Becke's Line, RI Testing, and Silicate Mineral Notes

• Becke's Line and Refractive-Index Testing in Petrographic Microscopy

  • the mineral has a higher or lower RI than the mounting medium.

  • Becke's effect (Becke's line) is used to compare the refractive indices (RI, n) of a mineral with its mounting medium to obtain relative RI values.

  • The technique was developed by Sir Friedrich John Karl Becke, Austrian mineralogist and petrographer.

  • Principle: By observing movement of light at the mineral–mounting medium interface under concentred illumination, you can deduce whether

• Central illumination method is commonly employed for determinations of RI (n) of a mineral.

• The Becke line is a bright line seen just inside the boundary of the mineral–liquid interface when focus is adjusted. Its movement indicates the relative RI of mineral vs mounting medium.

Step-by-step Becke test (as described in the transcript)

1) Focus the mineral in contact with mounting medium under reduced illuminated area (iris diaphragm) to sharply focus the mineral–medium contact.
2) Observe the Becke line: a bright narrow line inside the mineral–liquid boundary.
3) Focus adjustment: raise the focusing tube by increasing the distance between the mineral and objective.
4) Watch the Becke line carefully as you move the focus.
5) If the Becke line moves toward the mineral body, the mineral has a higher RI than the mounting medium (positive relief).
6) If you then lower the stage or observe movement away from the mineral boundary and toward the margin, the mineral has a lower RI than the mounting medium (negative relief).

• Key interpretation: movement of the Becke line toward the mineral indicates n(mineral) > n(mounting medium); movement toward the mounting medium indicates n(mineral) < n(mounting medium).

• For an accurate RI determination, an immersion method with liquids of known RI is used. The immersion method is especially favored for precise n determinations.

Becke line and immersion liquids (examples and RI values)

• The following liquids are used in Becke immersion tests with approximate RI values given in the transcript:

  • Kerosene: $n \approx 1.448$

  • Clove oil: $n \approx 1.530$

  • x-monobromnapthalene: $n \approx 1.658$

  • Methylene iodide: $n \approx 1.740$

  • Methylene iodide saturated with sulfur: $n \approx 1.778$

• A note from the transcript: Becke's line movement is used to refine the RI determination by replacing the mounting medium with an oil of slightly varying RI until the oil RI is just above or below the mineral RI to establish the mineral's RI relative to the liquid.

• Immersion method: used for accurate determination of the mineral grain RI; the grain is placed in an oil with RI that brackets the mineral's RI until the mineral appears to have the same RI as the immersion liquid (low relief) or a higher RI (positive relief).

Surface relief in thin sections (Becke context)

• Surface relief depends on the difference in RI between the mineral and mounting medium.

• When the RI difference is large, the surface relief is high; when the RI of the mineral equals the mounting medium, relief is low.

• Example: Canada balsam (RI ≈ $1.54$) and Halite (RI ≈ $1.54$ in the notes) would show low relief due to equal RI with the mounting medium.

• The Becke line method helps in estimating relative RI and thus the optical properties of minerals in thin section.

Olivine Group (Nesosilicate family context in transcript)

• General description: Olivine is a granular, orthorhombic mineral composed of independent tetrahedra (SiO4) linked as a single-tetrahedron species in the olivine structure.

• Mineral formulas and varieties mentioned:

  • Forsterite: {Mg}2 {SiO}4

  • Fayalite: {Fe}2 {SiO}4

  • Monticellite: $ext{CaMgSiO}_4$ (mentioned among related olivine-type minerals)

  • Liebenbergite, Kirschsteinite, Glaucochroite, Tephroite, Laibunite are listed as related members/chemistries (various Fe/Mg-Ca substitutions).

• General olivine formula (as per transcript): ({Mg,Fe})2 {SiO}4

• Physical properties (typical values provided in notes):

  • Streak: Colorless to pale

  • Luster: Vitreous

  • Cleavage: Absent

  • Hardness: $6 \text{ to } 7$

  • Specific Gravity: $3.2 \text{ to } 4.3$

  • Crystal system: Orthorhombic

• Optical properties and relief:

  • Polarized light: Faint in plane-polarized light (PPL)

  • Relief: High

  • Pleochroism: Absent

• Paragenesis and occurrence:

  • Common in ultrabasic to basic igneous rocks (e.g., peridotites, gabbros, basalt)

  • Associated minerals include amphibole, pyroxene, and sometimes metamorphic rocks.

