Question

List and define the principal physical properties used to identify a mineral.

Short answer

The principal physical properties used to identify a mineral are: 1. Color: The visual appearance of a mineral as it interacts with light. Color may not be reliable due to impurities or weathering. 2. Luster: Describes the way light reflects from the mineral's surface, either metallic or non-metallic (vitreous, pearly, greasy, or dull). 3. Streak: The color of the powder created when a mineral is rubbed on a streak plate, often more reliable for identification than the overall color. 4. Hardness: The mineral's resistance to being scratched, often described using the Mohs Scale of Hardness (1-10). 5. Cleavage: The tendency of a mineral to break along flat, smooth surfaces, indicating a specific orientation. 6. Fracture: How a mineral breaks when it does not follow a specific cleavage plane; types include conchoidal, splintery, and irregular. 7. Density: The mass of a substance per unit volume, typically measured in g/cm³, helpful for identifying similar-looking minerals. 8. Crystal Habit: The shape and arrangement of crystal forms in a mineral, including prismatic, cubic, acicular, and platy types, helping to classify minerals in the same crystal system.

Step-by-step solution

1. Color

The color of a mineral refers to the visual appearance of a mineral as it interacts with light. Although color can be an easily observable property, it may not be a reliable characteristic for identifying minerals due to impurities or weathering altering a mineral's color.

2. Luster

Luster describes the way light reflects from the mineral's surface. The two main types of luster are metallic (shiny and reflective like a metal) and non-metallic (which includes vitreous, pearly, greasy, and dull subcategories).

3. Streak

The streak of a mineral refers to the color of the powder created when a mineral is rubbed across a streak plate (usually made from unglazed porcelain tiles). Streak color can often be more reliable for identification than the overall color of the mineral.

4. Hardness

Hardness is a measure of the mineral's resistance to being scratched. The Mohs Scale of Hardness, which ranges from 1 (the softest, talc) to 10 (the hardest, diamond), is commonly used to describe and compare the hardness of minerals.

5. Cleavage

Cleavage refers to the tendency of a mineral to break along flat, smooth surfaces. Minerals with good cleavage have a specific orientation and create smooth, flat surfaces when broken.

6. Fracture

Fracture describes how a mineral breaks when it does not follow a specific cleavage plane. Types of fractures include conchoidal (shell-like), splintery, and irregular.

7. Density

Density is the mass of a substance per unit volume, typically measured in grams per cubic centimeter (g/cm³) for minerals. It can be a helpful property for identifying different minerals with similar appearances.

8. Crystal Habit

Crystal habit refers to the shape and arrangement of crystal forms in a mineral. Different crystal habits include prismatic, cubic, acicular (needle-like), and platy (flat, plate-like). Crystal habit can help to classify minerals in the same crystal system.

Key concepts

Mineral Properties

Understanding the physical properties of minerals is essential for their identification and classification. These properties are determined by the innate qualities of minerals and how they form, as well as their chemical composition and crystal structure. By examining characteristics such as color, luster, streak, hardness, cleavage, fracture, density, and crystal habit, geologists and enthusiasts alike can distinguish between different minerals, even when visually similar.

When identifying a mineral, one must consider the entirety of its properties. The combination of traits tends to be unique for each mineral and aids in its identification, differentiating it from others that might look alike.

Mohs Scale of Hardness

The Mohs Scale of Hardness is an ordinal scale that ranks minerals based on their scratch resistance, relative to other minerals. It spans from 1 to 10, with 1 being the softest (talc) and 10 being the hardest (diamond).

Soft Minerals:

• Talc (Mohs hardness: 1)
• Gypsum (Mohs hardness: 2)

Hard Minerals:

• Corundum (Mohs hardness: 9)
• Diamond (Mohs hardness: 10)

It's important to note that each step on the scale represents a significant increase in hardness; for example, a mineral ranked at 6 can easily scratch minerals with lower numbers.

Crystal Habit

Crystal habit refers to the typical external shape displayed by crystals of a mineral, and it can provide insight into the conditions under which the mineral formed.

Some common crystal habits include:

• Prismatic: Crystals are elongated and prism-like.
• Cubic: Crystals have a cube-like shape.
• Acicular: Crystals are fine and needle-like.
• Platy: Crystals form in flat, plate-like structures.

Each habit reflects specific growth conditions and can help in identifying a mineral by examining its geometric arrangement and the symmetry of its crystal forms.

Mineral Density

Mineral density, often expressed in grams per cubic centimeter ((g/cm³)), is a measure of how much mass a mineral contains in a given volume. This physical property can be crucial for identification because it remains relatively constant for a mineral, irrespective of size or external conditions.

For instance, gold has a high density ((~19.32 g/cm³)), which is markedly different from minerals that appear similar, such as pyrite ('fool's gold'). In contrast, lower density minerals like pumice can float on water.

Mineral Color

Mineral color can be very striking and one of the first features noticed, yet it is often the most misleading property when used for identification. Color variations can occur due to impurities within a mineral or due to the presence of trace elements, which can dramatically change a mineral's hue.

Some minerals have a characteristic color, like the blue of azurite, while others, such as quartz, come in a variety of colors. When using color as an identifying property, it is always best to consider it alongside other more consistent properties.

Mineral Luster

Luster pertains to the way light interacts with the surface of a mineral – the quality and intensity of the reflection.

Types of Luster:

• Metallic luster: The mineral surface shines like metal.
• Vitreous luster: The mineral has a glassy appearance.
• Pearly luster: The mineral exhibits an iridescent surface like a pearl.
• Greasy luster: The mineral appears as if it is covered in a thin oil layer.

Luster is a more reliable property than color because it's less subject to variation among a given mineral type.

Mineral Streak

The streak of a mineral is the color of its powder when it is dragged across an unglazed porcelain tile known as a streak plate. Unlike the overall color of a mineral, which can be affected by environmental factors and impurities, the streak typically does not vary and provides a clue to the mineral's true identity.

For example, hematite may appear silvery, black, or red in hand-sample, but it invariably leaves a reddish streak. Streak is particularly useful for distinguishing minerals with a metallic luster and a similar appearance.

Mineral Cleavage

Cleavage is the tendency of a mineral to break along specific planes of weakness in its crystal lattice. These planes are related to the atomic arrangement within the mineral and result in flat, smooth surfaces upon fracture.

Cleavage Quality:

• Perfect: The mineral breaks with smooth, flat surfaces along planes of weakness.
• Fair: The mineral has a tendency to break along certain planes but not as smoothly.

Recognizing cleavage and the number of planes along which a mineral can cleave is an invaluable skill in mineral identification.

Mineral Fracture

When minerals do not cleave along defined planes, they fracture. The pattern of the fracture can be distinctive and help in identification.

Fracture Types:

• Conchoidal: The fracture surface is smooth and curved, similar to broken glass.
• Splintery: The mineral fractures into splinter-like pieces.
• Irregular: The mineral breaks with a rough or uneven surface.

While fracture is less predictive than cleavage, it is particularly useful for minerals that do not have a cleavage or have an indistinct cleavage.