Optical Midterm 2

how do we characterize light traveling through biaxial minerals?

• maximum birefringence (delta) = n_y-n_a

• fast ray (n_a<n_a’<n_B)

• slow ray (n_B<n_y’<n_y)

optic normal

perpendicular to X-Z (optic) plane (maximum IC)

acute bisectrix (Bxa)

indicatrix axis that bisects 2V angle (low to moderate IC)

obtuse bisectrix (Bxo)

indicatrix axis that bisects obtuse angle (low to moderate IC)

optically neutral

2V=90, OA are perpendicular to each other

optically negative (Bxa = x axis)

2Vx < 90, 2Vz >90

optically positive (Bxa = z axis)

2Vx > 90, 2Vz < 90

= 180

2Vx + 2Vz

parts of a biaxial IF

• 2 melatopes

• isochromes (bands of IC)

• isogyres (regions of extinction)

flash IF (optic normal)

OA parallel to stage

centered optic axis figure

2V large enough that other optic axis is out of view (minimum IC)

how is the 2V angle of a biaxial mineral determined

orient optic plane to 45º NE-SW and look at degree of curvature of isogyre and how far the isogyres split apart

XYSi3O8

chemical composition of feldspars

NaAlSi3O8 (feldspar)

albite

KAlSi3O8 (feldspar)

k-spar

CaAl2Si2O8 (feldspar)

anorthite

albite, oligoclase, andesine, labradorite, bytownite, anorthite

compositions of feldspar

What are high and low feldspars?

• disorder - random distribution of cations in tetrahedral sites (volcanic)

• order - cation occupies specific sites systematically (plutonic)

highly disordered feldspar

• sanidine

• monoclinic

• volcanic rock

somewhat disordered feldspar

• orthoclase

• monoclinic

• shallow plutonic rocks

• some metamorphic rocks

completely ordered feldspar

• microcline

• triclinic

• deep plutonic rocks

• high-grade metamorphic rocks

endmembers change (albite, k-spar, anorthite)

why do the optical properties of feldspar vary with chemical composition?

what are distinguishable optical properties for feldspar

• twinning

• extinction

• habit/angle

• cleavage/fracture

carlsbad, albite/polysynthetic, crosshatch

what types of twins are present in feldspar?

how does the electron probe work?

fires a focused beam of electrons at a sample, causing the atoms within the sample to emit characteristic X-rays that are then detected and analyzed to determine the elemental composition of the sample at a very precise location

secondary electrons (SE), backscattered electrons (BSE), x-rays

types of signals produced by the interaction of the electron beam with the sample surface

proportional to the mean atomic number of the sample

what does the intensity of a mineral in a BSE image correspond to?

a higher atomic number means it will appear brighter

what minerals will be very bright in BSE? what will be darker?

how are characteristic x-rays generated?

high energy electron collides with an inner shell electron of an atom in a target material, knocking it out and creating a vacancy; an electron from a higher energy level then fills this vacancy, releasing the energy difference as an x ray photon with a specific energy characteristic of the element involved

what is energy dispersive spectroscopy (EDS; how does it tell about the composition of your mineral)

determines the elemental composition of a material by analyzing the energy of x-rays emitted from a sample when bombarded with a focused electron beam

what is wavelength dispersive spectroscopy (WDS; how does it tell you about the composition of your mineral)

used to analyze the elemental composition of a material by measuring the specific wavelengths of x-rays emitted by the sample when bombarded with electrons

what are standards needed for EPMA?

materials with well-known elemental compositions, typically similar in matrix to the samples being analyzed, used to calibrate the instrument and enable accurate quantitative analysis

the sum should be around 100% (given error)

how do you determine if the total weight percent oxide is reasonable?

chemistry and crystal structure of olivine

• (Mg,Fe)2SiO4

• forsterite (Fo) - Mg2SiO4

• fayalite (Fa) - Fe2SiO4

• orthorhombic (2/m 2/m 2/m)

how do orthopyroxenes and clinopyroxenes differ

orthopyroxenes (orthorhombic), clinopyroxenes (monoclinic), this difference in structure arises from the presence of larger cations like calcium in clinopyroxenes, which alters the arrangement of the silicate tetrahedra chains within the crystal lattice

distinguishing optical properties of pyroxenes

• olivine

  • colorless in PPL

  • high relief and positive

  • third order IC

  • no cleavage, irregular fracture

  • alteration in cracks (iddingsite, serpentine)

• orthopyroxene

  • 50–132° 2v

  • 2 cleavages at 90º

  • extinction is parallel to both length of crystal and cleavages

  • first order colors

• clinopyroxene

  • 25–70° 2v

  • second order colors

  • inclined extinction

how do the optical properties vary with chemical composition in olivine

• Mg rich – higher 2v, + optic sign

• Fe rich – Higher relief, higher IC, - optic sign

staurolite, chlorite, chloritoid, cordierite, garnet, muscovite, biotite, quartz, feldspar, kyanite, sillimanite, andalusite

minerals commonly found in pelitic rocks

distinguishing characteristics of pelitic minerals

• andalusite

  • chiastolite

• kyanite

  • single or multiple twins

  • elongate/bladed

• silimanite

  • slender prismatic/fibrolite

  • second order colors

• staurolite

  • honey yellow/brown pleochroic

  • poikiloblastic

• cordierite

  • yellow pleochroic halos

  • numerous inclusions

• chlorite

  • pleochroic green

  • anomalous blue/khaki

• chloritoid

  • pleochroic green

  • hourglass shape inclusions

talc, tremolite, diopside, forsterite, wollastonite, periclase, monticellite, melilite, garnet, vesuvianite, calcite, dolomite, quartz, epidot

minerals commonly found in calc-silicate rocks

distinguishing optical properties of calc-silicate minerals

• talc

  • third order IC

  • perfect cleavage on (001)

• tremolite-actinolite

  • colorless-pale/dark green

  • elongate/bladed

• wollastonite

  • bladed

  • up to first order yellow

• titanite

  • wedge shaped

  • creamy white IC

• clinozoisite - epidot

  • anomalous blue/green

• vesuvianite

  • up to first order white

A0-1B2C5T8O22(OH)2

general chemical formula for amphibole

optical properties for amphiboles

• commonly pleochroic

• two good cleavages intersecting at 56° and 124°

• slender prismatic to bladed crystals, diamond-shaped

variance in optical properties between amphiboles

• sodic amphiboles can be blue/purple

• inclined extinction

  • gudrite-anthophyllite is parallel

• hornblende-clinoamphibole

  • second order IC