Lecture Week 4 Mineral structures

Review:

What is the difference between ionic and covalent bonding

In the formula Be₃Al₂Si₆O₁₈ how many atoms of silicon are there (Si)?

What is an isotope

What is a polymorph

What are the three most abundant elements in the crust of the earth?

Most Abundant elements in crust?

Oxygen (O) 46.6%

Silicon (Si) 27.7%

Aluminum (Al) 8.1%

Iron (Fe) 5.0%

Calcium (Ca) 3.6%

Therefore, most minerals are made up of what elements?

7 mineral groups

Each group is defined by a specific chemical constituent

First 6 groups make up the ore minerals

Last group is the rock forming minerals

What determines which mineral will form?

Which elements are available

Most minerals on earth have silicon and oxygen in them

How abundant are the elements

Conditions of formation

Polymorphs..

What are the elements sizes and charges

Each atom has a size (bowling ball vs pingpong ball

if they are ions they have charges

Ionic substitution

Ions will/can substitute with each other in a mineral structure if

The ions are the same charge

The ions are a similar size

Example: Olivine: (Mg, Fe)2SiO4

Group 1 Carbonates

Carbonate ion combines with cation

Carbonate ion (CO32-)

Calcite (CaCO3)

Malachite

(CuCO3 (OH)2)

Group 2 Oxides:

Oxygen combines with a metal

Oxygen ion (O2-)

Hematite Fe2O3

Bauxite Al(OH)3

Group 3 Sulfides:

Sulfur ion combines with a metal

Sulfur ion (S2-)

Galena (PbS)

Group 4 Sulfates:

Sulfate ion (SO42-)

ex// Barite BaSO4

Group 5 Halides:

A metal and non-metal ionically bonded

ex// Flourite CaF2

Group 6 Native elements

(gold, silver, copper etc…)

Group 7 Silicates:

Based on the silica tetrahedron

(SiO₄ ^4-)

Most common and abundant mineral group

includes gemstones and most rock forming minerals

Silicate Structure is based on Silica Tetrahedron

1 silicon atom bonded to 4 oxygen atoms

Creates an anion with a -4 charge

Silicon has a +4 charge

Oxygen has a -2 charge

1 Si = +4 4O is -8 therefore net charge is -4

To get rid of charge two possible things can happen…

Bonds ionically with + charged ions to create stable compound

Or bonds covalently with other tetrahedons

Silicate Structures

Silicate minerals have different properties based on how the tetrahedron join together

Silicate minerals have cleavage along ionic bonds

Silicate minerals have fracture along covalent bonds

Silicate structures

There are 6 different silicate structures

Each structure is different due to the way the tetrahedron bond

Individual tetrahedron connected by cations

Ionic bonding between tetrahedrons and cations but no “planes” of ionic bonding

No cleavage only fracture

Example: Olivine

Tetrahedrons join together in rings (Cyclosilicates)

# of tetrahedron forming ring can vary from 3 to 8

Tetrahedron bond into ring covalently then rings bond together ionically

Examples: Beryl (emeralds and aquamarine), Tourmaline

Tetrahedrons join together in chains (single chain silicates)

Tetrahedron are joined together in chains covalently

Chains of tetrahedron are joined to other chains by cations ionically

Two directions of cleavage at 90o

Example: Pyroxenes

Tetrahedrons join together and create double chains

Tetrahedron are joined together in chains covalently

Chains of tetrahedron are joined to other chains by cations ionically

Two directions of cleavage at 60o/120o

Example: Amphiboles

Review:

List the seven groups of minerals and indicate which are the ore minerals and which are the rock forming minerals

What elements form the silica tetrahedron and how many of each are there?

What is the charge on the silica tetrahedron?

How are the atoms in the Silica tetrahedron bonded?

What is a mineral example for:

Single tetrahedrons, Single chain tetrahedrons, Double chain tetrahedrons, sheet silicates, frameworks silicates

Which of the structures above have cleavage and why?

Tetrahedrons join together and create sheets (sheet silicates)

Sheets are joined by cations

One cleavage plane

Tetrahedrons join covalently (framework silicates)

Includes quartz and feldspar

Primarily fracture

Feldspars are exceptions

Quartz

Tetrahedrons bond with other tetrahedrons covalently

Chemical formula is SiO2

Covalent bonding causes quartz to have fracture…no cleavage

Where does quartz form?

Found in all types of rocks.

Igneous rocks

Volcanic

Thunder eggs

Magma

Metamorphic rocks

veins of milk quartz

Recrystallization

Sedimentary rocks

Cement holding seds together

Precipitates from supersaturated solution

Common Properties of Quartz

Hardness of 7

Some quartz can be a bit softer due to water in the structure

Fracture not cleavage

Quartz Can be ANY color

Forms of Crystal quartz (SiO2)

Milky quartz

Rock quartz

Smoky quartz

Blue quartz

Citrine

Amethyst

Rose quartz

Tigers eye

Aventurine

Rutilated quartz

Microcrystalline quartz

Agate

Jasper (iron oxides)

Onyx

Petrified wood

Carnelian (iron oxides)

Flint

Chert

Feldspars

Aluminum ions can substitute for Silicon ions in tetrahedron structure

Aluminum Ions have a +3 charge where silicon has a +4 charge

Net charge on molecule with aluminum is a -5

Alumina substitutes for every 4th silicon

Combine with cations and other silicate tetrahedrons

Some covalent bonds between silica tetrahedron

Some ionic bonds with Aluminate tetrahedrons

Cleavage planes (2)

Feldspars Composition

Range in composition from calcium, sodium or potassium rich

Ca-Na range is a solid solution series

Plagioclase feldspar…
Solid solution series

Anorthite Ca rich

Albite Na Rich

Orthoclase feldspar (microcline) Potassium rich