5 Mineral Characteristics: Spot Them Like A Geologist!

Decoding Earth’s Secrets: Identifying Minerals Using 5 Key Characteristics

This guide will help you understand the fundamental "5 characteristics of minerals" that geologists (and aspiring rockhounds!) use to identify these fascinating building blocks of our planet. We’ll break down each characteristic in a clear and approachable way, allowing you to spot them like a pro.

What Makes Something a Mineral?

Before diving into the "5 characteristics of minerals," let’s clarify what exactly defines a mineral. A mineral isn’t just any shiny rock; it needs to meet specific criteria to earn that title.

Defining the Mineral: The 5 Rules

To be officially considered a mineral, a substance must meet all of the following requirements:

1. Naturally Occurring: It must be formed by natural geological processes, not manufactured in a lab. (Think of table salt versus a synthetic diamond.)
2. Inorganic: It cannot be made up of organic (carbon-based) material produced by living organisms. Coal, for example, is derived from ancient plant matter and is not a mineral.
3. Solid: It must be in a solid state at standard temperature and pressure. Water (liquid) and natural gas (gas) are, therefore, not minerals.
4. Definite Chemical Composition: It must have a specific chemical formula or a range of formulas within defined limits. Quartz (SiO2) always has two oxygen atoms for every silicon atom.
5. Ordered Crystalline Structure: Its atoms must be arranged in a highly ordered, repeating pattern. This internal structure is what gives minerals their unique properties and shapes.

Characteristic 1: Color

Color is often the first thing we notice about a mineral, but it’s also one of the least reliable identification tools.

Why Color Can Be Deceiving

• Impurities: Tiny amounts of impurities can drastically change a mineral’s color. For example, pure quartz is colorless, but even a small amount of iron can turn it purple (amethyst).
• Surface Alteration: Weathering and other surface processes can alter the color of a mineral’s surface without affecting its internal composition.
• Variable Composition: Some minerals have a range of chemical compositions, and these variations can lead to different colors.

Using Color Wisely

While not definitive on its own, color can still be a useful clue. Descriptive terms such as "metallic," "earthy," "vitreous," and "pearly" are often used to describe the way a mineral interacts with light and, thus, affect its apparent color.

Characteristic 2: Streak

Streak is the color of a mineral in powdered form. It’s a more reliable indicator than the color of the mineral sample itself because it is less affected by surface alterations and minor impurities.

How to Test for Streak

• Rub the mineral across a streak plate (an unglazed porcelain tile).
• The powder left behind is the mineral’s streak.
• Observe the color of the streak.

Streak Examples

Mineral	Color	Streak
Hematite	Reddish-brown	Reddish-brown
Pyrite	Brass-yellow	Black
Galena	Lead-gray	Lead-gray
Gold	Gold	Gold

Characteristic 3: Luster

Luster describes how light reflects from the surface of a mineral. It’s the quality of the reflected light, not necessarily the amount.

Types of Luster

• Metallic: Looks like polished metal (e.g., pyrite, galena, gold).
• Nonmetallic: Doesn’t look like metal. There are several subcategories:
  • Vitreous: Glassy (e.g., quartz, obsidian).
  • Pearly: Iridescent, like a pearl (e.g., talc, muscovite).
  • Silky: Fine, parallel fibers create a sheen (e.g., satin spar gypsum).
  • Adamantine: Brilliant, like a diamond (e.g., diamond).
  • Dull/Earthy: Lacks any noticeable shine (e.g., kaolinite).
  • Resinous: Looks like resin (e.g., sphalerite).

Characteristic 4: Hardness

Hardness is a mineral’s resistance to being scratched. It’s a relative property, determined by comparing the mineral to a standard scale known as the Mohs Hardness Scale.

The Mohs Hardness Scale

The Mohs scale ranges from 1 (softest) to 10 (hardest).

Hardness	Mineral	Common Test
1	Talc	Easily scratched by a fingernail
2	Gypsum	Scratched by a fingernail
3	Calcite	Scratched by a copper coin
4	Fluorite	Easily scratched by a knife
5	Apatite	Scratched by a knife with some effort
6	Orthoclase	Scratches glass, scratched by a steel file
7	Quartz	Scratches glass easily
8	Topaz	Scratches quartz
9	Corundum	Scratches topaz
10	Diamond	Scratches everything

Performing a Hardness Test

• Try to scratch the mineral with various objects of known hardness (fingernail, copper coin, steel knife, glass plate).
• Note which object scratches the mineral and which the mineral scratches.
• Compare the results to the Mohs scale to estimate the mineral’s hardness.

Characteristic 5: Cleavage and Fracture

Cleavage and fracture describe how a mineral breaks. This property is related to the arrangement of atoms within the mineral’s crystalline structure.

Cleavage: Breaking Along Planes

Cleavage is the tendency of a mineral to break along specific planes of weakness, creating smooth, flat surfaces. Cleavage is described by the number of cleavage planes and the angles between them. For example:

• Mica: Has one perfect cleavage plane, resulting in thin, easily separated sheets.
• Halite (Salt): Has three cleavage planes at 90-degree angles, forming cubes.
• Calcite: Has three cleavage planes that are not at 90-degree angles, creating rhombohedral fragments.

Fracture: Irregular Breakage

Fracture describes how a mineral breaks when it doesn’t cleave along a smooth plane. Common types of fracture include:

• Conchoidal: Smooth, curved surfaces like broken glass (e.g., quartz, obsidian).
• Irregular: Uneven, rough surfaces.
• Earthy: Crumbly or powdery.
• Hackly: Jagged, with sharp edges (e.g., metals).

Hopefully, you can now spot the 5 characteristics of minerals like a pro! Keep exploring and rock on!