• Gem and industrial use:

  • Peridot is the gem variety of olivine; green to olive color; important gemstone mining locations include Egypt, Burma (Myanmar), and Brazil.

• Immersion and Becke context: Olivine-rich rocks are used in discussions of refractory materials (see later sections).

Refractory Materials and Gem Varieties (Olivine discussion context)

• Refractory brick uses: Olivine is used in refractory brick manufacture.

• Gem variety: Peridot is a gem variety of olivine; attractive for its pale green color.

• Other notes: Pure forsterite can be found in metamorphosed impure dolomite and can be serpentinized to ophicalcite (Calcite + Serpentine).

Silicate Structures (Nesosilicate, Sorosilicate, Cyclosilicate)

• Nesosilicate (island silicates): Isolated tetrahedra, e.g., Olivine, Garnet, Zircon.

  • General description: Each SiO4 tetrahedron is isolated (no shared oxygens) → high density, compact packing, high specific gravity, hard minerals.

  • Greek origin: “Nesos” means island.

• Sorosilicate: Silicate tetrahedra linked in pairs by sharing one oxygen each, forming pairs that connect via cations (Ca, Na, Al^3+, Fe^3+, Mn).

  • Examples: Epidote, Idocrase.

• Cyclosilicate: Silicate tetrahedra linked in rings; Si:O ratio is 1:3 in rings.

  • Examples: Wollastonite, Beryl.

  • Examples notes: Be aware of the ring structure and the crystalline frameworks formed.

• In the transcript, the above three categories are summarized with the petrological examples for each family.

Pyroxene Group (Silicate Structures – Single Chain), in transcript context

• General description: Pyroxenes are an important group of rock-forming ferromagnesian silicates. They commonly include Fe, Mg, and Ca with occasional Na, Li, Mn.

• Structural feature: The silicate tetrahedra form a single chain by sharing two of their four oxygens, producing a chain that runs parallel to the c-axis.

• Classification by crystal system and composition:

  • Orthopyroxene (usually orthorhombic) vs Clinopyroxene (usually monoclinic).

  • End-member representation (approximated in the transcript): end-members in a triangular diagram include Wollastonite (CaSi2O6), Enstatite (MgSiO3), and Ferrosilite (FeSiO3) as end points guiding solid solution across the group.

• Notation and character: The transcript presents a ternary-diagram style context where apices correspond to 100% Wollastonite, Enstatite, and Ferrosilite; orthopyroxenes lie toward the base triangle, with clinopyroxenes occupying other regions depending on Ca vs Mg vs Fe substitution.

• Practical notes: Pyroxenes are central to igneous and metamorphic rocks and show characteristic cleavage and interference colours under polarized light (not detailed here but implied by the standard petrographic context).

Garnet Group (Nesosilicate) – General Properties and Variants

• Garnet as a rock-forming mineral in metamorphic and igneous rocks; generally occurs as trace to small amounts in many rocks (noted in schists and gneisses).

• General formula: X3Y2 {Si}3 {O}12 where X is a divalent cation (e.g., Ca, Mg, Fe, Mn) and Y is a trivalent cation (e.g., Al, Fe, Cr, Ti).

• Physical properties (typical ranges given in notes):

  • Crystal system: Cubic (Isometric)

  • Habit/Form: Dodecahedral crystals (Euhedral)

  • Luster: Vitreous; Transparency: Transparent to translucent

  • Streak: Colorless

  • Hardness: $6.5 \text{ to } 7.5$ (often ~7)

  • Specific Gravity: $3.1 \text{ to } 4.3$

  • Cleavage: Absent; Fracture: Uneven

  • Colour: Varies with type (overall colorless to various colors in gem varieties)

• Common mineral types (end-members/varieties):

  • Grossular {Ca}3 {Al}2 {Si}3 {O}12 (varies)

  • Pyrope {Mg}3 {Al}2 {Si}3 {O}12 (Bloody/ pinkish red)

  • Almandine {Fe}3 {Al}2 {Si}3 {O}12 (brownish red)

  • Spessartine {Mn}3 {Al}2 {Si}3 {O}12 (orange)

  • Andraolite {Ca)3 ( Fe}2 {Si}3 {O}12 (varies)

  • Uvarovite {Ca}3 {Cr}2 {Si}3 {O}12 (green)

• Optical features and paragenesis:

  • Garnets occur in various rock types, including meta- and igneous rocks.

