1. Minerals: Physical Properties and Crystal Forms
From:

2. Minerals: the building blocks of rocks
Definition of a Mineral:
naturally occurring
inorganic
solid
characteristic crystalline structure
definite chemical composition
Definition of a Rock:
A solid aggregate (mixture) of minerals

4. Mineral characteristics
Definition of a Mineral:
naturally occurring
inorganic
solid
characteristic crystalline structure
definite chemical composition
steel plastic sugar table salt mercury ice coal
no, #1
no, #1
no, #1,2
YES!
no, #3
YES!
no, #2
basalt obsidian mica gold paper chalk coral
no, #5
no, #4
YES!
YES!
no, #1,2
no, #2
no, #2

5. Mineral characteristics
Naturally formed
No substance created artificially is a mineral.
examples: plastic, steel, sugar, paper
Inorganic
Anything formed by a living organism and containing organic materials is not a mineral.
examples: wood, plants, shells, coal
Solid
Liquids and gases are not minerals.
examples: water, petroleum, lava, oxygen

6. Inorganic Calcite CaCO3 calcium carbonate
Calcium carbonate, or CaCO3, comprises more than 4% of the earth’s crust and is found throughout the world. Its most common natural forms are chalk, limestone, and marble, produced by the sedimentation of the shells of small fossilized snails, shellfish, and coral over millions of years. Although all three forms are identical in chemical terms, they differ in many other respects, including purity, whiteness, thickness and homogeneity. Calcium carbonate is one of the most useful and versatile materials known to man.
CaCO3 calcium carbonate

7. Mineral characteristics
Characteristic crystalline structure
must have an ordered arrangement of atoms
displays repetitive geometric patterns in 3-D
glass not a mineral (no internal crystalline structure)
Definite chemical composition
must have consistent chemical formula
examples: gold (Au), quartz (SiO2), orthoclase (KAlSi3O8)
basalt (like many other rocks) contains variable ratios of different minerals; thus, has no consistent formula

8. e.g., Atomic Structure of Diamond
Minerals are …
Composed of ordered arrangements of atoms….
Repeated patterns
Uniform spacing
atoms arranged in an orderly, repeating,
3-D array.
e.g., Atomic Structure of Diamond

9. How many minerals are there?
Nearly 4,000 types of minerals
Only ~30 occur commonly
Why not more?
Some combinations are chemically impossible
Relative abundances of elements don’t allow more

10. Mineral Formation Crystallization from a magma
quartz, feldspar, mica in granite
Crystal growth in the solid state
mica, garnet, feldspar in schist
Precipitation from solution
calcite in marine organism shells
silica in agate

11. Quartz Crystals

12. Element abundances in the crust
All others: 1.5%

13. Ions
When an atom loses or gains an electron to or from another atom it is called an ion.
Positively charged ions (loss of electron) are called cations.
Negatively charged ions (gain of electron) are called anions.
Ions of opposite charge attract (net charge = 0):
> Ionic Bonding
90% of all minerals are ~ ionic compounds.

14. Ionic Bonding
Cation (+)
Anion (-)

15. Ionic Compounds
Fig.3.4
NaCl
Halite
Table Salt

16. Important Ions in Minerals
anions charge cations charge
Si +4
K +1
Ca +2
Na +1
Al +3
Mg +2
Fe +2 or +3
O -2
Cl -1
S -2

17. Atomic Structure of Diamond
Covalent Bonding
Electrons are shared between atoms.
Covalent bonds are much more stable and stronger than ionic bonds.
Atomic Structure of Diamond

18. Ionic Radius and Charge
Crystal Structure
Anions are generally larger than cations.
The structure of the mineral is determined largely by how the anions are arranged, and how the cations fit between them.
Ionic Radius and Charge

19. Predictable Interface Angles
Crystal Structure
Perfect Crystals:
Predictable Interface Angles

20. Polymorphs Identical chemical compounds Different atomic structure
Generally stable under different conditions.

21. “Polymorphs” (although different minerals)

22. Physical properties of minerals
We know that minerals are composed of atoms, arranged in a specific order, with a well defined chemical composition.
We might expect then that the microscopic variations in bond environment will also be manifested in macroscopic physical and chemical properties.
This is indeed the case.

