MINERALS - CLASSIFICATION AND IDENTIFYING PROPERTIES

We will be looking at the properties of minerals and how we can use them to identify which mineral we have.

Minerals are defined as naturally occurring inorganic crystalline materials with distinct chemical and physical properties. So any manmade materials even if they are otherwise identical aren't minerals really, nor are any materials made by other living things. Although we generally include the carbonate minerals since we only recently discovered that in most cases they are actually made by living things. In addition there are a number of "mineraloids" that get lumped in - despite not fitting the exact definition above *G* .

Minerals have a crystal structure - that is the atoms they are made of are arranged in a definite and repeating pattern. These patterns form shapes, and these shapes can be grouped so that we can see symmetries in them. Symmetries are repeating identical faces as we turn the material or view it in a mirror. It turns out that all the different shapes possible fall into 6 groups. Each group can be identified by the number, lengths, and angles between the axis of the crystal's faces the 6 groups are:

Hexagonal crystals: with 1 axis at 90* to the other 3 which are at 60* to each other

the 3 axis at 60* are of equal length but the 1 at 90* is of a different length (either shorter or longer).
There is a subgroup of the hexagonals called the trigonals where the symmetry breaks down some and may leave only 3 rotations that are identical instead of the normal 6.

Isometric crystals: -3 equal sized axis of rotation all set at 90 degrees to each other (like a cube).

Tetragonal crystals: -3 axis at right angles to each other with 2 equal in length and the third different.

Orthorhombic crystals: - 3 axis of symmetry all at 90* - but all of different lengths (like a shoe box).

Monoclinic crystals: - 3 axis of different lengths; 2 at 90* and the third at some other angle.

Triclinc (3 angles) crystals: -3 axis of different lengths with the angles between them different and not 90*

{t} fine so far.

{a} are there exceptions to this rule?

* {a} is trying to fit opals into these

a: actually no - all the different possible closed flat faced shapes fall into one of the groups.
Opals aren't minerals - they are stacks of balls of silica and can take any shape - more on opals and the other mineraloids at the end <G>

{a} k

All minerals have a their own characteristic crystal shapes (yes they can have more than one but they all will have the same type of symmetry). The crystal shapes are one of the most important and useful tools for identifying minerals. In many cases the the crystal shape is enough to identify the mineral or mineral group all by its self - like with garnets .

{W} What about sulfur? It comes in several shapes, Rhomboidal, and prismatic!

W: - yes it comes in several shapes - but all the shapes have the same crystal symmetry.

We also use other properties including color (not that useful in most cases really), streak (the color of the powdered mineral) - also not very useful in most cases, and hardness.

While there are a number of different scales that can be used, in mineralogy we use a simple one called the Mohs scale (after the guy that created it).

MOHS SCALE of RELATIVE HARDNESS

10 - diamond



In addition to hardness we also use the density (or specific gravity -essentially the same) while we do measure the actual density for field purposes we can split them into ranges:

very light - density < 2.0

Most minerals fall into the light or heavy range but a few are identified by there density.

We can also use the way a mineral breaks to help ID it

{W} Cleavage and fracture?

Yes - if it breaks with a series of smooth flat planes it is said to cleave ( like the faces had been cut loose with a knife), if the surface is rough or curved its is called fracture.

Glass and glass like materials generally have curving fractures called chonchoidal. The angles between the cleavage planes is often very useful in identifying the mineral as is the shape of the pieces that brake off. In addition there are a number of other properties that are very useful for selected minerals but not for most these include:

acid dissolution - if it dissolves easily its calcite if you have to gently heat it its dolomite.

radioactivity - if a Geiger counter clicks a lot its a uranium or thorium mineral

dissolves in water - its probably salt - if it tastes salty it is a salt if not its probably borax or gypsum.

And we can even use some optical properties

{W} Like double refractivity?

Yes

A few minerals like calcite have distinctive optical properties like double refraction (:P) Others will fluoresce - rereadiate light if an ultraviolet light is held over them in the dark. While the different physical properties are used for lab and field identification the minerals are defined on their chemical compositions and their crystal habits.

