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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

Mineral

Colour

Augite

White to greenish-grey (augite is close to the hardness of the streak plate)

Andalusite, Apatite and Anhydrite

White

Arsenopyrite

Dark greyish black

Azurite

Light blue

Barite  And Benitoite

White

Bauxite     

White, sometimes discoloured to brown, pink, or red by iron staining.

Bornite

greyish black

Beryl

 

colourless (even harder than the streak plate)

Calcite Clinozoisite

White

Chromite   

Dark brown

Cassiterite

colourless

Chalcopyrite

Greenish black

Chlorite

Greenish to greenish-black to white

Chalcocite

greyish black

Cinnabar

Red

Copper      

Metallic copper red

Cuprite

Brownish red

Cordierite and Corundum 

colourless (harder than the streak plate).

Diamond   

colourless (even harder than the streak plate)

Dolomite

White

Epidote and Euclase

White or colourless

Fluorite and Fuchsite     

White

Gold

Metallic gold yellow

Garnet

colourless

Gypsum

White

Graphite

Black

Halite

White

Hematite

Red to reddish-brown

Ilmenite

Black

Jadeite

colourless

Kyanite

White or colourless

Limonite

Yellowish-brown

Magnesite Monazite, And Muscovite

White

Magnetite

Black

Marcasite

greyish Black

Malachite

Green

Nepheline

White

Olivine

White or colourless

Orthoclase

White

Plagioclase and Prehnite

White

Pyrrhotite

greyish black

Quartz

colourless

Rhodochrosite and Rhodonite

White

Rutile

Pale brown

Scapolite, Serpentine and Sylvite       

White

Spinel       

colourless

Silver

Silvery white

Sodalite

White or light blue

Sulfur

Yellow

Talc

White to pale green

Titanite

White

Topaz

colourless

Turquoise

White, bluish, greenish

Uraninite

Brownish black, greyish

Vanadinite

Pale yellow to yellowish-brown

Witherite and Wollastonite

White

Zircon

colourless

Zoisite       

White


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 (Frequently Asked Questions)

1. What is the Use of a Streak Test?

Answer: The streak test is quite worth seeing that a large variety of minerals occur in apparent, different colours - but all specimens of that mineral share the same streak colour. For example specimens of hematite mineral can be red, black, brown, or silver in colour and take place in a huge variety of habits; but, all specimens of hematite generate a streak with a reddish colour. This is quite a valued test for hematite. It can be conducted to distinguish hematite from a large number of other opaque minerals having high specific gravity and identical colour and habit.

2. How to Reduce Error When Doing a Streak Test?

Answer: A general error that is most commonly made by people who are conducting the streak test for the very first time is that they tend to lightly rub the specimen backwards and forwards on the surface of the streak plate. This will not generate an accurate streak. Some mineral specimens are so hard that very solid pressure and resolution are needed to produce a mineral powder.

3. Which Popular Mineral Differs From its Streak Colour?

Answer: Fluorite is another mineral whose apparent colour is distinguished from the colour of the streak. Specimens of fluorite can be blue, yellow, green, purple, or colourless. However, all specimens of fluorite are with a white streak.


Specimens of pyrite always contain a brassy yellow colour; however, all specimens of pyrite yield a black streak.

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