EarthMat Midterm 1

The hell is a mineral

-Inorganic

-Naturally occuring

-Specific atomic structure

-Definite (but not fixed) chemical composition

-Homogenous

-Solid

Cleavage vs fracture vs crystal face

• Cleavage- breakage along weak points

• Fracture- breakage along irregular faces (because strength is equal in all directions (isotropic))

• Crystal face- planar surface that forms because of how a crystal grows

Isotropic vs anisotropic

• Isotropic vs anisotropic

  • Isotropic- same RI in all directions, never pleochroic, extinct in XPL

  • Anisotropic- different RI in different directions

What controls substitution

-Valence electrons

-Radius

-Temperature

-Availability

(most important 2 are _____)

Miscibility gap

Region where a material exists as 2 or more phases

Cubic & hexagonal crystal systems

Cubic- all axes same length & 90 degrees (isometric, every axis is c-axis)

Hexagonal- all axes same length & 120 degrees (uniaxial)

Tetragonal & Orthorhombic crystal systems

Tetragonal- 2 axes same length & 90 degrees (uniaxial)

Orthorhombic- all axes different length & 90 degrees (biaxial)

Monoclinic & Triclinic crystal systems

Monoclinic- all axes different lengths & only one 90 degree angle (one tilted axis causes inclined extinction) (biaxial)

Triclinic- all axes different lengths & different angles (biaxial)

Polymorphism

Some minerals have the same atoms but different structure because of PT differences in formation (ex: coal and diamond)

Explain what XPL is

2 analyzers oriented perpendicular to each other so only light that has been refracted off a mineral can make it thru the second analyzer

E=hc/λ

• h= planck’s constant

• c= velocity of light

• λ= wavelength

• E= energy

+ because speed & wavelength are related, speed & color are related

Pleochroism

When you change the angle that the light hits a mineral, you change the wavelength (color)

Refractive index (RI)

[velocity of light in air] / [velocity of light in mineral] 

3 circumstances of extinction

-Isotropic mineral

-Looking down the optic axis (so mineral appears isotropic)

-Either slow or fast ray is aligned with the vibration direction

types of extinction

Parallel

Inclined

Symmetrical

Undulatory

Birefringence

Decomposition of a light ray into a fast and slow ray

-Different minerals have different max birefringence

Interference colors

Result of the recombination of fast & slow rays at the analyzer

Optic axis

The direction of light propgation where RI is the same in all directions for a specific mineral

Optically postive= OA runs parallel to long side

Optically negative= OA runs parallel to squat side

Optical indicatrix

An ellipsoid model where the axes describe the variation of RI values in a mineral

Uniaxial- 1 optical axis

Biaxial- 2 optical axes

Methods of identifying minerals

• Low tech

  • Streak color

  • Luster

  • Cleavage

  • Habit

  • Hardness

  • Specific gravity

  • Refraction

• Equipment required

  • Refractive index

  • Effervescence

  • Radioactivity

  • Petrography/optical (thin sections)

• Major equipment required

  • Mass spectrometry/spectroscopy

  • Electron microscope

3 types of bonding

Covalent- strongest

Ionic- medium

Van der Waals- weakest

Common elements in the crust

-Oxygen ~50%

-Silicon ~25%

-Magnesium ~25%

Factors that control substitution

-Atomic radius

-Temperature

-Radius

-Availability

Coordination number

The number of anions pulling the cation in the center