Mineral by Mark Rollins

Geology and Soil Mechanics

Wigan and Leigh College

COLOUR This is the colour in natural light TRUE COLOUR :of a pure mineral depends on the nature and arrangement of constituent ions. Minerals containing Al, Na, Ca, Mg and Ba= GENERALLY COLOURLESS or LIGHTLY COLOURED. Mineral containing:- Fe, Cr, Mn, Ni, Ti, Va and Cu= ARE COLOURED OFTEN DEEPLY. VALENCY:- can influence colour eg:DIVALENT Fe2+ PRODUCES COMMONLY GREEN COLOURS TRIVALENT Fe3+ PRODUCES RED, BROWN AND YELLOW COLOURS.

BONDING:can affect colour for example Carbon Atoms:DIAMOND = COLOURLESS GRAPHITE = BLACK. Colour may exist in identification, but it can often be unreliable, for example:QUARTZ ROCK CRYSTAL = COLOURLESS ROSE QUARTZ = PINK(stained by Ti) AMETHYST = PURPLE (stained by Mn) CAIRNGORM = BROWN( stained by Fe). However other minerals are more reliable:GALENA = lead grey colour CHALCOPYRITE = Brass yellow colour.

THE COLOUR OF A MINERAL SHOULD ALWAYS BE DESCRIBED WITH THE STREAK, SOMETIMES AS A CONTRAST. COLOUR VARIETIES ARE OBSERVED IN THE FOLLOWING MINERALS:FLUORITE, CALCITE, HEMATITE, HALITE.

Geology and Soil Mechanics

Wigan and Leigh College

HARDNESS The hardness of a mineral (or crystal) is determined by:CHEMICAL STRUCTURE:-. CLEAVAGE:-

WEATHERING:-

IMPURITIES:-

ie the strength of mineral bonds or weaknesses in the structure.

remember that the cleavage planes are weaknesses and therefore do not represent the true hardness.

any mineral which has been affected by weathering will have undergone change and therefore its hardness will be affected.

any minerals which is closely associated with fragments of a weaker/stronger mineral will have a false hardness.

MOHS SCALE OF HARDNESS

When you have scratched a mineral with another mineral, or one of the standard implements, always wipe away any powder in order to see what has been scratched.

Geology and Soil Mechanics

Wigan and Leigh College

FRACTURE To break a mineral in a direction other than cleavage. Fracture can be divided into:CONCHOIDAL:-

EVEN :-

mineral breaks with a CURVED CONCAVE or CONVEX fracture(found in minerals with no cleavage.)

fracture surface is nearly FLAT eg chert.

UNEVEN:-

fracture surface is rough because of minute irregularities.

IRREGULAR:-

a jagged tear eg MICAS

HACKY:-

break composed of sharp jagged elevations not very common.

EARTHY:-

crumbs easily eg LIMONITE.

Most other minerals which display good cleavage will display either UNEVEN or SUB-CONCHOIDAL fractures.

Geology and Soil Mechanics

Wigan and Leigh College

HABIT the mineral is the form which crystals commonly assume. Not necessarily reflecting the “CRYSTAL SYSTEM.” Divided at two:CRYSTALLISED:-

minerals occur as well developed crystals, some minerals display a habit based on crystal systems. CUBIC HABIT eg HALITE, FLUORITE, GALENA, PYRITE. OCTAHEDRAL HABIT:- Eg FLUORITE, MAGNETITE PRISMATIC HABIT:- QUARTZ, HORNBLENDE. TABULAR HABIT:- FELDSPAR, BARITE.

MASSIVE:-

mineral has no discernible shape. Crystalline is term denoting that no definite crystals are developed, but an aggregate of partially formed crystal grains, Cryptocrystalline is a term denoting only traces of crystalline structure.

Geology and Soil Mechanics

Wigan and Leigh College

CLEAVAGE “ THE TENDENCY TO SPLIT ALONG CERTAIN DEFINITE PLANES ‘CLEAVAGE PLANES’ POSSESSED BY MANY MINERALS IS CLOSELY RELATED TO CRYSTALLINE FORM AND INTERNAL STRUCTURE OF THE CRYSTAL.” •

The plane is parallel to the Crystal Plane

•

The cleavage plane is the plane of least cohesion therefore splitting or cleavage easily occurs.

•

Cleavage surfaces generally have smooth, flat surfaces.

Quality of Cleavage:• • • • •

excellent good distinct poor indistinct,

Number of sets of cleavage -NONE - MASSIVE. • • • •

One set two Sets three Sets Four Sets

eg MICA FELDSPAR HALITE, GALENA FLUORITE.

Sets of cleavage need not be equally developed:- eg GYPSUM has Three sets 1 EXCELLENT and 2 POOR.

