Luster and streak are two important properties used in the identification of minerals. Both properties provide valuable clues for identifying minerals and are fundamental in the field of geology. This article focuses on the difference between luster and streak.
What is the difference between luster and streak? Luster is the quantitative measure of the light-reflective ability of a mineral, whereas streak is the reflected colour.
Key Areas Covered
1. What is Luster
– Definition, Features
2. What is Streak
– Definition, Features
3. Similarities Between Luster and Streak
– Outline of Common Features
4. Difference Between Luster and Streak
– Comparison of Key Differences
5. FAQ: Luster and Streak
– Answers to Frequently Asked Questions
Key Terms
Luster, Streak, Minerals
What is Luster
Luster is the way that light interacts with the surface of a mineral. Luster is determined by the reflection of light off the surface of a mineral, which can result in various visual effects. It is an important property that helps to identify and classify minerals. There are two main types of lusters: metallic luster and non-metallic luster.
Metallic luster is the shiny reflective surface that resembles the appearance of metals such as gold, copper, and silver. This type of luster is usually observed in minerals containing a metallic composition. For example, pyrite and galena show some luster. This reflective nature is seen when the light that is incident on the surface bounces back in a specular manner.
Figure 1: Metallic Luster
Non metallic luster is the opposite of metallic luster. Non-metallic luster can be further classified into several subcategories, including vitreous (glassy), pearly, greasy, resinous, silky, and dull, each subcategory is based on the distinct visual qualities as a result of the changes in the texture, refractive index and the transparency. For example, minerals with a vitreous luster, such as quartz or calcite, exhibit a bright, glass-like sheen, while those with a pearly luster, like talc or gypsum, display a soft, iridescent glow reminiscent of pearls.
Luster is also used to find a mineral’s physical properties, such as formation environments. For example, minerals with a metallic luster are often associated with ore deposits. Minerals with non metallic luster indicate specific geological conditions and alterations of processes.
What is Streak
Streak is the colour of the mineral in its powdered form. This colour could be obtained by rubbing the mineral on a hard and white surface, such as on unglazed porcelain. This is a qualitative property. For a given species of minerals, this streak is unique. However, the colour of a streak may differ considerably from the colour of the unpowdered mineral. The presence of these different colours is due to the presence of trace impurities within the mineral or due to the macroscopic crystal structure.
Streak is a very accurate method of finding the species of mineral. If no streak is made, the streak of the mineral is said to be white or colouress. Hence, the streak is a very useful diagnostic method to identify opaque and colourful minerals. Nevertheless, this method cannot be applied to silicate minerals as they are less prone to powdering and having a white streak.
A streak plate, such as unglazed porcelain, is the surface across which the mineral is being dragged. The relevant streak to be found is usually identified and compared with another streak plate.
Figure 2: Streak Plate with Pyrite and Rhodochrosite
Some minerals leave a streak similar to their natural colours, such as cinnabar, lazurite, and native gold. Flourite has a white streak even though it appears in colours purple, blue, yellow, or green in the crystalline form. Hematite, which is black in colour, leaves a red streak.
Similarities Between Luster and Streak
- Luster and streak describe the visual characteristics of a mineral.
- Both are useful as diagnostic tools in mineral identification.
Difference Between Luster and Streak
Definition
- Luster describes the appearance of a mineral’s surface when light reflects off it, while streak refers to the color of the powdered form of the mineral.
Determined by
- Luster is observed by visually examining the surface of a mineral, while streak is determined by rubbing the mineral against a streak plate to produce a streak of powder.
Type of Measure
- Luster is a quantitative measure, while streak is a qualitative measure.
Conclusion
Luster refers to the way light interacts with a mineral’s surface. It has two types as metallic and non-metallic types, offering insights into a mineral’s composition and formation. On the other hand, streak is the color of a mineral’s powdered form, and it is determined by rubbing it against a surface. This is the main difference between luster and streak. Although both help in mineral identification, luster is a quantitative measure reflecting light, whereas streak is qualitative.
FAQ: Luster and Streak
1. What are 2 types of luster?
The two main types of luster are metallic luster and non metallic luster. Metallic luster gives minerals a shiny, reflective appearance similar to metals, while non-metallic luster includes various appearances such as glassy, diamond-like, pearly, silky, and dull.
2. How do you test for luster?
Follow the steps below to test for luster:
- Hold the sample to orient it with any planar (flatter) surfaces facing an overhead light.
- Rotate the sample back and forth to reflect light from a source.
- Observe the reflection of light from any flat surface.
3. What are the terms used to describe luster?
Luster can be described using various terms, including earthy, metallic, greasy, silky, vitreous (glassy), adamantine (diamond-like), pearly, and dull. Each term reflects a different quality of the mineral’s surface appearance.
4. Is iron a shiny luster?
Yes, iron typically exhibits a shiny luster. When polished or freshly cut, iron surfaces can appear shiny and reflective, adding to their metallic appearance.
5. What causes luster?
Luster is mainly caused by the reflection of light off the surface of a material. In metals, this reflection occurs due to the presence of mobile electrons, which allow for efficient light reflection. This also contributes to the metallic luster characteristic of metals.
Reference:
1. “Lustre.” Encyclopedia Britannica.
2. “Strak.” Encyclopedia Britannica.
Image Courtesy:
1. “Pyrite_3” By JJ Harrison – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Streak plate with Pyrite and Rhodochrosite” By Ra’ike – Own work, (CC BY-SA 3.0) via Commons Wikimedia
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