Chptr17

• Grain texture


  Size, shape, orientation and intergranular relationships of the rock's constituents
• Metamorphic textures

  • May be due to the metamorphic changes induced by the temperature and pressure conditions of metamorphism or relict textures inherited from the protolith
• Relict features derived from igneous rocks

  • Volcanic flow contacts
  • Intrusive contacts
  • Vesicles
  • Phenocrysts
• Relict features derived from sedimentary rocks

  • Ripple marks
  • Mudcracks
  • Graded bedding
  • Fossils
  • Tool marks
• Equant grains


  Possess nearly equal diameters in all directions and assume forms approximated by spheres or cubes
• Inequant grains


  Contain at least one direction in which the grain diameter is not equal to the other grain diameters
• Inequant grain forms


  • Tabular, disc-shaped (pancake or paper-like)
  • Bladed (prismatic)
  • Acicular (needle-like) or prolate (cigar-shaped)
• Aphanitic grains


  Diameters of less than 1mm
• Phaneritic grains


  Diameters of 1 mm or greater
• Porphyroclasts


  Large relict grains from the protolith that have experienced deformation but have retained their original composition
• Augen


  Oval-shaped feldspar porphyroclasts that resemble the shape of an eye
• Flaser


  Composed of quartz porphyroclasts
• Porphyroblasts


  Large grains that have experienced neocrystallization and growth in response to favorable temperature and pressure conditions during metamorphism
• Common porphyroblast minerals


  • Garnet
  • Staurolite
  • Cordierite
• Euhedral, subhedral, anhedral


  Degree to which crystals are bounded by crystal faces
• Tendency of common metamorphic minerals to develop complete crystal forms

  • Most euhedral: Titanite, rutile, pyrite, spinel
  • Garnet, sillimanite, staurolite, tourmaline
  • Epidote, magnetite, ilmenite
  • Andalusite, pyroxene, amphibole
  • Mica, chlorite, dolomite, kyanite
  • Calcite, vesuvianite, scapolite
  • Feldspar, quartz, cordierite
  • Least euhedral
• Random grain orientation


  No preferred orientation of inequant grains is visible
• Preferred grain orientation


  Inequant grains are oriented sub-parallel to one another
• Lineations


  Line-like features with preferred orientation of acicular, bladed or rod-like grains with sub-parallel long axes
• Foliations


  Metamorphic layers oriented parallel to one another with preferred orientation of tabular grains, especially phyllosilicate minerals, with sub-parallel long axes
• Metamorphic rock groups based on grain orientation

  • Rocks with non-foliated textures
  • Rocks with foliated textures
• Non-foliated textures


  Lack metamorphic layering as defined by preferred mineral orientations
• Hornfelsic texture


  Fine-grained (<1.0 mm diameter), non-foliated fabric that develops by contact metamorphism
• Granoblastic textures


  Characterized by large (>1.0mm diameter) equant grains or large inequant crystals that lack preferred orientation
• Metaquartzite


  Contains >90% quartz and is derived from quartz-rich sandstone or chert protoliths
• Metaquartzite


  • Recrystallization, pressure solution and intracrystalline plastic deformation mechanisms produce interlocking quartz grains with granoblastic texture
• Marble


  Granoblastic metamorphic rocks rich in calcite and/or dolomite, derived by recrystallization of limestone or dolostone protoliths
• Common accessory minerals in marble

  • Graphite
  • Brucite
  • Diopside
  • Forsterite
  • Wollastonite
• Quartzarenites


  • Quartz is stable over a wide range of metamorphic temperatures and pressures
  • Metaquartzite, also referred to as quartzite, is a common metamorphic rock
  • Metaquartzites are hard, durable rocks that produce angular surfaces when fractured
  • Metaquartzites are not ideal for road construction because of the tendency to tear rubber tires
  • Metaquartzites are used for rock walls, railroad ballast and drainage culverts
• Marble


  Granoblastic metamorphic rocks rich in calcite and/or dolomite
• Formation of marble


  Recrystallization of limestone or dolostone protoliths via dynamothermal, deep burial or contact metamorphism
• Accessory minerals in marble


  • Graphite
  • Calcium- and/or magnesium-bearing minerals such as brucite, diopside, forsterite, wollastonite, epidote, serpentine, idocrase (vesuvianite), tremolite and grossular garnet
• Accessory minerals in marble

  Provide distinctive hues that allow marble to assume a wide variety of colors
• Marble colors

  • Reddish or yellow from iron oxides
  • Gray to black from graphite
  • Green from serpentine
• Marbles can either retain some relict sedimentary structures and fossils or exhibit total recrystallization so that all prior sedimentary textures and structures are obliterated
• Marbles


  • Relatively soft and easy to cut
  • Great color diversity and softness have led to extensive use for sculptures and building stone for thousands of years
• Classic sculptures made from marble


  • Venus de Milo
  • Michelangelo's David
• Skarn


  Granoblastic calc-silicate rocks formed by contact metamorphism of carbonate country rocks such as limestone or dolostone
• Formation of skarn


  Release of silica and volatiles from the magma results in extensive metasomatism, generating calc-silicate mineral assemblages and/or metallic ore deposits
• Common skarn minerals

  • Carbonate minerals such as calcite, dolomite and ankerite
  • Silica group minerals such as quartz
  • Calcium-magnesium silicates such as wollastonite, tremolite, diopside, talc, epidote, grossular garnet, phlogopite and idocrase (vesuvianite)