  • Paragenesis indicates garnet formation in metamorphic zones; particular garnet types are common in pelitic rocks and calc-silicate rocks.

  • Some garnet varieties occur as gem materials (e.g., Bohemian garnet, Cape Ruby, Cinnamon stone).

• Gem and industrial uses:

  • Garnet is mined for use as an abrasive in industrial applications.

  • Gem varieties are valued in jewelry and collection pieces.

• Specific notes on end-member occurrence:

  • Pyrope occurs in ultrabasic rocks (garnet peridotites) often with olivine and pyroxene; associated occurrences include Saxony and the Czech region.

  • Almandine is common in medium-grade metamorphic rocks (schist, gneiss) and pelitic sediments or in some granites.

  • Grossular and Andradite are common in metamorphic igneous rocks and skarns; demantoid (green) and other gem varieties are noted within Andradite types.

• Uses and distribution:

  • Garnet is mined for industrial abrasives and for gemstones; gem varieties have historic and regional significance (e.g., Bohemian Garnet, Cape Ruby).

  • Occurrence is widespread in metamorphic rocks and some igneous contexts.

Feldspar Group (Alkali Feldspar and Plagioclase)

• General framework: Feldspars are tectosilicates (framework silicates) in which each SiO4 tetrahedron shares all four oxygens with adjacent tetrahedra, forming a 3D framework. Large interstices between the framework are occupied by alkali or alkaline-earth ions (e.g., K+, Na+, Ca2+) depending on the feldspar type.

• Alkaline feldspars (Orthorhombic) examples:

  • Orthoclase (KAlSi3O8)

  • Sanidine (KAlSi3O8, high-temperature polymorph)

  • Microcline (KAlSi3O8, low-temperature, cross-hatching twinning)

  • Albite (NaAlSi3O8) is often considered alkali-feldspar end-member in some contexts but listed separately here as Na-dominant plagioclase component

• Plagioclase feldspars (Triclinic) include a solid-solution series from Albite to Anorthite:

  • Albite (NaAlSi3O8)

  • Oligoclase

  • Andesine

  • Bytownite

  • Labradorite (intermediate composition, often labradorite is a specimen-related name)

  • Anorthite (CaAl2Si2O8)

• Structural and compositional notes:

  • The tectosilicate framework features tetrahedra linked by shared oxygens; alkali or calcium/aluminium ions occupy interstitial sites.

  • The alkali feldspars crystallize in the orthorhombic system; plagioclase feldspars crystallize in the triclinic system.

• Physical and optical properties (as described in transcript):

  • Color: Albite (white); Plagioclase typically gray; Orthoclase pink or salmon depending on composition; Labradorite shows labradorescence in some varieties; Microcline often pale green (Amazonite).

  • Luster: Vitreous; Form: Tabular as two cleavage directions.

  • Cleavage: 2 distinct sets; Fracture: Even to uneven.

  • Relief: Low to moderate in many instances; Pleochroism: generally absent in Albite; Variations exist among feldspars.

  • Under crossed nicols (BxN): Anisotropism observed; Twinning features differ: Plagioclase commonly shows lamellar twinning; Microcline shows cross-hatching.

  • Interference colours: Feldspars exhibit grey to white interference colours (low order).

• Paragenesis and occurrences:

  • Plagioclases are abundant in most igneous rocks, metamorphic rocks (except ultrabasic rocks), and arkose in sedimentary contexts.

  • Alkali feldspars appear prominently in granitic and felsic rocks and pegmatites.

  • In Deccan Traps context (regional note): plagioclase phenocrysts are abundant; alkali feldspar grains are large in pegmatites; feldspars occur in schists and gneisses in metamorphic settings; Arkose (sedimentary) contains feldspars.

• Uses:

  • Industrial: Feldspars are used in the manufacture of glass, porcelain, sanitary ware, and as fillers in paints, plastics, rubber, and adhesives.

  • Gem varieties and their common names:

  • Orthoclase as Moonstone

  • Labradorite as Sunstone

  • Microcline as Amazonite (green)

• Notes on zoning and polymorphism:

  • The alkali-plagioclase solid-solution is temperature dependent; alkali feldspars develop very large grains in pegmatites.

  • The plagioclase feldspar solid-solution shows a compositional range between Albite and Anorthite with intermediate members (Oligoclase, Andesine, Bytownite, Labradorite).

