Minerals
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Minerals. What is a Mineral. A mineral is a natural, usually inorganic solid that has a characteristics chemical composition, an orderly structure, and a characteristic set of physical properties. Characteristics of a Mineral.

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Minerals

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Minerals

Minerals


What is a mineral

What is a Mineral

  • A mineral is a natural, usually inorganic solid that has a characteristics chemical composition, an orderly structure, and a characteristic set of physical properties.


Characteristics of a mineral

Characteristics of a Mineral

Four question to determine if a substance is a mineral or nonmineral.

1. Is the substance inorganic? If it is organic as in coal it is not a mineral

2. Does the substance occur naturally? Brass is a metal that is manufactured and not occurring in nature


What is a mineral continued

What is a Mineral Continued

3. Is the substance a solid in crystalline form? Obsidian is naturally occurring substance, but the atoms in obsidian are not arrange in a regularly repeating crystalline structure. Obsidian is not a mineral

4. Does the substance have a consistent chemical composition? If you say yes to all four answers the it is a mineral


Kinds of minerals

Kinds of Minerals

Silicate Minerals: A mineral that contains a combination of silicate mineral and oyxgen. Quartz has only silicon and oxygen. Other minerals have one or more additional elements like Feldspars.

Nonsilicate Minerals: 4% of Earth’s crust is made up of minerals that do not contain compounds of silicon and oxygen


Identifying minerals

Identifying Minerals

  • ColourThe colour of a mineral is a result of the mineral's light absorbing and light reflecting properties.  These may vary greatly in vitreous minerals with the presence of traces of impurities.  Colour is therefore not always an indication of identity in a vitreous specimen, although it is a more reliable indicator with opaque minerals.


Minerals

  • Flourite not the colour Quartz clear no colour


Different color of quartz

Different Color of Quartz

  • An excellent example of the above is quartz.  Six different varieties of quartz are each a different characteristic colour despite having identical chemical compositions (SiO2):

  • Rock Crystal - colourless Amethyst - purple Citrine - yellow, to orange-brown Smokey Quartz - brown or grey Rose Quartz - pink Milky Quartz - white

    It is also worth remembering that completely different minerals may be the same colour.


Streak for classification

Streak for Classification

  • StreakThe streak of a mineral is the colour of it's powder when rubbed along an unglazed porcelain plate (streak-plate) and may be different from the colour of the mineral itself. Powder may also be produced by scratching the mineral with a knife.  The streak of any given mineral is consistent for that mineral despite any differences in colour.  The six different varieties of quartz above all have the same white streak.


Luster

Luster

  • LusterThe mineral's appearance due to the amount and quality of light reflected from it's surfaces.  Depending on the quality of light a mineral reflects it may appear:

  • Adamantine - the lustre of diamond Vitreous - the lustre of broken glass, e.g.. quartz Subvitreous - as vitreous, less well developed Resinous - the lustre of resin, e.g.. amber and opal Pearly - the lustre of pearl Silky - the lustre of silk in fibrous minerals such as satin spar gypsum Metallic - the lustre of metal Submetalic - as metallic, poorly displayed

  • Depending on the quantity or intensity of light a mineral reflects it may appear:

  • SplendentShiningDull


Mineral luster

Mineral Luster

Galena has a Metallic Mica has a pearly

LusterLuster


Cleavage

Cleavage

Cleavage is the tendency of a mineral to split in certain preferred directions when struck.  These directions are parallel to sheets of atoms in the mineral's atomic lattice. Cleavage is described in terms of: (1) the ease of cleavage, (2) the number and orientations of cleavage planes.  For example:

Gypsum has 'easy' cleavage in one direction Calcite has good cleavage in three directions parallel to its rhombohedral habit and is therefore said to have rhombohedral cleavage.Fluorite has a cubic habit, but it has four cleavage directions which cut across it's corners to leave an octahedral core.  Therefore fluorite has octahedral cleavage.


