FELDSPARS 1
Orthoclase (K)
Albite (Na)
Anorthite (Ca)
TWINNING
- Polysynthetic: Plag, Albite
- Tartsun (Plaid): Microcline
- Simple/Contact: Ortho
- Penetration: andalusite
ANISOTROPIC1
Anisotropic Xerals behave different w/ light. Light ray will split into 2 rays
Types of Extinction
- Random: extinct 4x upon rotation, no well-defined cleavage
- Inclined: most common. extinct when NOT in E-W or N-S.
- Parallel: extinct when long axis paralell to E-W or N-S. Usually uniaxial
- Symmetric: If xeral has symmetric outline and it goes extinct when symmetry is aligned to E-W or N-S
- Undulatory: band of darkness as stage is rotated
INDICATRIX
THE VARIATION OF LIGHT VELOCITY WITHIN XERAL (x, y, z)
* Isotropic xeral: a sphere
Uniaxial vs (Biaxial)
ANISOTROPIC
- Optical axis with singular (double) direction in which light does not split.
- axis is the specific direction in an aniso where the light ray behaves as isotrop.
- UNI: Tetragonal, Hexagonal
- BI: Orthorhombic, Monoclinic, Triclinic
Uniaxial
- light ray enters, splits in one fast and one slow ray. vibrate perpend. to ea. other
- ALL ANISO
- Double refraction. 2 images bc of different speed of rays
- orientation where light does NOT split is when ray (optic axis, extinct) is parallel. to C axis
- ordinary ray follows Snell law. extra. does not (on long axis)
- acc. plate to change the retardation of the mineral grain. > increase in color when slow ray in xeral parall. to slow ray on plate
BIAXIAL
- indicatrix with axes of different lengths (X:Fast, Z: Slow, Y: interm). POS: optic axis acute about “Z”. NEG: optic axis acute about “X”
- two optic axes and two circular sections (extinct)
- ALL ANISO
- 2 optic axes NOT coincident with xtallography axes
- split isogyres
- 2V angle: acute angle bw optic axes
- 2E angle: apparent 2v angle
Uniaxial Optic Sign
= v(ordinary ray) - v(extra ray)
E = extinct direction. if BIGGER, sign is POSITIVE
w = max. birefring. if BIGGER, optic sign is NEGATIVE
max. biref = w-E
Interference Fig
- Bertrand Lens to align optical components and focal planes
- Melatope: emergence of optical axis(es)
- Isogyres: cross. extinction angles
- Isochromes: circle center
- BURP
Si Tetrahedron
SiO4
- Isolated: olivine. Nesosil
- Pairs: epidote. Sorosil.
- Rings: beryl. Cyclosil
- Single Chain: pyroxenes. Inosil
- Double Chain: amphiboles. Inosil
- Sheets: micas/clays. Phyllosil
- 3D rotating network: quartz/feldspars. Tectosilicates
Silicate Characteristics
NESO (ol. garn. Al-silicates, Zirc. stauro. topaz. SiO4)
* bond strength equal in all directions
* Equidimensional habit
* No cleavage
SORO (rare. Si2O7)
* accessory xerals, widely distributed
INO (fiber. pyroxenes(SI2O6) amphib(Si4O11))
* Pyroxenoids: distorted/twisted chains
CYCLO (usually hexagonal. Si6O18)
PHYLLO (leaf. Si2O5)
TECTO (building. SiO2)
Olivine
Mg2SiO4 <> Fe2SiO4
Forsterite <> Fayalite
Non-Silicates
CARBONATES
SULFATES
PHOSPHATES
Gibbs
- you want negative (stable) to nucleate
- surface area overwhelms volume in small embryo then you increase radius…
- ^ increasing radius & volume you outpace DELTAG(surface area)
- CRITICAL RADIUS needed for xtallyzation
- embryo will only form if total free energy of embryo is less than free energy of melt
- DELTAG = DELTAG(surf) + DELTAG(volume)
- Heterogenous Nucleation when xeral uses pre-existing xeral surface for nucleation site
- rapidly growing faces grow themselves out of existence. most prominent faces are the slow ones
