Metamorphic Rocks Part 3 CONTACTREGIONAL AND METASOMATIC ROCKS

Metamorphic Rocks, Part 3 CONTACT/REGIONAL AND METASOMATIC ROCKS Marble, Quartzite, and Serpentinite 1

Marble, Quartzite, and Serpentinite • Marble and quartzite may be either regional or contact metamorphic • Serpentinite is formed by metasomatic alteration of mafic rocks • Marble may also involve metasomatism • Therefore these rocks do not fall into neat categories 2

Marble • Marble is usually the product of metamorphism of limestone or dolomitic limestone • Limestones often contain silicate impurities, and the impurities may be converted to minute crystals of sericite, chlorite, etc • These crystals may impart a slightly silky luster to the marble, similar to the process that occurs during the formation of phyllite 3

Metamorphic Grade of Marble • Marbles range in grade from slates to schists • Foliation may be visible in hand specimen § Foliation may be due to plastic flow during metamorphosis, or § Foliation may be relict sedimentary 4

Naming Marble • Marbles may be named for their color, for example pink or white marble • White marble is often dolomitic • Marble may also be named for accessory minerals such as brucite, grunerite, pyrrhotite, etc 5

Pink Marble • Nonfoliated Marble 6

Relict Sedimentary Bedding • Relict sedimentary bedding in marble 7

Photomicrograph of Marble • Marble, CN • The photo shows strongly twinned and highly cleaved calcite 8

Weathering in Marble • Weathering in a marble tombstone (left) • Lichens secrete acid, which help to dissolve marble (right) 9

Brecciated Marble • Angular fragments in carbonate matrix 10

Acid Reaction in Marble • Marble usually retains at least some carbonate component • If calcite is present, the marble will react to acid vigorously • Dolomitic marbles react very slowly to cold hydrochloric acid • Acid solutioning of marble may lead to cave formation 11

Cavities in Marble • The metamorphic process often releases large quantities of carbon dioxide • This gas escapes though the marble and may lead large fractures and cavities in the rock, in a manner similar to the formation of vesicular basalt • Marble is used as a decorative stone, and the presence of cavities is often undesirable 12

Filling Cavities • For decorative purposes, the cavities may be filled with epoxy colored to match the background color of the marble • This is often done and is generally a satisfactory solution 13

Testing for Epoxy Filling • Acid etching of limestone marble will quickly expose the epoxy as topographically high regions • The use of mineral-specific stains, for either calcite or dolomite, will leave the epoxy uncolored 14

Load-Bearing Marble • For load-bearing structures, such as marble columns, the marble should be dense, with little or no cavities • Before marble is used in critical loadbearing applications, representative sample must be tested, and these tests should include testing for epoxy filling 15

Mineralogy of Marble • Common non-carbonate minerals in marble include tremolite, actinolite, diopside, epidote, phlogopite, scapolite, and serpentine • Epidote (along with albite) occur in lower grade marbles • Hornblende, plagioclase, some mica, and, in the higher grades, diopside are common • Sphene, apatite, and scapolite are present in amphibolite facies marbles 16

High-Grade Marble Mineralogy • Under higher grade conditions, dolomite will disappear • It decomposes to yield periclase (Mg. O) or brucite (Mg(OH)2) • Dolomite present in high-grade metamorphics is probably due to retrogressive metamorphism 17

Epidote and Actinolite • France, from the Ecole de Mines 18

Actinolite in Thin Section • (Upper - CN) Actinolite in a groundmass of Mg-rich chlorite. The photo shows the upper first-order to mid second-order interference colors of actinolite • (Lower - PP) Actinolite in a groundmass of Mg-rich chlorite 19

Photo of Apatite • Apatite from Durango, Mexico • Photo: Monique Claye, Ecole de Mines 20

Photomicrograph of Apatite • Large apatite end section (indicated by arrows) • Note: hexagonal shape • Green phenocryst is hornblende • Width of view is 0. 85 mm 21

Quartzite • Quartzites are often the metamorphic product of quartz sandstones • During metamorphism, the quartz grains become interlocking due to compression and recrystallization • If shearing forces are large enough, the quartz grains elongate and interlocking grain boundaries granulate • The granulation of the boundaries can only be seen in thin section 22

