Rocks and Minerals in the Field

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Rocks and Minerals in the Field

• First step in interpreting an outcrop is to
  identify the minerals and rocks.
• Need to look for diagnostic physical properties
  that allow us to identify minerals and rocks.
• Observation and experience

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Minerals in the Field

• Set of rock-forming minerals everyone should be
  able to identify includes
• Quartz
• Feldspars
• Micas
• Olivine
• Pyroxenes
• Amphiboles
• Garnets
• Carbonates
• Common Fe, Pb, Cu oxides and sulfates
• Common halides and sulphides

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Rocks in the Field

• Identification begins with deciding if it is
  sedimentary, igneous, or metamorphic
• Further classification based on
• Texture (qualitative observation)
• Composition (quantitative observation
• Genetic process (interpretation)

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Specimen Identification

• Break up into groups and gather around mineral
  specimen boxes.
• Using our prior knowledge and simple
  observations, as a class well talk about and
  identify as many minerals as we can (hopefully
  all ?)

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Igneous Rocks

• Important textures are those that imply
  crystallization from a melt
• Glass
• Well-formed (euhedral) crystals
• Interlocking textures with overlap relationships
  that match expected mineral crystallization
  sequence

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Igneous Rocks

• Phaneritic large mineral grains
• Aphanitic small mineral grains (needs
  magnification)
• Glassy
• Fragmental

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Igneous Rocks

• Phaneritic Textures
• Granular no mineral alignment
• Fluidal alignment of platy and elongated grains
• Porphyritic phenocrysts are larger than
  groundmass (bi-modal)
• Seriate continuum of phenocryst sizes

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Igneous Rocks

• Phaneritic Textures
• Hypidiomorphic suggestive of a crystallization
  sequence (continuum of euhedral to anhedral
  crystals)
• Allotriomorphic all grains are anhedral due to
  simultaneous growth, prolonged crystallization,
  or recrystallization

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Igneous Rocks

• Glassy Textures
• Pitchstone waxy
• Obsidian massive
• Perlite pelletal, fractured glass
• Vitropheric glass with phenocrysts

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Igneous Rocks

• Aphanitic Textures lt0.25 mm
• Trachytic planar or linear fabric
• Porphyroaphanitic visible phenocrysts
• glomeroporphyritic visible phenocrysts in clots
• Vesicular frothy
• Dikytaxitic vesicles form interstices between
  phenocrysts
• Devitrification textures flow banding,
  spherulites, lithophysae, granophyre, varioles

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Igneous Rocks

• Fragmental and Pyroclastic Textures
• Protoclastic breccia, conglomerate, gneissose,
  schistose, mylonitic textures in intrusive rocks
  (typically at margins of pluton) due to late
  state crystallization processes, vapor escape,
  collapse, deformation
• Pyroclastic due to extrusive, volcanic processes
  (eruptions, explosions)
• Tephra pyroclastic particles of a wide range of
  sizes
• Welded hot pyroclastic material that has been
  softened by heat and compacted to form a dense
  rock (tuff)

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Igneous Rocks

• Named according to both texture and mineral
  composition
• Special terms
• Aplite equigranular, fine-grained, felsic
• Pegmatite inequigranular, coarse-grained
• Granophyre feathery to graphic intergrowths of
  feldspar and quartz
• Porphyry abundant phenocrysts in a fine-grained,
  but phaneritic groundmass
• Lamprophyre mafic porphyry

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Sedimentary Rocks

• Texture
• Clastic
• Crystalline
• Primary mineral growth
• Diagenetic mineral growth

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Sedimentary Rocks

• Grainsize can be uni- or bi-modal
• Sorting how uniform in size
• Rounding how angular gives clue to environment
• Fabric imbrication, etc. can give clues to
  environment
• Maturity mineral composition - can give clues
  to provenance

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Sedimentary Rocks

• Other textures
• Hypidiomorphic interlocking euhedral crystals
• Oolitic pisolitic precipitation around
  pre-existing cores
• Pelletal infilling precipitating material
• Stromatolites and fossils
• Diagenetic features (compaction, dissolution,
  sytolites, polymorphs, cements, etc.)

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Mature
Immature (polymictic)
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Sedimentary Rocks

• Rudites coarse-grained rocks
• Conglomerate rounded clasts
• Breccia angular clasts
• Clast-supported well-sorted, no matrix
• Matrix-supported poorly-sorted or bi-modal with
  matrix separating clasts

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Sedimentary Rocks

• Lutite fine-grained sedimentary rocks
• Siltstone grains can be seen with hand-lens
• Claystone microscopic grains smooth/waxy feel
  not gritty in mouth
• Mudstone mix of clay and silt
• Shale flaky cleavage and thinly bedded lutite
• Argillite tough lutite that fractures into
  angular fragments

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Sedimentary Rocks

• Wacke silt and clay detritus between sand grains
• Arenite no matrix generic sandstone
• Graywacke crystallized, tough matrix sandstone

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Sedimentary Rocks

• Carbonates (limestones) carbonate or dolomite
  based chemical sediments
• Calcirudite (gt 2mm)
• Calcarentie (2 1/16 mm)
• Calcilutite (micrite) (lt1/16 mm)
• Grainstone, packstone, wackestone, mudstone
  (progressively more matrix)

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Sedimentary Rocks

• Other chemical sediments
• Phosphorites (apatite-based)
• Siliceous muds (ooze, diatomites, radiolarites,
  chert, etc.)
• Volcaniclastics
• Evaporites
• Fe-rich rocks (BIF, laterite, etc.)
• Carbonaceous (kerogen, coal, petroleum)

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Metamorphic Rocks

• Minerals tend to grow during deformation at high
  temperature and pressure.
• Texture is simultaneous growth at some
  equilibrium condition rather than a sequential
  order as with igneous rocks (not hypidiomorphic).
• Mineral compositin indicative of P-T conditions
• Texture indicative of deformation

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Feldspars usually euhedral in igneous rocks but
are anhedral in metamorphic rocks
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Metamorphic Rocks

• Granoblastic phaneritic and granular
  (equidimensional grains of about the same size)
• Hornfelsic relict texture of parent rock
  recognizable under pervasive recrystallization to
  aphanitic metamorphic grains very tough hackly
  fracture
• Schistose platy fissility tabular or elongated
  outcrops
• Semischistose platy, but not well-developed

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Metamorphic Rocks

• Cataclastic brittle deformation brecciated
• Mylonitic aphanitic groundmass with strong
  foliation and ductile deformational fabric
  grainsize reduction
• Polymetamorphic complex texture due to more than
  one deformational event

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Metamorphic Rocks

• Porphyroblastic larger grains that grew during
  metamorphism in a fine groundmass
• Poikiloblastic porphyroblasts include many
  smaller grains
• Porphyroclastic large grains are relicts in a
  mylonite
• Blastoporphyritic large grains are relict
  phenocrysts from an igneous protolith

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Metamorphic Rocks

• Granofels (granulite)
• Skarn (calcsilicate)
• Marble, quartzite, amphibolite
• Hornfels
• Schist
• Phyllite
• Slate
• Semischist
• Gniess
• Fault rocks (gouge, breccia)
• Mylonite, protomylonite, orthomylonite,
  ultramylonite, blastomylonite
• Meta-

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