Minerals with cleavage

Minerals with Cleavage

  • Notice how the sides are smooth where it has broken off on both samples.


Fracture

Fracture

  • The fracture of a mineral is how it breaks other than along cleavage planes.  The fracture may be described as:

  • Conchoidal - a 'shell-like', convex or concave fracture displaying curved fracture or undulation rings concentric to the point of impact and lines or fractures radial from the point of impact, as in quartz, flint and obsidian. Even - a flat fracture, as in chert Uneven - a rough fracture surface.  This is the most common type of fracture. Hackly - jagged sharp ridges, such as in native copper.


Hardness

Hardness

  • HardnessThe hardness of a mineral is measured on Moh's scale.  The scale lists hardness values from 1 to 10.  The numbers may be treated as relative values except for diamond; i.e. fluorite(4) is twice the hardness of gypsum(2).  Diamond(10) is about ten times the hardness of corundum(9).  Each value has a corresponding mineral of thathardness.  Therefore the hardness of a mineral can be tested relative to the minerals on Moh's scale by scratching them with those minerals and other household items of known hardness.


Moh s scale of hardness

Moh’s Scale of Hardness


Minerals harder than glass

Minerals Harder than Glass


Density specific gravity

Density/Specific Gravity

  • The relative density of a mineral is its mass divided by it's volume.  The specific gravity of a mineral is it's mass divided by the mass of an equal volume of water.  In the field it is adequate to simply 'heft' a specimen to determine whether it is of low, high or moderate weight compared to it's size. Silicates and other non-metallic minerals are the least dense with SGs of 2.5 to 3.5 Metallic minerals are denser with SGs from 5 upwards (typically 5 to 8).  Gold has an SG of 19 to 20.


Sgs of some common minerals

SGs of some common minerals


Acid reaction

Acid Reaction

  • Acid ReactionCarbonate minerals react with dilute hydrochloric acid:

  • Calcite effervesces strongly in dil. HCl Malachite also reacts strongly Dolomite reacts weakly in warm dil. HCl or if scratched to produce a little powder prior to applying the acid Siderite reacts weakly


Other special properties

Other Special Properties

  • Magnetism.  Magnetite and pyrrhotite are magnetic and will be affected by a bar magnet.  Other iron minerals are magnetic to a lesser extent, but cannot be tested by an ordinary magnet in the field.  Large iron-bearing masses may affect the orientation of compass needles.  A story I once heard from a petrology lecturer described how they once stopped for lunch on a large magnetite-bearing outcrop and then set off in completely the wrong direction and wasted the rest of the day.

  • Taste.  Halite NaCl, is rock-salt and therefore tastes of salt.  There are other minerals with distinctive tastes, most of them somewhat less pleasant than salt.

  • Odour.  Pyrite, sphalerite and chalcopyrite give a sulphurous 'rotten egg' smell when struck or rubbed on a streak plate.  haematite and limonite may give off an 'earthy' smell (the smell of damp earth) when breathed upon.

  • Pyrite sparkes when struck with a geological hammer.  I have also experienced this effect with haematite.

  • Feel.  Cryslalline minerals will feel rough.  Talc and serpentine often feel unctuous (greasy) or soapy.  Graphite and satin spar gypsum may feel smooth, unctous or soapy.  Graphite is a good conductor of heat and will therefor feel cold.

  • Graphite is also a good condcuctor of electricity (it is used a brushes on electric motors), but this property would not be tested in the field.


Density and specific gravity of minerals

Density and Specific Gravity of Minerals

  • DensityThe relative density of a mineral is its mass divided by it's volume.  The specific gravity of a mineral is it's mass divided by the mass of an equal volume of water.  In the field it is adequate to simply 'heft' a specimen to determine whether it is of low, high or moderate weight compared to it's size. Silicates and other non-metallic minerals are the least dense with SGs of 2.5 to 3.5 Metallic minerals are denser with SGs from 5 upwards (typically 5 to 8).  Gold has an SG of 19 to 20.


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