• Microstructure and textures:

  • Under PPL, feldspars appear colorless to pale; cross-hatched twinning is characteristic of Microcline; twinning and zoning are common, particularly in plagioclases.

Metastructure: Silicate Classes and Distinguishing Features

• Nesosilicates (island silicates): Isolated SiO4 tetrahedra; e.g., Olivine, Garnet, Zircon.

  • High density and hardness due to isolated tetrahedra.

• Sorosilicates: Pairs of SiO4 tetrahedra sharing one oxygen; cationic linkage via Ca, Na, Al^3+, Fe^3+, Mn; e.g., Epidote, Idocrase.

• Cyclosilicates: Tetrahedra linked in rings; Si/O ratio 1:3; e.g., Wollastonite, Beryl.

• Pyroxenes: Single-chain silicates (SiO3 units linked into chains); key members and their end-members were discussed in the Pyroxene section.

• Garnets: Nesosilicates with X3Y2Si3O12 structure; various end-members (grossular, pyrope, almandine, spessartine, andradite, uvarovite) with gem varieties noted.

Related Notes: Practical Contexts and Implications

• Becke’s line technique provides a practical, microscopic approach to compare RI values and identify minerals in petrographic thin sections.

• Immersion liquids with known RI enable accurate RI determination and help resolve ambiguous cases when mounting media RI is close to that of the mineral.

• Surface relief and Becke line behavior depend on RI contrasts; this is essential for interpreting mineral textures in thin sections.

• Olivine, feldspars, pyroxenes, and garnets form the core silicate minerals of magmatic and metamorphic rocks; understanding their RI, relief, and optical properties helps identify rock types (e.g., ultrabasic vs basic rocks, granitoids, pelites, skarns).

• Gem varieties and industrial uses of feldspars, olivine, and garnets highlight the economic and aesthetic relevance of mineralogy beyond mere identification.

Quick Reference: Selected Formulas and Key Values from the Transcript

• Olivine general formula: ({Mg,Fe})2 {SiO}4

• Forsterite: {Mg}2 {SiO}4$; Fayalite:$ {Fe}2 {SiO}4

• Garnet general formula: X3Y2 {Si}3 {O}12

• Garnet end-members (examples):

  • Grossular: {Ca}3 {Al}2 {Si}3 {O}12

  • Pyrope: {Mg}3 {Al}2 {Si}3 {O}12

  • Almandine: {Fe}3 {Al}2 {Si}3 {O}12

  • Spessartine: {Mn}3 {Al}2 {Si}3 {O}12

  • Andraolite: {Ca)3 (Fe}2 {Si}3 {O}12

  • Uvarovite: {Ca}3 {Cr}2 {Si}3 {O}12

• Feldspar group general concepts:

  • Alkali feldspar: Orthoclase, Sanidine, Microcline (KAlSi3O8 family); Albite (NaAlSi3O8) is often cited as a plagioclase end-member (Na-dominant).

  • Plagioclase feldspar series: Albite, Oligoclase, Andesine, Labradorite, Bytownite, Anorthite (NaAlSi3O8 to CaAl2Si2O8 series).

• SI unit and classifier notes:

  • Feldspars form the basis of igneous rock classification due to their abundance and mineralogical significance.

  • Silicate structure terminology (Nesosilicate, Sorosilicate, Cyclosilicate) helps categorize minerals by the way SiO4 tetrahedra are linked.

The Aluminosilicate Group includes important polymorphs—Andalusite, Kyanite, and Sillimanite—all sharing the same chemical formula ($ext{Al}2 ext{SiO}5$) but possessing distinct crystal structures. These differences arise from their formation under specific pressure (P) and temperature (T) conditions, making them crucial index minerals in metamorphic petrology for defining metamorphic facies and zones.

Andalusite ($ext{Al}2 ext{SiO}5$)

1. Physical Properties:

• Crystal System: Orthorhombic

• Habit/Form: Typically forms euhedral prismatic crystals, often nearly square or rectangular in cross-section. It can also be massive. A distinctive variety, chiastolite, contains cruciform inclusions of carbonaceous material.

• Color: Commonly reddish-brown, pink, gray, or white; chiastolite is often gray with dark inclusions.

• Luster: Vitreous to sub-vitreous.

• Cleavage: Good cleavage parallel to the c-axis (${110}$), often appearing splintery.

