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HARDNESS

What is hardness?

The hardness of a mineral is its ability to resist scratching.

Where did the hardness scale originate?

Friedrich Mohs, a German mineralogist, developed a hardness scale over 100 years ago. The hardestmineral known, diamond, was assigned the number 10.

How does the hardness scale work?

The Mohs ardness !cale ranks the order of hardness of minerals and some common ob"ects. For

e#ample, your fingernail can scratch the minerals talc and gypsum, with a hardness of $ or lower. %

copper penny can scratch calcite, gypsum, and talc.

% common misunderstanding of how to identify a diamond is that it will scratch glass. &hile this is true,

other minerals can scratch glass too as long as they have a hardness ' (.

STREAKWhat is streak?

The streak of a mineral is the color of the powder left on a streak plate (piece of

unglazed porcelain) when the mineral is scraped across it. The streak plate has a

hardness of glass, so minerals with a Mohs Hardness >7 will scratch the streak

plate and won't powder the mineral.

Where do you observe streak?

mineral!s streak is determined "# ru""ing it on a streak plate, which is a piece of

unglazed porcelain. The streak plate is essentiall# a t#pe of glass, so it isn't used

on minerals with hardness greater than 7.

How useful is streak in identifying minerals?

$treak can "e useful for identif#ing metallic and earth# minerals. %onmetallic

minerals usuall# gi&e a white streak "ecause the# are &er# lightcolored. ther

minerals ma# ha&e &er# distincti&e streaks. Hematite, for eample, alwa#s gi&es a

reddish"rown streak no matter what t#pe of luster it displa#s.

CLEAVAGE

What is cleavage?

)leavage is the ability of a mineral to break along preferred planes.

!ome minerals show different colors along different crystallographic a#es. This is known as

pleochroism.

How is cleavage described?

Minerals that have *perfect* cleavage almost always break in a preferred direction. Minerals that have

*good* cleavage will sometimes break in a particular direction, and other times, they may not.

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CLR

What ca!ses color in "inerals?

Minerals are colored because certain wave lengths of light are absorbed, and the color results from a

combination of those wave lengths that reach the eye.

!ome minerals show different colors along different crystallographic a#es. This is known as pleochroism.

Which "inerals is color !se#!l #or identi#ication?

For some minerals, color is directly related to one of the ma"or elements and can be characteristic, thus

serving as a means of identification. Malichite is always green+ aurite is always blue+ and rhondonite is

always red or pink.

Most metallic minerals- color is constant, such as the brassyellow of chalcopyrite and the copperred of

niccolite. These minerals may also tarnish, which is especially true of the mineral bornite. /t is called the

*peacock ore* because of the bluevelvet tarnish that develops on the surface.

L$S%ER

What is L!ster?

uster refers to how light is reflected from the surface of a mineral. The two main types of luster are

metallic and nonmetallic.

What is &etallic L!ster?

Minerals e#hibiting metallic luster look like metal, such as a silvery appearance or that of a flat piece of

steel.

How "an' t'(es o# non"etallic l!ster are there?

itreous2 The luster of glass

3esinous2 The luster of resin.

4early2 The luster of pearls.

Greasy2 ooks like it is covered in a thin layer of oil.

!ilky2 The luster of silk.

%damantine2 % hard, brilliant luster.

%nother common nonmetallic luster is called transl!centluster, where you can see into the mineral, butnot completely through it. % mineral that displays a trans(arentluster transmits light completely

through it, resembling glass.

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S)EC*+*C GRAV*%,

What is s(eci#ic gravit'?

!pecific gravity is the *heaviness* of a mineral. /t is defined as a number that e#presses the ratio

between the weight of a mineral and the weight of an e5ual volume of water. &ater has a specific

gravity of 61.7

What is s(eci#ic gravit' de(endent !(on?

The specific gravity depends on2

The kind of atoms that comprise the mineral.

ow the atoms are packed together.

8ou can easily distinguish minerals that have unusually high or unusually low specific gravities "ust by the

way they *feel* when you hold a sample in your hand.

How is s(eci#ic gravit' "eas!red?

The steps in measuring the specific gravity are2

&eigh the mineral in air 9&a:.

&eigh the mineral in water 9&w:.

;se the following e5uation2

Wa/ ( Wa- Ww)

&*SCELLANE$S )R)ER%*ES

!everal properties can be used to identify particular minerals.

&agnetite-The mineral magnetite is magnetic. odestone, a variety of magnetite, is a natural magnet. To

test for magnetism, suspend the magnet from a string and then draw it near the sample. /f magnetic, the

magnet will be deflected toward the sample.

)lagioclase #elds(ar-The mineral plagioclase feldspar commonly shows very small, parallel lines on some

of the cleavage planes. These are called striations. /n some cases, these striations are the only way todistinguish plagioclase feldspar from orthoclase feldspar.

Gra(hite-as a greasy feel. %fter feeling it, check your fingers. They will be dirty since graphite also has

a hardness of 61.7 8ou can also see if you can mark on paper with the sample. /t is 6pencil lead.*

S!l#!r-/s a native mineral with a chemical formula of !. /t smells like rotten eggs when streaked or

scratched. /t is also bright yellow in color.

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Calcite-)alcite will effervesce 9fi: when a drop of concentrated lemon "uice is placed on its surface. /t

also has a hardness of