Minerals can be identified on the basis of a set of certain observable diagnostic properties:

Such diagnostic properties reflect the internal structures and compositions of minerals.

When a mineral crystallizes without impediment to its growth, it assumes a characteristic shape or euhedral crystal habit which reflects its internal crystal structure.

e.g.: mica has a bookish habit that reflects its silicate sheet structure

e.g.: halite has a cubic habit that reflects the cubic arrangement of its atoms

Cleavage is the tendency of some minerals to break (cleave) along flat surfaces called cleavage planes.

Cleavage planes are planes of weak chemical bonds within mineral crystals. The planes fail when force is applied to them.

Minerals can have 1 to 6 different directions of cleavage, forming a variety of different cleavage shapes.

Fractures are rough and nonplanar breakage planes. They can be:

• conchoidal (smooth dish-shaped)
• fibrous (splintery and wood-like)
• uneven (plain rough and irregular)
  

Minerals that fracture (rather than cleave) have crystal structures with equally strong bonds in all directions. There are no preferred planes of weakness within them.

Luster is the way in which light is reflected from the surface of a mineral:

a. metallic luster
b. nonmetallic lusters
i. pearly
ii. resinous
iii. silky
iv. vitreous (glassy)
v. waxy
vi. earthy

Hardness is the resistance of a mineral to scratching. It is measured relatively with Mohs' scale.

• The hardness of a mineral sample is measured by scratching it with a standard mineral or object.
• It is typical to try first to scratch the mineral with your fingernail (H=2.5):
• If you cannot, repeat with progressively harder standards until you can.
• If you can, repeat with progressively softer standards until you cannot.
  
  

Some generalizations about color:

• Si and Al rich minerals like quartz and feldspar are typically light colored
• Fe and Mg rich minerals like olivine and pyroxene are typically dark colored

However, color is sometimes deceptive because:

• it can be altered by weathering
• it can be influenced by trace elements:
• e.g.: pure quartz is light colored, but the variety amethyst tinted purple by Fe
  

Streak: color of a mineral in powdered form.

• Grind a mineral sample against an unglazed porcelain streak plate and examine color of the powder or streak it leaves behind.
• Streak is very useful for oxides, sulfides, and native metals.
• However, it cannot be used for a mineral with a hardness > 7 (hardness of good porcelain).

Specific gravity:

• ratio of the weight of a mineral to the weight of an equal volume of water
• measures the density of a mineral
• particularly diagnostic of native metals such as gold (sg=19) and copper (8.9)
• also distinguishes light Si and Al-bearing minerals from dense Fe and Mg-bearing minerals

Magnetism:

• attract to magnets, steel clips, etc.
• characteristic of the mineral magnetite

Effervescence: the reactivity of minerals to dilute HCl

e.g. calcite and powdered dolomite both react to cold HCl

CAUTION: HCl is highly reactive even when it is dilute. It can cause serious burns to your clothes, skin, nose, and eyes. Handle it carefully and wash your hands afterwards.