23. The Physical Properties of Minerals
Color
Streak
Luster
Hardness
External Crystal Form
Cleavage

24. The Physical Properties of Minerals (cont.)
Fracture
Specific Gravity
Special Properties
Other Properties
Chemical Tests

25. Important Physical Properties
Color - Although an obvious feature, it is often unreliable to use to determine the type of mineral.
Color arises due to electronic transitions, often of trace constituents, in the visible range of the EM spectrum. For example, quartz is found in a variety of colors.
Color of a mineral may be quite diagnostic for the trace element and coordination number of its bonding environment.

26. Citrine Amethyst Crystal Quartz Smokey Quartz Quartz Gems
Color Some minerals have more than one color for example; purple amethyst and yellow citrine are both varieties of quartz.  In contrast, yellow is the only color of sulfur and is therefore a useful tool in identifying this mineral.
Citrine
Amethyst
Crystal Quartz
Smokey Quartz
Agate, Calcedony, Jasper
Quartz Gems

27. Important Physical Properties
Streak - The color of a mineral in its powdered form; obtained by rubbing the mineral against an unglazed porcelain plate.
Useful for distinguishing between minerals with metallic luster.

28. Streak - The color of a mineral in its powdered form;
Hematite may look black, but it will always produce a RED/BROWN streak on a streak plate. Care must be taken if the mineral being tested is harder than the porcelain, the result will be a powder produced by the porcelain plate being scratched and will always be white.

29. Important Physical Properties
Luster - This property describes the appearance of reflected light from the mineral's surface. Nonmetallic minerals are described using the following terms: vitreous, pearly, silky, resinous, and earthy.

30. Luster Metallic Non-metallic Vitreous Resinous Dull Pearly Silky
Greasy
Glassy

31. Hope Diamond: 44.5 carats

32. Hardness: We measure the hardness of a mineral by how easy we can scratch it using different tools like finger nails, piece of glass and piece of copper (usually a penny). 
MOH’s
hardness scale

33. External Crystal Form Crystal form
Minerals are grouped into systems according to their crystal symmetry (regularity of form).
The figure below shows the six main systems.

34. Cleavage Cleavage occurs when a mineral
has a preferred direction of breakage.
Cleavage like most other properties
is a function of the crystal structure
and the nature of the bonding. When
a mineral is broken, it breaks into
pieces that resemble one another,
then it is said to have perfect cleavage.
Perfect cleavage is due to a higher order
of symmetry and is more prevalent in
minerals with strong ionic (therefore
weak) bonding. Cleavage surfaces
usually look almost polished and are
very flat. The angles between cleavage
surfaces is related to the crystal structure
and hence diagnostic of the particular
mineral.

35. Planer Cleavage in Mica

36. Weak Bonding Yields Planer Cleavage

37. Fracture
Fracture is the tendency of a mineral to break along curved surfaces without a definite shape. These minerals do not have planes of weakness and break irregularly.

38. Cleavage in three directions at right angles (90o). Cubic cleavage.
Mineral
Type of Breakage
Halite
                                     
Calcite
CLEAVAGE
Cleavage in three directions at right angles (90o). Cubic cleavage.
CLEAVAGE
Cleavage in three directions not at right angles (120o and 60o). Rhombohedral cleavage.

39. Cleavage in two directions at right angles.
Mineral
Type of Breakage
Quartz
                                     
Feldspar
FRACTURE
Mineral does not exhibit cleavage, it breaks or fracture in an irregular manner.
CLEAVAGE
Cleavage in two directions at right angles.

40. Rhombohedral Cleavage in Calcite

41. Conchoidal Fracture in Glass

42. Density and Specific Gravity
Density - Defined as the mass divided by the volume and normally designated by the Greek letter, rho, 
mass/volume; SI units: kg/m3 or kg m-3, but geologists often use g/cm3 as the unit of choice.
Specific Gravity - Ratio of the mass of a substance to the mass of an equal volume of water. Note that water = 1 g cm-3. S.G. is unitless.
Examples - quartz (SiO2) has a S.G. of 2.65 while galena (PbS) has a S.G. of 7.5 and gold (Au) has a S.G. of 19.3.