Geology and Soil Mechanics

Wigan and Leigh College

STREAK THE STREAK OF A MINERAL IS DEFINED AS:“THE FINE GROUND POWDER LEFT BEHIND BY A MINERAL WHEN SCRAPPED ACROSS A STEAK PLATE*”.

*(A PIECE OF WHITE UNGLAZED PORCELAIN.)

“ALTERNATIVELY THE FINE POWDER OF A MINERAL CAN BE SCRAPPED OFF USING A HARDER OBJECT IE KNIFE.” This property can remove the effect of impurities contained within a mineral and give a TRUE COLOUR. This will provide two advantages:• •

the streak is consistent through one mineral type it can be quite different from the mineral colour.

COLOUR AND STREAK SHOULD ALWAYS BE EXAMINED TOGETHER, eg PYRITE:-

HAEMATITE:-

BRASS - YELLOW COLOUR BROWN BLACK STREAK.

RED TO RED-BLACK COLOUR BRIGHT RED STREAK.

ORTHOCLASE is too hard to be scratched by a streak plate but may be powdered by a steel blade.

BEWARE when testing for streak make sure that it is the mineral that is powdered NOT the streak plate.

Geology and Soil Mechanics

Wigan and Leigh College

DENSITY:DENSITY = MASS/VOLUME THE UNITS ARE g/cm3

An alternative is SPECIFIC GRAVITY: this is only a relative measure ie the number of times heavier that substance is than equal volume of water.

SPECIFIC GRAVITY = DENSITY ie if a mineral has a density of 4 g/cm3 then its has a Specific Gravity of 4. Density/specific gravity can be determined by the following method: • weigh specimen, then calculate volume by displacement of water. •

By using heavy liquids and observing whether mineral floats. This method is not usually used because of dangerous liquids used.

•

The easiest way is to estimate density based on comparisons between minerals. Remember to allow for size of specimens.

AS A GUIDE: WHITE MINERALS have a lower densities between 2-3 * exceptions are Barite and Witherite. DARK MINERALS have a moderate densities between 2.8 and 3.5 METALLIC MINERALS have densities between 4-9.

WHY DO MINERALS POSSESS VARIOUS DENSITIES? Basically the heavier the mineral eg Mg Olivines have a molecular weight of 149.68(density 3.2), whilst Fe Olivines have a molecular weight of 203.74(density of 4.3) The more densely packed the atoms the higher the density. Eg calcite and aragonite are both CaCO3 but calcite has a density of 2.71 and Aragonite a density of 2.94.

Geology and Soil Mechanics

Wigan and Leigh College

SOLUBILITY Some minerals will react to aqueous chemical solutions. The compounds which make the minerals will DISSOCIATE- (compounds will chemically separate.) Chemical solutions are:• WATER H2O • HYDROCHLORIC ACID HCL • SULPHURIC ACID H2SO4 We are concerned with Dilute Hydrochloric Acid HCl. ALL CARBONATES(eg CALCITE) will react with HCL and give off CO2 gas, with the creation of Calcium Chloride Salts. HOWEVER we deal with very dilute HCL and the only reactions to concern us include:• CALCITE • DOLOMITE(warm dilute HCL) The reaction is obvious - CO2 is released violently and bubbling is seen - accompanied by a fizzing noise. MANY SULPHIDES (eg GALENA) will react with HCl and give off H2S(Hydrogen Sulphide) Gas, with the creation of salts, eg • GALENA - PbS ----- H2S + PbCL2 + O2 SPHALERITE - ZnS------H2S + ZnCl2 + O2 • The reaction above will occur with dilute HCL - a stronger reaction is noticed with Sphalerite. You will not see a reaction but H2S gas has a ROTTEN EGG ODOUR.

Geology and Soil Mechanics

Wigan and Leigh College

LUSTRE

This is the DEGREE OR INTENSITY of REFLECTED LIGHT.

Basically divided into two types:• METALLIC • NON METALLIC Metallic lustre means a reflection similar to a metal- always bright. Minerals displaying this property are always ORE MINERALS. • • •

GALENA (lead ore) SPHALERITE ( Zinc Ore) MAGNETITE (Iron Ore)

Non metallic lustre can be very varied - from BRILLIANT to DULL. Eg DIAMOND has BRILLIANT or ADAMANTINE lustre. FLUORITE Have a GLASSY or VITREOUS CALCITE lustre. HALITE has WAXY lustre. The lustre of any mineral specimen will be at its maximum “BRILLIANCE” along crystal cleavage surfaces. Where the mineral is massive, or where it is broken along a fracture, the lustre will not be at its most intense. Eg GYPSUM can have a VITREOUS lustre on crystal surfaces or it can have a SILKY lustre in fibrous forms. Other types of lustre include: RESINOUS, PEARLY, SPLENDENT. Minerals showing no discernible lustre are said to be DULL.

Geology and Soil Mechanics

Wigan and Leigh College

Geology and Soil Mechanics

Wigan and Leigh College