Streak Meaning
Steak is a colour of a mineral when powdered and observed against unglazed porcelain (white ceramic plate). Streak plates that have been used rigorously will be laminated with streaks and powdered minerals. However, they can easily be cleaned with water and damp or dry 220 grit sandpaper. In addition, Aluminum oxide or silicon carbide sandpaper works best to clean streaks since the granules are hard enough to smoothen out the surface of the streak plate. The sanding should be carried out wet to control dust.
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How to Determine the Colour of the Streak?
A "streak test" is conducted to identify the colour of a mineral in its powdered form. The colour of a mineral's powder is commonly a very crucial property for determining the mineral.
How is a Streak Test Conducted?
The streak test is carried out scraping a specimen of the mineral covering the piece of unglazed porcelain termed as a "streak plate." The streak test should be undertaken on clean, unworn, or freshly broken specimens of the mineral. This is conducted in order to minimize the possibility that a foreign matter, pollutant, weathered coating, or corrosion will influence the outcome of the test.
This can yield a little amount of powdered mineral on the surface of the plate. The powder colour of that mineral is what we call a "streak."
Streak Colours of Common Minerals
Uses of Streak Plates
Additionally to their usage in conducting the streak test, streak plates can be used any time you require a little amount of powdered mineral. In performing the acid test to differentiate calcite from dolomite, dolomite might need powdering in order to exhibit effervescence with dilute hydrochloric acid. Simply use the streak plate in order to create some powder of your specimen and add a small amount of acid to it right on the streak plate. For this test, a black streak plate makes monitoring easier since powdered dolomite is white.
A few minerals will yield a stench (odour) upon being powdered or fragmented. For example, sphalerite releases an odour of sulfur when it is powdered or broken. Scraping it across a streak plate is the easiest way to undertake this test.
Traces to other mineral properties can be acquired while conducting the streak test. Minerals harder than the streak plate are rapidly determined. Experienced testers can approximate the hardness of a specimen by how complicated it is to mark the streak plate. augite often displays its splintery cleavage, olivine most commonly exhibits its granular characteristic, and black tourmaline displays its brittleness. Having said that When you conduct a streak test, look for just beyond the colour of a specimen's powder.
Fun Facts
Rubbing the mineral across an unglazed porcelain white plate determines and describes the colour of the powder left on the plate, i.e. the streak.
Mineral pyrite is gold-coloured, however, its streak is greenish-black.
Most transparent and pale coloured or translucent minerals have a non-identifying white streak.
Minerals having a hardness of above 6.5 will not display a streak since they are harder than a piece of unglazed porcelain.
FAQs on Streak
1. What is a 'streak' in the context of mineralogy and how is it observed?
In mineralogy, the streak refers to the colour of a mineral in its powdered form. This property is considered a more reliable identifier than the mineral's apparent surface colour. It is observed by rubbing a mineral specimen firmly across an unglazed piece of porcelain, known as a streak plate. The coloured line of powder left behind on the plate is the mineral's streak.
2. Why is a mineral's streak considered a more reliable identification property than its external colour?
A mineral's streak is more reliable because the external colour of a mineral can vary significantly due to trace impurities or weathering on its surface. For example, Quartz can be purple (Amethyst), yellow (Citrine), or clear. However, when powdered, the mineral's fundamental chemical composition dictates the colour, which remains consistent. The streak test reveals this consistent internal colour, making it a more dependable diagnostic tool for mineral identification.
3. What is a streak plate and what are its key properties for testing minerals?
A streak plate is a standard tool used in geology, made of unglazed porcelain tile. Its key properties are:
Hardness: It has a hardness of approximately 6.5 to 7 on the Mohs scale. This allows it to powder any mineral that is softer than it.
Colour: It is typically white or black, providing a neutral background that makes it easy to see the true colour of the mineral's powder.
Abrasive Surface: Its unglazed, slightly rough texture is effective at grinding a small amount of the mineral into a powder when the specimen is rubbed against it.
4. Explain the correct procedure for performing a streak test on a mineral sample.
To perform a streak test correctly, follow these steps:
Select a fresh, unweathered surface on the mineral specimen.
Hold the streak plate firmly on a flat surface.
Press the mineral firmly against the plate and drag it across the surface to create a line of powder about one or two centimetres long. It is important to apply consistent and firm pressure; lightly rubbing the mineral will not produce an accurate streak.
Observe the colour of the powder left on the streak plate. This is the mineral's streak.
5. Provide examples of common minerals where the streak colour is very different from the mineral's visible colour.
Several common minerals show a stark difference between their external colour and their streak. Key examples include:
Hematite: Can appear black, silver-grey, or reddish-brown, but its streak is always a consistent cherry-red or reddish-brown.
Pyrite (Fool's Gold): Has a metallic, brassy-yellow colour, but it leaves a distinct greenish-black streak.
Fluorite: Occurs in a wide range of colours like purple, green, blue, and yellow, but its streak is always white.
6. For which types of minerals is the streak test not effective, and why?
The streak test is not effective for minerals that are harder than the streak plate (i.e., having a Mohs hardness greater than 7). When a harder mineral like Topaz, Corundum, or Diamond is rubbed against the plate, it will not leave a powdered streak. Instead, it will scratch the plate, leaving a white line that is actually the powder of the porcelain plate itself. Therefore, the test is inconclusive for very hard minerals.
7. How does the concept of streak relate to a mineral's hardness?
Streak and hardness are interconnected properties. A successful streak test can only be performed if the mineral is softer than the streak plate (which has a hardness of about 6.5-7). Therefore, if a mineral leaves a coloured streak, it indirectly tells you that its hardness is less than 7. If the mineral scratches the plate, it tells you its hardness is greater than 7. This interaction makes the streak test a dual-purpose observation, providing clues about both a mineral's true colour and its relative hardness.
