens that fracture, rather than having cleavage, are shown below.

 

 

a. Both the black obsidian and white quartz samples below will demonstrate conchoidal fracture meaning the fracture surfaces are smooth and curved like broken glass.

2. Some specimens will have flat surfaces which are not cleavage; that is breakage, surfaces. Instead, these flat surfaces are the original external crystal faces that grew when the mineral first formed in the Earth. The photograph below shows a quartz crystal that has numerous external crystal faces. However, where the mineral has broken in the lower right-hand corner, the surface is irregular. That is, the mineral fractures rather than cleaves.

 

                                   

 

If a flat face does not have other breaks running parallel to it within the specimen, or if on broken surfaces the mineral obviously fractures, the mineral probably doesn’t cleave.

B. Cleavage

1. One way to recognize a cleavage face is to hold the mineral in good light. Light will reflect off cleavage faces, but will look dull if the face is only fractured. Some specimens may seem to have flat cleavage surfaces, but always look for

 

                                           

other flat surfaces running the same way within or on the other side of the specimen.  Cleavage planes usually occur in groups. In the picture above many  planes parallel to the plane of the page appear as flat, shiny surfaces. You can see these on several sides and the top of this specimen. These are cleavage planes.

 

                                           

 

The specimen above shows many stepped cleavage planes in the lower left-hand corner of the photograph.

2. When you describe cleavages you will need to include both the number of cleavage directions and the angles between those cleavage directions. Examples of different types of cleavage follow.

a. One direction of cleavage – gypsum and biotite

 

 

 

1) When a mineral has only one direction of cleavage it often occurs as flat plates or sheets.

b. Two directions of cleavage at a  90o angle to one another – feldspar

 

                                               

 

1) The two directions are parallel to the base of the specimen and vertical, such as the plane running along the left side.

2) These two directions are represented by four different planes (two on the top and bottom of the sample and another two on the left and right sides). However, because the two members of each pair of  surfaces are parallel to each other it is described as a single direction of cleavage. Obviously, if many more than two parallel cleavage surfaces exist it is still considered a single cleavage direction.

c. Two directions of cleavage NOT at  90o angles – hornblende

 

                               

 

1) In the specimen above the two cleavage directions are at 60o and 120o to one another. You can see the 120o angle between cleavage planes along the top of the photograph.

d. Three directions of cleavage at a  90o angle to one another – halite and galena

 

 

 

 

 

 


 

In official mineralogical terminology the cleavage of halite and galena is called “cubic” because the three directions parallel the sides of a cube.

e. Three directions of cleavage NOT at  90o angles – calcite

 

                  

 

f. Four directions of cleavage – fluorite

 

                       

 

The photo above shows a cleaved fragment with four cleavage planes oriented down into the picture away from the point of a pyramid.

 

                                               

            

With fluorite it is often difficult to distinguish all four of the cleavage directions – especially in a sample such as that above that includes numerous fluorite crystals. However, looking down into the interior of various crystals you can see many planes cutting into the mineral. In your quest to identify minerals it will normally be enough in a case like this to recognize that a specimen has 2 or more directions of cleavage. The other properties will be enough to permit an identification.

 

g. Six directions of cleavagesphalerite

 

                               

 

As for fluorite, it will be sufficient to recognize that this specimen has many directions of cleavage. The other properties will be enough to permit identification.

III.  Crystal habit

A. To describe the external appearance and habit of minerals you should be looking for obvious shapes such as the following:

1. cubes

2. sheets (called micaceous or floliated)

3. elongate prisms with 3, 4 or 6 sides

4. prismatic crystals with pyramids on the end

5. blocks

6. blades (knife-like)

7. homogenous masses (no obvious flat surfaces)-called massive

 

IV. Hardness

A. The “scratchability” of a mineral is described on a relative scale called Moh’s hardness scale. This property is determined by trying to scratch the mineral with common objects such as your fingernail, a penny, or a nail. You will also try to scratch a glass plate with your minerals to decide if they are harder or softer than glass. When the first object is scratched or gouged by a second object the first object is softer than the second. Once the hardness of your mineral is established relative to the common objects, its hardness on the Moh’s Scale can be estimated:

1 – talc

2 – gypsum

A bit harder than 2 – fingernail

3 – calcite

A bit harder than 3 – penny

4 – fluorite

5 – apatite

A bit harder than 5 – nail

5.5 – glass

6 – orthoclase (K-feldspar)

7 – quartz

About 7 – porcelain streak plate

8 – topaz

9 – corundum

10 - diamond

1. Sometimes it may help to compare the hardness of minerals to each other.

2. Describe hardnesses with numbers from Moh’s hardness scale by bracketing the hardness as precisely as possible.

a. For example, if your fingernail will not scratch (leave a gouge in) the specimen, then the mineral is harder than your fingernail and, therefore, has a hardness greater than 2.5. In turn, if the mineral can be scratched by a penny, it is softer than the penny and has a hardness less than 3.5. You would then describe the hardness of the mineral as follows:

                        2.5 < Hardness < 3.5

B. Sometimes samples may be covered with a coating of an alteration mineral that is much softer than the original mineral underneath. Try to find a fresh surface to test. Also test several spots on a specimen to get a good average value.

C. A few minerals (kyanite, for example) may show different hardness depending on whether you scratch parallel to the length or perpendicular to it.

 

V. Luster

A. The appearance of a specimen in reflected light is a useful property that is often used in tables and flow charts to initially divide minerals into two groups.

B. The basic distinction to be made is that of “metallic” versus “non-metallic”.

1. A sample, such as galena, with metallic luster looks bright and shiny like polished metal.

 

                                   

 

2. Many samples with non-metallic luster are shiny, but it is the shine of glass rather than metal. The sample of calcite below has what’s called glassy luster.

 

3. Earthy luster is applied to samples that look powdery or soil-like. Kaolinite is an example (http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm)

 

 

                                  

 

 

VI. Streak

A. The color of a mineral in powdered form may be different from that of the mineral in reflected light. However, even if the color changes from specimen to specimen, the streak will not. Both the specimens below are hematite. The colors in reflected light are quite different – dark gray and rusty red. The streak is always rusty red.

 

http://core.ecu.edu/geology/harper/

mineral/mineral_new.cfm

 

 

 

 

VII. Other properties

A. A mineral exhibits effervescence if it reacts to weak acid by generating bubbles of gas. Calcite, CaCO3, gives off carbon dioxide gas, CO2, when it is exposed to a tiny drop of acid. These bubbles are visible at the top of the specimen shown below.

 

                             

 

B. Dolomite, CaMg(CO3)2, will effervesce if powdered on the streak plate before acid is applied.

C. Magnetite, Fe3O4, will attract a magnet.

D. Color is a distinctive property for a few minerals such as sulfur (always yellow), malachite (always green), kyanite (always pale blue), etc. Many minerals, however, can occur in a wide variety of colors. Also, there are hundreds of white, black, brown and green minerals.

E. Some minerals show straight lines running across or along cleavage or crystal faces. These are called striations and can be readily seen on the left half of this specimen.

 

                                   

 

http://core.ecu.edu/geology/harper/mineral/mineral_new.cfm

 

F. Density, odor, iridescence, and feel may be helpful in identifying some of the minerals in your kit.