Quartzite • Sioux Quartzite, South Dakota • Nonfoliated 23

Photomicrograph of Quartzite • Quartzite CN • Quartzite is metamorphosed quartz-rich sandstones • All of the grains are quartz; black spaces are in extinction or are holes in the thin section (plucking) • Note that all grains are xenoblastic (anhedral), typical of quartz in metamorphic rocks 24

Use of Quartzite • Quartzite is highly resistant to physical and chemical weathering, so it does well in applications like this rip-rap 25

Sheared Quarzites • In highly sheared quartzites, the quartz grains become lenticular • All trace of the original sandstone disappears • The quartz grains may also show strain effects under the optical microscope 26

Strain in Argillaceous Sandstones • If the parent rock is an argillaceous sandstone, strain is taken up primarily in the fine-grained argillaceous phases and quartz and feldspar grains will be relatively undeformed 27

Mineralogy of Quartzites • Impure quartz sandstones are likely to produce sericite during metamorphism • Arkoses and feldspathic sandstones typically produce quartz-mica schists • They will differ from a schist produced from an argillaceous sandstone or graywacke due to their lack of chlorite or biotite 28

Kyanite Photomicrographs • (Upper - CN) Kyanite is surrounded by biotite and muscovite • The cleavage, relief, and bladed form of kyanite are clearly visible • Maximum first order red interference colors; inclined extinction that can almost be parallel. • (Lower - PP) Colorless to pale blue in plane polarized light • Tabular crystals; 2 cleavages; high relief 29

Kyanite Photomicrographs • (Upper - CN) Kyanite is surrounded by quartz • (Lower - PP) 30

Serpentinite • Product of metasomatic alteration of ultramafic igneous rocks • Serpentine minerals are usually pseudomorphous after the minerals they replace • Serpentines replacing olivine even retain the irregular curving fractures typical of olivine • The fractures may fill with very fine-grained magnetite produced during the serpentinization process 31

Serpentinite • Serpentinite marble • Nonfoliated 32

Photomicrographs of Serpentine • (Upper CN , Lower PP) The rock shown is almost 100% serpentine • The equant crystal forms seen are serpentine pseudomorphs after olivine • All members of the group have low birefringence (first order yellow maximum) and parallel extinction • The mineral habit is fibrous, and in plane polarized light grains are colorless to pale green • Grain size is typically too small to determine many optical properties 33

Serpentine in Mafic Rocks • (Upper, CN) The picture (1. 5 mm field of view) shows light gray stringers of serpentine altering clinopyroxene (at extinction) • (Lower, CN) Clinopyroxene grain surrounded by gray serpentine 34

Serpentinite Photo • Serpentinite from California Mother Lode country, in the Sierra Nevadas • Metallic mineral appears to be pyrite 35

Pseudomorphism in Serpentinite • The outlines of the crystals are also visible because of the magnetite grains which define the outline of the crystal • Pseudomorphs in serpentine are often among the finest pseudomorphs found in any rock • Serpentine replacing pyroxene may retain the cleavage, parting, or Schiller luster of the pyroxene 36

Biotite after Garnet The brown patch at the center of the field of view has the regular outline of a garnet • It consists of a more-or-less random aggregate of biotite flakes • Elsewhere in the thin section relics of garnet remain in these aggregates, which are said to pseudomorph the original mineral • This is the result of polymetamorphism - a thermal overprint on a regionally-metamorphosed 37 rock

Serpentinite Mineralogy • Magnesite, in minute grains, inevitably accompanies the serpentine minerals magnesite is a product of the metasomatic alteration • Other minerals found in serpentinites include tremolite and anthophyllite, usually as fibers or prisms on the borders of former olivine crystals 38

Serpentinite Mineralogy continued • Talc is another common alteration product • Talc may occur to the exclusion of all other secondary minerals • Resulting structures are unusual, possibly due to volume expansion during metasomatism • Slickensides are sometimes seen on serpentinites • Another fairly rare mineral is brucite 39

Talc in Serpentinite • France 40

Brucite Photo • Closeup of brucite 41