Minerals

What is a mineral?

What is a mineral?

A homogenous, naturally occurring, solid formed by geologic processes with a definable chemical composition and a crystalline structure (orderly arrangement of atoms)

• More than 5,000 are known

• Around 50-100 new minerals are discovered annually

What is biomineralization?

• Biologic activity can facilitate mineral formation 

• Calcite, argonite common biogenic minerals: CaCO3

• Living organism acting in a way to produce minerals 

What is Crystaline?

• Regular atomic order/arrangement inside the mineral 

  • Why is it important?

    • minerals can take many forms

    • Ex: Carbon when formed like sheet makes graphite, and another formation is diamond

• Amorphous-randomly organized, no regular arrangement, mineraloids  (solids) quartz

• Polymorphs-  Minerals who’s compositions are the same but structures are different (multiple forms) calcite

colour

• The part of visible light that is not absorbed by a mineral

• Varieties often reflect trace impurities 

• Not very diagnostic 

Streak

• Color of powdered mineral and can be a different color than the non powdered mineral

• Obtained by scraping a mineral on unglazed porcelain

• Streak color is less variable than crystal color 

Luster

• Quality of light reflected or scattered from the mineral surface 

• 2 subdivisions

  • Metallic–looks like metals

    •  Pyrite

  • Non Metallic-may look shiny 

    • Quartz–vitreous (glass)

    • Sub-vitreous

    • earthy

Hardness

• measures the ability of a mineral to scratch other substances

  • A measure of the strength of atomic bonds 

• Scratching  resistance of a mineral 

  • Harder than a mineral–should be able to scratch it 

• Mohs scale of hardness–talc softest, diamond hardest 

Specific Gravity

• Density relative to equal volume of water 

• Represents the density of a mineral 

• Mineral weight over the weight of an equal water volume 

• We measure specific gravity is “heft”--how heavy it feels

  • Pyrite–heavy (SG~5)

  • Quartz-light (SG 2.65)

• Pyrite “feels” heavier than quartz 

Crystal Habit

• how their crystals grow and appear in rocks

  • Crystal shapes are determined by the arrangement of the atoms within the crystal structure

• External shape/form of single crystals or aggregate of crystals

  • well-formed faces, or poorly formed faces

• External forms can be different than general structure 

• striations (dark and light parallel lines on a crystal face) are used to identify minerals

Cleavage

• planes are smooth, flat, parallel planes within the crystal

  • may show as reflective surfaces on the crystal (parallel cracks that penetrate into the crystal/on the edge or side of the crystal

• Tendency to break along planes of weaker atomic bonds

• Is sometimes mistaken for crystal habit–only outside form 

  • Crystal faces only occur in external surfaces 

• Cleavage is repetitive through the mineral; it often forms parallel steps–continuous throughout the mineral  

Examples of cleavage

• one direction

  • Mica

  • Smoothe plane 

  • Perfect cleavage 

• 2 directions at 90 degrees

  • Feldspar

  • Good cleavage–has 2 at an angle of 90

• 3 directions at 90 degrees

  • Intersect at 90

• 3 directions not at 90 degrees

  •  Calcite

  • Wrong shapes when broken 

• Multiple cleavages–angle can be diagnostic 

Fracture

• A description of the quality of surface other than a cleavage plane along which a mineral break 

• Conchoidal fracture

How do minerals form?

• precipitation directly from a solution or fluid

• crystallization from a molten magma

• precipitation by organisms

Precipitation directly from a solution or fluid

• Could also be gasses–volcanic active region-crystals of sulfur

• Least soluble will form first

• First one to form when you evaporate sea water-calcite

• Further evaporation concentrates them

• Crystallized from open seas-evaporite

  • Mineral deposits formed through evaporation 

Crystallization from a molten magma

• As magmas cool below melting temp, form crystal minerals

• Order of crystallization based on melting point/temp of mineral 

  • Highest goes first 

Precipitation from organisms

• Precipitate minerals from solution–ammonite 

• Ammonite shell made of calcium carbonate or aragonite calcite (polymorphy)

• Diatoms–crystalize shells made of silica–quartz 

What is the composition of the earths crust?

• Oxygen and silicon are the 2 most abundant elements in earth's crust 

• most of the minerals are made of these 2 elements bonded together in various proportions (silicate minerals)

• Compounds form from a cation (silicon) and a anion (oxygen), need to have to form most minerals

  • Most abundant group of minerals we have (Oxygen and silicon)

  • 3500 of 5000 minerals  

What are silicate minerals?

• cations (si^4+) and anions (0²-) typically bond together ionically

  • silicon gives up 4 electrons and oxygen needs to borrow 2 electrons

• so when these 2 bond together, the shape that forms is called a tetrahedral shape which s the fundamental unit of silicate minerals

• Silicon bonded to 4 oxygen 

  • Fundamental unit

Classification of silicate minerals

• Isolated silicate structure–Connected to other atoms of iron and magnesium (olivine)

  • No oxygen shared between tetrahedras 

• Chain silicate-Connected to other tetrahedra to form a chain structure 

  • Share oxygen (2 are shared) 

• Double chain silicate-Chain and combining with another chain

  • Alternates tetrahedras (can share 2 or 3 oxygens) 

• Sheet silicate-All tetrahedras are sharing 3 oxygens with the neighbour tetrahedra

  • Arranged as 2 dimensional sheet–sheet silicate structure (Mica)

• Framework silicates-Forms 3 dimensional framework–share all 4 corners of tetrahedra with other tetrahedra (quartz)

Classification of non-silicate minerals

• Based on anion they contain when they are more complex compound than a simple element 

• Name groups based on anion or anionic group 

• carbonate=carbonates (group is oxygen which are oxides)

• Sulfide-should have sulfur and so on