• Hardness: $6.5 ext{ to } 7.5$

• Specific Gravity: $3.1 ext{ to } 3.2$

Facture: Uneven
Transparancy: Translucent to Opaque

2. Optical Properties (in thin section):

• Plane-Polarized Light (PPL): Colorless to pale pink/reddish in thin section, sometimes with faint pleochroism (pink to green). Exhibits moderate to high relief. Chiastolite shows characteristic dark cross patterns from carbon inclusions.

• Crossed-Polarized Light (XPL): Anisotropic. Displays low-order interference colors, typically gray to yellow. Extinction is parallel to the cleavage. It is biaxial negative ($(-)2 ext{V}$ often around $80-85 ext{°}$).

3. Paragenesis (Occurrence):

• Forms under relatively low-pressure (P) and moderate- to high-temperature (T) metamorphic conditions.

• Commonly found in contact metamorphic aureoles around igneous intrusions and in regional metamorphic terrains formed at shallow crustal depths.

• Typical host rocks include pelitic (clay-rich) schists, gneisses, and hornfels. Often associated with muscovite, biotite, quartz, cordierite, and potassium feldspar.

Kyanite ($ext{Al}2 ext{SiO}5$)

1. Physical Properties:

• Crystal System: Triclinic

• Habit/Form: Distinctive bladed, elongated, or columnar crystals. Often found in radiating or fibrous aggregates. Crystals are typically flat.

• Color: Commonly blue, but can be white, gray, or green. The blue color can be patchy or zoned.

• Luster: Vitreous to pearly.

• Cleavage: Perfect cleavage parallel to the length of the blades (%${100}$) and good cleavage (${010}$), often appearing step-like.

• Hardness: Highly anisotropic hardness: $4.5 ext{ to } 5$ parallel to the length of the blades (c-axis) and $6 ext{ to } 7$ across the blades. This is a key diagnostic feature.

• Specific Gravity: $3.5 ext{ to } 3.7$

  Facture: Uneven
  

2. Optical Properties (in thin section):

• Plane-Polarized Light (PPL): Colorless to pale blue in thin section, often non-pleochroic or with very faint blue pleochroism. Exhibits high relief, particularly noticeable against quartz or feldspar. Cleavage traces are prominent.

• Crossed-Polarized Light (XPL): Anisotropic. Displays low-order interference colors, typically gray to first-order yellow. Extinction is oblique (inclined) to the cleavage. It is biaxial negative ($(-)2 ext{V}$ large, typically around $82 ext{°}$).

3. Paragenesis (Occurrence):

• Forms under highly medium- to high-pressure (P) and moderate- to high-temperature (T) metamorphic conditions.

• Typically found in regionally metamorphosed pelitic schists and gneisses, often indicating significant burial depth.

• Associated minerals include staurolite, garnet, muscovite, biotite, and quartz.

Sillimanite ($ext{Al}2 ext{SiO}5$)

1. Physical Properties:

• Crystal System: Orthorhombic (similar to Andalusite, but different structure)

• Habit/Form: Characteristically forms fibrous or acicular (needle-like) crystals, often aggregated into bundles, referred to as "fibrolite." Can also occur as coarse prismatic crystals.

• Color: Colorless, white, gray, or pale brown.

• Luster: Vitreous to silky (especially the fibrous variety).

• Cleavage: Perfect cleavage parallel to the c-axis (${100}$), typically observed as individual fibers or bundles.

• Hardness: $6.5 ext{ to } 7.5$

• Specific Gravity: $3.2 ext{ to } 3.3$

2. Optical Properties (in thin section):

• Plane-Polarized Light (PPL): Colorless in thin section. Exhibits high relief. Fibrous habits are highly distinctive.

• Crossed-Polarized Light (XPL): Anisotropic. Displays moderate to high-order interference colors, often second- or third-order blues, greens, and reds, due to its relatively high birefringence. Extinction is parallel to the length of the fibers/prisms. It is biaxial positive ($(+)2 ext{V}$ small, typically around $20-30 ext{°}$).

3. Paragenesis (Occurrence):

• Forms under conditions of high-temperature (T) and moderate- to high-pressure (P), often a product of continuous heating of rocks already metamorphosed to Kyanite or Andalusite grades.

• Typically found in high-grade regional metamorphic rocks such as schists, gneisses, and granulites.

• Associated minerals often include garnet, biotite, muscovite (which may break down into sillimanite), quartz, and potassium feldspar. It marks environments transitional to partial melting.