43. Steps to determine the density of a mineral.
                                                   
1) Use a balance. In this example the balance to be used is a triple beam balance.                                                      
2) Place the specimen in the weighing pan.                                                      
3) Record the weight of the specimen, in this case grams.

44.                                                          
4) Record the level in a graduated cylinder before you put the specimen in. In this case 900ml.
  5) Record the level after the specimen was placed under water. In this case 920ml.
Density = 155.8grams / 20 cc
6) Divide 155.8/20 = 7.79 g/cc. (in this case 20ml = 20cc, because the amount of units displaced are equivalent). So, the density of the minerals is 7.79g/cc.
7) The closest mineral having this density is galena with a density of 7.60 g/cc. (In order to get a precise density sophisticated equipment with a tolerance of five to seven decimals is used).

45. Special and Other Properties
Striations - Commonly found on plagioclase feldspar. Straight, parallel lines on one or more of the cleavage planes caused by mineral twinning.
Magnetism - Property of a substance such that it will spontaneous orient itself within a magnetic field. Magnetite (Fe3O4) has this property and it can be used to distinguish it from other non-magnetite iron oxides, such as hematite (Fe2O3).
Double Refraction - Seen in calcite crystals. Light is split or refracted into two components giving rise to two distinct images.

46. Plagioclase striations

47. Calcite Double Refraction

48. Important Mineral Groups
Name Important constituents
(other than O)
Olivine Si, Fe, Mg
Pyroxene Si, Fe, Mg, Ca
Amphibole Si, Ca, Mg, Fe, Na, K
Micas Si, Al, K, Fe, Mg
Feldspars Si, Al, Ca, Na, K
Carbonates C, Ca, Mg
Evaporites K, Cl, Ca, S

49. Color and Density
Two broad categories are ferromagnesian and nonferromagnesian silicates, which simply means iron and magnesian bearing or not. The presence or absence of Fe and Mg strongly affects the external appearance (color) and density of the minerals.
Ferromagnesian silicates - dark color, density range from g/cc
Olivine - high T, low silica rocks; comprises over 50% of upper mantle
Pyroxenes - high T, low silica rocks
Amphiboles - esp. hornblende; moderate T, higher silica rocks
Mica - esp. biotite; moderate T, higher silica rocks
Garnet - common metamorphic mineral
Nonferromagnesian silicates - light color, density close to 2.7 g/cc
Mica - exp. muscovite; moderate T, higher silica rocks
Feldspars - plagioclase and orthoclase; most common mineral in crust; form over a wide range of temperatures and melt compositions
Quartz - low T, high silica rocks; extremely stable at surface, hence it tends to be a major component in sedimentary rocks.
Clay - esp. kaolinite; different types found in different soils

50. Important Silicates
                                                  
The Olivine Group is made up of two "end members" one with iron (Fe) and the other with magnesium (Mg). In the real world there is probably never a pure olivine, most are combinations of the two end members. Fe2SiO4
( Fayalite ) and Mg2SiO4 (Foresterite).
A combination formula might look like this: (Mg,Fe)2SiO4
Olivine occurs in the crust, ocean crust, and upper mantle and is not rare, except as clean crystals. It normally grows in small granular forms. As a gem it is known as peridot.
                                       

51. The Garnet Group is another with the isolated silicate structure
The Garnet Group is another with the isolated silicate structure. As with olivine it is made up of different substitution patterns, but unlike olivine it has many "end members" and thus a variety of different chemistries.
The general formula is
A3B2(SiO4)3
                                                                                              
Where A can be any of the following: Mg+2, Fe+2, Ca+2, and Mn+2. The B elements can be any of the following: Al+3, Fe+3, Cr+3 or a mixture thereof.
Because it has a wide variety of possible chemical formulas, it can be found in a rainbow of colors.
One of the important uses for garnet is the manufacture of wood sandpaper.
Grossular (Ca,Al) tsavorite, rhodinite, pyrope                                                             
Andradite (Ca,Fe) Almandine (Fe,Al)