Mineral Chemistry

All the known minerals (over 2500 types) are divided into 8 groups with a ninth for the mineraloids (inorganic mineral like substances that don't fit the exact definition for one reason or another), and a tenth - organics - for those materials (clearly) created by living things.

These groups are:

1) Native Elements and alloys (also carbides, nitrides, & phosphides)

Essentially pure elements or simple alloys of two metals or a metal with carbon, nitrogen, or phosphorous.

2) Sulfides (and selenides, tellurides, arsenides, antimonidies, and bismuthides)

combinations with elemental sulfur, tellurium, arsenic, antimony, selenium, or bisthmuth.

3) Halides (salts :>)

combinations of a alkaline earth or metal and a halogen gas.

4) Oxides and Hydroxides ( no not the cookies)!

Combinations with oxygen or with oxygen and water.

5) Nitrates, Carbonates, and Borates

Combinations with nitrate (NO₃), carbonate (CO₃), or borate (BO₃)

6) Sulfates (and chromates, molybdates, and Wolframates)

combinations with sulfate (SO₄) or with chromate (CrO₄), molybdate (MoO₄), or vanadate (VO₄)

7) Phosphates, Arsenates, and Vanadates (no they aren't rocks that dated Vanna White!)

Combinations involving phosphate (PO₄), arsenate (AsO₄), or vanadate (VO₄)

8) Silicates (6 subtypes based on the arrangements of the silica molecules)

any combination involving the silica molecule (SiO₂). The silicates form 80% of all the minerals and so we need to look at their subgroups a bit. Silica forms tetrahedra (four sided pyramids (including the base as a side), the tetrahedra can be linked at their points in several ways and these are the means of classifying the silicate subgroups.

Nesosilicates - unlinked tetrahedra

Sorosilicates - 2 linked tip to tip

Cyclosilicates - 3, 4 or 6 linked to form rings

Inosilicates - chains there are actually 2 types

single chains where the tetrahedra flip back and forth,

double chains where they form a chain of 6 sided rings

Phyllosilicates - which form sheets (micas are the best examples)

Tectosilicates - which form 3 dimensional networks

And now that I have completely confused you all into silence I'll take questions <G>

{a} can you give samples of the others like you did with phyllosilicates?

Nesosilicates - topaz

Sorosilicates - epidote

Cyclosilicates - beryl (emerald/Aquamarine)

Inosilicates - single chain - amphibole (hornblende); double chain - pyroxene (augite)

Tectosilicates - Quartz

9) Mineraloids - including opal - inorganic materials that don't have a crystal structure.

10)Organics: including jet, coral, amber, and a few others - created by living things and generally still containing the organic carbon.

Rock Forming Minerals
Accessory Rock Forming Minerals
Economic Minerals
Gems and Decrative Minerals.

* DH checks the pulses of his students - ok not dead just asleep <G>

* An chuckles

* P grins

* K thinks he understood most of it pretty okay, but can always read back later :)

{P} DH, is sapphire a Beryl?

no sapphire is a corundum - aluminum oxide just like ruby

{P} OH RIGHT! DOH! :>

{An} Hmmm, what about emerald?

Emerald is beryl as is Aquamarine

Actually ruby is red gem corundum and sapphire is any other color gem corundum

{P} I've only ever heard of it coming in blue or white... and yellow I think...

you ladies prolly keep some corundum handy for working on your talons - the low grade stuff is called emery. Well anytime you hear them talking about an "oriental" <gem> its actually sapphire of the appropriate color

* P nodnods

{P} OH! I know... I once heard someone refer to "yellow" diamonds... is that correct or would they have meant a topaz?

Diamonds come in all colors so there are yellows.

{A} do metallic minerals all fall into the above classes (am thinking of mercury)

Mercury is found as a native element - group one

* anna grins ... should have thought of that *grin*

Over the next few weeks we will go over minerals in groups looking at the specific properties that allow us to identify them. So if you check out the classroom webpage on my complex you can see the upcoming lessons and download the previous ones *G*

{a} where on the page (have lynx and have looked but could not find)

* Sol nods.
it should be no problem for lynx users as its strictly downloadable files or text of upcoming stuff.

Website created: January 22, 1998
Website last updated: October 11, 1998