Question

List and explain minerals" physical properties that are most useful for identification.

Short answer

Color, streak, hardness, luster, cleavage, fracture, and specific gravity are key physical properties used to identify minerals.

Step-by-step solution

Understanding Mineral Properties

Minerals have distinct physical properties that help in their identification. These properties are observable and can usually be tested with simple tools or even estimated visually.

Color

Color is the most noticeable property but can be misleading as some minerals come in multiple colors. Nonetheless, it provides an immediate hint and should be noted as part of the identification process.

Streak

Streak refers to the color of the powdered mineral, usually tested by scraping the mineral on a porcelain streak plate. The streak of a mineral is often more consistent and reliable than its surface color.

Hardness

Mineral hardness is measured using the Mohs scale, which rates minerals from 1 (softest, talc) to 10 (hardest, diamond). Hardness tests determine a mineral's resistance to being scratched and help distinguish minerals with similar appearances.

Luster

Luster describes how light reflects from a mineral's surface. Common descriptors include metallic, vitreous (glassy), pearly, and dull. Luster can provide important clues about a mineral's identity.

Cleavage and Fracture

Cleavage is the tendency of a mineral to break along flat, even surfaces, while fracture refers to more irregular breakage patterns. Some minerals cleave easily in one or more directions, while others fracture.

Specific Gravity

Specific gravity measures a mineral's density, comparing its weight to an equal volume of water. This property requires more sophisticated equipment, but is useful for distinguishing between minerals with similar appearances and compositions.

Key concepts

Physical Properties of Minerals

Minerals have a variety of physical properties that make them unique and help in their identification. Identifying minerals is often likened to being a detective, where these properties serve as clues. Some properties are more subtle, requiring experience, while others can be easily observed by beginners with simple tools.
Color is often the first property noticed. However, it can be deceiving, since many minerals can have multiple color varieties due to impurities. Streak, the color of a powdered mineral, remains consistent regardless of surface impurities, making it a more reliable indicator.
Hardness is another critical property, determining how resistant a mineral is to scratching. This property is compared using the Mohs hardness scale. Luster and texture are about the shine and feel of the mineral, adding to how a mineral might be identified in hand samples. Last but not least, specific gravity reflects how a mineral's density compares to water, which helps in differentiating minerals similar in appearance but different in weight.

Mohs Hardness Scale

The Mohs hardness scale is a relative scale that ranks the hardness of minerals on a numerical value from 1 to 10.
At the softest end of the scale is talc, rated 1, while diamond sits at 10, being the hardest mineral known. This scale is not linear; the difference in hardness between, say, diamond and corundum (rated 9) is much larger than between gypsum (rated 2) and talc.

• To determine hardness, a scratch test is performed, which involves scratching the mineral against an object of known hardness or vice versa.
• For example, a mineral that can scratch calcite (hardness 3) but is scratched by fluorite (hardness 4) would have a hardness between 3 and 4.

This testing helps classify minerals into a hierarchy of hardness, making it easier to distinguish them.

Mineral Luster

Luster describes how a mineral reflects light from its surface. It is one of the first observable characteristics noticed in hand specimens and can suggest clues about the mineral's identity.
Mineral luster is classified into two main categories: metallic and non-metallic. Metallic luster is shiny and reflective, akin to metal surfaces. Non-metallic can vary widely, described by terms like:

• Vitreous – having a glass-like shine.
• Pearly – resembling the sheen of pearls.
• Resinous – similar to the look of resin.
• Earthy or dull – a lack of shine.

This nuanced description is important, especially when combined with other properties, to accurately identify minerals.

Cleavage and Fracture

Cleavage and fracture describe how minerals break when force is applied. This property indicates a mineral's structure at a microscopic level.

• Cleavage occurs along planes where atomic bonds are weaker, producing smooth, flat surfaces. A good example is mica, which cleaves into thin sheets.
• Fracture, on the other hand, refers to mineral breakage in an irregular pattern when no cleavage planes are present. Quartz exhibits conchoidal fracture, with curved surfaces reminiscent of glass edges.

Recognizing whether a mineral breaks along cleavage planes or fractures is crucial for identification and understanding its internal structure.