Question

Will a diamond leave a streak on a streak plate? Explain.

Short answer

A diamond will not leave a streak; it would scratch the streak plate instead.

Step-by-step solution

Understand the Properties of a Diamond

To determine whether a diamond will leave a streak on a streak plate, we need to consider the properties of both the diamond and the streak plate. Diamonds are known for their hardness, which is the highest on the Mohs scale of mineral hardness. This scale rates minerals from 1 (talc) to 10 (diamond), making diamonds capable of scratching almost any other substance.

Understand the Properties of a Streak Plate

Streak plates are typically made of unglazed porcelain and are used to identify minerals by the color of their powdered form. The hardness of a streak plate is around 7 on the Mohs scale. Therefore, it is softer than a diamond, which ranks as 10 on the scale.

Analyze the Streak Test Process

The streak test involves dragging a mineral across the streak plate to observe the color of the streak left behind. For a mineral to leave a streak, it must be softer than the streak plate so that it can be powdered by the plate.

Determine the Outcome for a Diamond

Since a diamond is harder than the streak plate, it will not be scratched or powdered by the plate. Instead, the diamond will scratch the streak plate. Therefore, a diamond will not leave a streak because it cannot be powdered by a softer material.

Key concepts

Diamond Properties

Diamonds are renowned for their unique and fascinating properties. At the forefront of these properties is their incredible hardness. On the Mohs scale of mineral hardness, diamonds are rated a perfect 10, which is the highest possible rank. This remarkable hardness allows diamonds to scratch virtually every other substance, a feature that is often utilized in industrial applications such as cutting, drilling, and grinding.
Another notable property of diamonds is their exceptional refractive index. This characteristic accounts for their sparkling appearance and brilliant light dispersion, often referred to as "fire."
Moreover, diamonds are known for their thermal conductivity. They can efficiently transfer heat better than most metals, which is why they are sometimes used in high-performance heat sinks and semiconductors.

• Highest hardness on Mohs scale (10)
• Exceptional light dispersion ("fire")
• Outstanding thermal conductivity

Understanding these properties is key to appreciating why a diamond behaves the way it does during a streak test.

Streak Test

The streak test is a simple yet effective method used in mineral identification. It is based on examining the color of the streak left by a mineral when it is dragged across a streak plate. Streak plates are usually made of unglazed porcelain, and they have a hardness of about 7 on the Mohs scale.
The color of the streak can provide valuable information as it often differs from the color of the actual mineral. This is particularly useful for minerals that appear in multiple colors. For instance, minerals that look different to the eye might leave the same streak on a plate.
Here's how to perform a streak test:

• Ensure the streak plate is clean and dry.
• Firmly hold the mineral specimen against the plate.
• Drag it across the plate with moderate pressure.
• Observe the color of the streak left behind.

The streak test requires that the mineral be softer than the streak plate. If a mineral is harder, like a diamond, it won't leave a streak, but will instead scratch the plate.

Mineral Identification

Mineral identification involves determining the characteristics that make each mineral unique. Several methods and tests are used by geologists and hobbyists alike to identify minerals, among which the streak test is a fundamental tool.
However, there are multiple facets to mineral identification beyond the streak test:

• Color: While color can sometimes be a giveaway, it can also be misleading due to variations.
• Hardness: Determined by the Mohs scale, hardness measures a mineral's ability to scratch others.
• Luster: Describes how a mineral reflects light (e.g., metallic, vitreous, or dull).
• Cleavage and Fracture: Refers to how a mineral breaks along specific planes or irregular surfaces.
• Density: Often measured as specific gravity, this tells us the "heaviness" of a mineral.

By combining these tests and observations, one can arrive at a reliable identification of a mineral. Each property adds a piece to the puzzle, making the whole picture clearer.

Hardness Scale

The Mohs scale of mineral hardness is a comparative tool that measures a mineral's ability to resist scratching. It ranges from 1 to 10, with 1 being the softest (talc) and 10 being the hardest (diamond).
This scale is crucial for testing and identifying minerals, as it helps determine their ranking in comparison to known benchmarks. For example:

• Talc - 1
• Gypsum - 2
• Calcite - 3
• Fluorite - 4
• Apatite - 5
• Orthoclase Feldspar - 6
• Quartz - 7
• Topaz - 8
• Corundum - 9
• Diamond - 10

To use the Mohs scale in practice, if a mineral can scratch another with a known hardness, it is harder or equal to that mineral. Conversely, if it is scratched by a known reference mineral, it is softer or equal to that hardness level. The Mohs scale is an intuitive and accessible system for understanding and testing mineral hardness.