Term

Definition
1) The velocity of light varies depending on direction through the mineral
2)They show double refraction (the light entering the mineral is split into two rays with different velocities)
3) Each Anisotropic mineral has either 1 (uniaxial) or 2 (biaxial) optic axes along which they act isotropic. 


Term
Optically Uniaxial Minerals 

Definition
1) Minerals that belong to the Hexagonal and Tetragonal systems
2) Have one optic axes 


Term
Optically Biaxial Minerals 

Definition
1) Minerals that belong to the orthorhombic, monoclinic, and triclinic systems.
2) Have two optic axes 


Term

Definition
1) The colors produced because of light split into two rays passing through a mineral under crossed polars. 


Term
Ray Retardation (Δ)
Δ = d(V/V_{s}V/V_{f}) 

Definition
1) The distance that the slow ray is behind the fast ray after both have excited the crystal.
2) The magnitude of the retardation depends on the thickness of the crystal plate (d) and the differences in the velocity of the slow ray (V_{s}) and the fast ray (V_{f}) 


Term
Anisotropic Double Refracted Light Rays 

Definition
1) Plane polarized and vibrate at right angles to each other.
2) Ray with lower index of refraction (n) is called the fast ray, and the ray with the higher index of refraction is called the slow ray (retarded). 


Term
Birefringence (δ)
δ = (n_{s}  n_{f}) 

Definition
1) The difference between the indices of refraction of the slow (n_{s})_{ }and fast (n_{f}) rays.
2) Numerical value depends on path of light through the mineral.
3) Paths along optic axes show zero Birefringence.
4) May vary depending on wavelength of light. 


Term

Definition
Two waves travel in the same plane and the same direction.
1) In Phase: two rays are vibrating at right angles, the resolved components are in opposite directions and therefore destructively interfere and cancel each other. No light passes through the polarizer and the mineral grain appears dark
2) Out of Phase: resolved components are in the same direction, light constructively interferes and light passes the upper polarizer. 


Term
Orders of Interference Colors 

Definition
Go through a repeating sequence, with change from red to blue occurring at retardations of approximately 550, 1100, and 1650 nm.
1st order: less than 550 nm
2nd order: between 550  1100 nm
3rd order: between 11001650 nm
The higher order, the more washed out the color 


Term

Definition
The mineral is extinct when the cleavage or length is aligned with one or the other of the cross hairs. The extinction angle is zero degrees 


Term

Definition
The mineral is extinct when the cleavage or length is at some angle to the cross hairs. 


Term

Definition
The mineral displays either two cleavage directions or two distinct crystal faces to which two extinction angles can be measured, one from each cleavage or crystal face. If the two extinction angles are the same, the mineral displays symmetrical extinction. 


Term

Definition
Mineral does not display an elongate habit or prominent cleavage. They go extinct once in every 90° of stage rotation, but no cleavage or elongation is available from which to measure an extinction angle. 


Term

Definition
Extinction in a grain follows an irregular or wavy pattern. Due to deformation of rocks, grains may be bent or strained and parts of a single grain are in slightly different orientations and therefore go extinct at different times. 


Term

Definition
Anisotropic minerals that display a change of color as the stage is rotated in plane light (upper polarizer removed). It is produced because the two rays of light are absorbed differently as they pass through the colored mineral and therefore have different colors. 


Term
Extraordinary Ray (ε Ray) 

Definition
Ray in Uniaxial Mineral that vibrates perpendicular to the ordinary ray (ω ray) and parallel to the caxis. Velocity varies upon direction.
Optically positive: index of extraordinary ray (n_{ε}) is greater than index of ordinary ray (n_{ω}).
In other words, if the ε ray is slow, the mineral is optically positive and therefore negative if fast.



Term

Definition
Ray in Uniaxial Mineral that vibrates perpendicular to the caxis. Ray has the same velocity regardless of path through mineral.
Optically Negative: index of Ordinary Ray (n_{ω}) is greater than index of Extraordinary Ray (n_{ε}) 


Term

Definition
Geometric figure constructed so that the indices of refraction are plotted as radii that are parallel to the vibration direction of the light. Includes a principle, circular, and random section. 


Term
Indicatrix: Principal Section 

Definition
A section through the indicatrix that includes the optic axis, which is an ellipse whose axes are n_{ω} and n_{ε}. 


Term
Indicatrix: Circular Section 

Definition
A section through the indicatrix perpendicular to the optic axis whose radius is n_{ω}. 


Term
Indicatrix: Random Section 

Definition
Any random cut through the indicatrix produces an ellipse whose axes are n_{ω} and n_{ε}' where n_{ε}' is between n_{ω} and n_{ε.} 


Term

Definition
Typically prismatic and either elongate or stubby parallel to the caxis. The faces are commonly combinations of prisms parallel to the caxis , pinacoids perpendicular to c, and pyramids, although other forms are possible. (See Tetragonal: case 13)



Term

Definition
The crystal is cut perpendicular to the optic axis (circular section) so that light follows the optic axis and travels with index n_{ω.}
If polarizers are crossed, the section should be uniformly dark on rotation, so no extinction angle can be measured. 


Term

Definition
Crystal is cut parallel to the optic axis (Principle Section with ε axis parallel to the length of the crystal and the ω axis perpendicular to the length). Light travels through the crystal perpendicular to the optic axis. Must be parallel extinction b/c vibration directions for ω and ε are parallel to the width and length. 


Term

Definition
Crystal is cut along a random angle. Birefringence is intermediate. All three cleavages are visible. n_{ε} intermediate. Extinction to the prismatic cleavage is parallel, symmetrical, or anything in between. The ω_{} ray vibrates parallel to the trace of the pinacoidal cleavage, so extinction is always parallel to cleavage. 


Term

Definition
Common forms: prisms, pinacoids, pyramids, and rhombohedrons, although a number of other forms are possible.
Common cleavages: prismatic, pinacoidal, and rhombohedral
See Hexagonal: Case 13 


Term

Definition
Three cleavage planes that intersect at angles other than 90°
[image] 


Term
Prismatic and Pinacoidal Cleavage 

Definition
Crystal with three prismatic {100} cleavages that intersect at angles of 60° and 120° and a {001} pinacoidal cleavage at right angles to the caxis. 


Term
Interaction of light with minerals 

Definition
1) Reflection: luster
2) Speed
3) Index of Refraction: n = velocity of light in vacuum/ velocity of light in mineral; measures how effective a mineral is at bending light as it passes from one material to another. 


Term

Definition
sinθ_{1}/sinθ_{2} = n_{2}/n_{1} = speed of light in mineral/ seed of light in air
large difference between n of mineral and n of its surroundings means that it will stand out and have high relief. Small difference means it will have low relief. 


Term

Definition
Increase in index of refraction (n) with decrease in wavelength (λ) 


Term

Definition
Decrease in index of refraction (n) with a decrease in wavelength (λ).
Example: Halides; some light absorbed by mineral 


Term

Definition
1) Periodic repetitions along vectors
2) Translation of ions  achieved in:
1D: Row Lattices
2D: Plane Lattices
3D: Space Lattices 


Term

Definition
An imaginary pattern of points (or nodes) that has an environment that is identical to any other node in the pattern; repetition of a patter 


Term

Definition
Produced by translations along two vectors repeated with constant distances and angles. 


Term

Definition
 Oblique Net
 Rectangular Net
 Centered Rectangular Net
 Diamond Net
 Hexagonal Net
 Square Net



Term
Plane Lattice: Oblique Net 

Definition
p = primitive, nodes only at corners 


Term
Plane Lattice: Rectangular Net 

Definition
P = primitive, nodes only at the corners



Term
Plane Lattices: Centered Rectangular
and
Diamond Net 

Definition
c = centered, node in the middle and at the corners (diamond is primitive) 


Term

Definition
Stacking of an oblique net (or plane lattice) at an arbitrary angle results in primitive triclinic lattices



Term
7 Lattice Systems
In order from least to most symmetric 

Definition
1) Triclinic: No symmetry
2) Monoclinic: 1 diad symmetry
3) Orthorhombic: 3 perpendicular diads
4) Rhombohedral: 1 triad
5) Tetragonal: 1 tetrad
6) Hexagonal: 1 hexad
7) Cubic: 4 triads 


Term

Definition
Described by vectors of unequal length, as in the orthorhombic system. All three vectors are not mutually orthogonal (δ,β,α ≠ 90°). Least Symmetric and is the only lattice type that has no mirror planes. Mineral examples include plagioclase, microcline, rhodonite,
turquoise, wollastonite and amblygonite, all in triclinic normal (bar 1). 


Term
Monoclinic Crystal System 

Definition
Described by vectors of unequal length (a≠b≠c) as in orthorhombic system. They form rectangular prism with a parallelogram as its base.(δ,α=90°,β≠ 90°). Two lattices exist: Primitive and Centered[image] 


Term
Orthorhombic Crystal System 

Definition
Lattices result from stretching a cubic lattice along two of its orthogonal pairs. (a≠b≠c) and (δ,α,β=90°). Lattices include: simple orthorhombic, base centered, body centered, and face centered. Common minerals include Olivine, Aragonite. 


Term
Tetragonal Crystal System 

Definition
Two Lattices: Simple Tetragonal, Centered Tetragonal. Common minerals: rutile, zircon, pyrolusite, mulfenite, chalcopyrite. (a=b≠c) and (δ,α,β=90°). 


Term

Definition
has only one lattice type: simple hexagonal. Common Mineral: Graphite.(a=b=c) intersecting at 120°. Has 4 Miller indices. 


Term

Definition
"cubic" crystal system where unit cell is a cube. The most common and simplest shapes found in crystals and minerals. Three varieties of crystals: simple cubic, bodycentered cubic, and facecentered cubic. Native minerals are common (lead, aluminum copper, gold, iron, chromium) 

