Mineralogy and Petrology Notes and Review
Mineralogy and Petrology Notes and Review Geol 225
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Date Created: 08/22/16
Mineralogy Notes: November 5, 2104 Chemical Sedimentary Rocks Evaporation of seawater • Calcite begins depositing when seawater ﬁrst begins to evaporate • Gypsum begins when ~75% of original water removed • Halite begins when ~90% of original water removed • Various Mg/K salts form when ~95% of original water removed Volume left Mineral precipitation begins - calcite 1/3 gypsum 1/10 halite 1/20 Mg & K salts Evaporite Minerals Common evaporites include: • Sulfates (gypsum, anhydrite) • Halides (halite, sylvite) • Rare to ﬁnd in outcrop - only common in arid climates • Common in subsurface - present under about 1/3 of US land surface • Permian is time of great abundance - Probably due to having all continents together near equator • Halite - Mechanically unstable: deforms by ﬂowing when deeply buried - Forms salt dome structures Phosphorites • Marine sed. rocks composed mostly of apatite with >20% P2O5 • apatite: - Phosphate mineral - Ca5(PO4)3(F,Cl,OH) • Collophane • massive cryptocrystalline variety of apatite found in phosphorites • major source of phosphate used in fertilizer Chert • Rock name for cryptocrystalline quartz • Occurs in bedded deposits and as nodules in sed sequences • Precipitates in water, mostly marine - Silica sources include • Some plankton • Sponge spicules November 7, 2014 Metamorphic Rocks Metamorphism • Metamorphism occurs when a protolith undergoes a solid-state change in response to a modiﬁcation of environment. - protolith, solid-state, change, modiﬁcation of environment • Rock seeks new equilibrium due to changing conditions Range of metamorphism • Lower boundary set at ﬁrst appearance of mineral(s) not normally occurring in sedimentary rocks - Ex; chlorite, epidote, certain zeolites • Gradual change from diagenesis to very low grade metamorphism • Upper boundary set at beginning of signiﬁcant melting’ Agents of Metamorphism • Heat - Temps b/t diagenesis and melting - Recrystallization results in new, stable minerals - Heat may be provided by: • Contact metamorphism-heat from magma • Geothermal gradient-increase in T with depth • Radioactive decay • Prograde changes - Happen when rocks are heating - Occur rapidly, b/c diffusion is faster • Retrograde changes - Occur during cooling - Much slower Conﬁning Pressure • - Increases with depth - Applies forces equally in all directions - May cause recrystallization to more compact XLline form • Differential Stress - Unequal in different directions - Can result in preferred orientation of mineral grains - Ex-convergent plate boundaries • Hydrothermal ﬂuids - Mainly water - Enhances migration of ions - Aid in recrystallization of existing minerals - Metasomatism: change in rock’s composition by reaction with hydrothermal ﬂuids • Different types based on: - Temperature/pressure conditions - Inferred processes active during formation • contact or thermal metamorphism - Magma invades a host rock - Temperature rises - Zone of alteration forms in rock surrounding the magma (limited areal extent) - High temps, but low pressures - Typically produces massive rocks Metamorphic Environments • Hydrothermal metamorphism - Chemical alteration caused when hot, ion-rich ﬂuids circulate through ﬁssures and cracks - Most widespread along axis of the mid-ocean ridge system, altering ocean-ﬂoor basalt • Regional metamorphism - Widespread metamorphic zones, producing greatest quantity of metamorphic rock - Associated with mountain building/convergence - Very high temps associated with depth and/or igneous activity - Higher pressures due to overburden weight, plus compressing and shearing - Foliation develops • High P, Low T- Subduction Zone metamorphism - Restricted to narrow belts(smaller than regional) - Formed at temps below average geothermal gradient, but very high pressure - Accretionary prisms at subduction zones is only site November 10, 2014 • Burial metamorphism - Associated with very thick sedimentary strata - Increasing T and P with additional burial/ subsidence - Required depth varies depending on prevailing geothermal gradient - Typically low-grade - Little to no internal deformation of rocks • Dynamic metamorphism - Occurs at depth and high temperatures along fault zones (localized) - Pre-existing minerals deform by ductile ﬂow - Result of shearing - Mylonite: • Meta rock produced in shear zones • Large grains in protolith recrystallize to very ﬁne-grained • Impact metamorphism - Occurs when meteorites strike earth’s surface - Rocks are called impactiles - Ultra high temp & pressure - Extremely limited occurrence Metamorphic Rocks • Contact vs. regionally metamorphosed rock textures - Contact is more about the temperatures to make changes - Regional is more exfoliated • Granoblastic textures - May form during recrystallization - XLs of roughly the same size - exhibit polygonal grain boundaries • Hornfels: generic name for ﬁne-grained, granoblastic, contact metamorphic rocks that are typically - Hard - Dense - Baked appearance - Gray to brown to black - Smooth to the touch Non foliated examples Certain non-foliated rocks with speciﬁc chemical compositions and/or mineral assemblages • are given speciﬁc names • Marble: - Composed mostly of calcite (less commonly dolomite) - Protolith of LS or DS - Usually granoblastic - Foliation may be present • Skarns: - Originate from contact metamorphism of (possibly impure) LS and DS - Show evidence of having exchanged constituents with intruding magma (metasomatism) - May form metallic ore deposits • Quartzites: - Metamorphism of quarts arenites and cherts - Recrystallization with interlocking crystals of quartz - Fractures across grain boundaries - Hard • Serpentinites: - Consist mostly of serpentine • Soapstones: - Mixture of serpentine and talc - Talc gives greasy feel • Both typically form by hydrothermal alteration of ultramaﬁc igneous rocks • Greenstone - Chlorite commonly formed, giving rock a green color - Maﬁc protolith, such as basalt • Amphibolites: - Medium to coarse grained - Dark colored - Principal minerals are hornblende and plagioclase • Porphyroblasts: metamorphic equivalent or phenocrysts - May occur in foliated and non foliated rocks - Ex; Garnet, Staurolite November 12, 2014 More on Metamorphic Rocks Metamorphic textures • Foliation-Parallel planes of aligned platy or elongated minerals or compositional layering produced by differential stress - Exhibited commonly by regionally metamorphosed rocks - Develops perpendicular to the direction of maximum stress - May assume any angle to original bedding • Ways foliation can form: - Rotation of platy and/or elongated minerals - Recrystallization of minerals in the direction of preferred orientation - Changing the shape of equidimensional grains into elongated shapes that are aligned • Analyzing foliated rocks includes observations of: - Intensity of foliating - Size of crystals - Degree to which segregated into light and dark bands - Metamorphic grade • Slate - Low grade metamorphism of shale/mudstone - Growth of very ﬁne-grained chlorite and micas result in • Slaty cleavage: splitting into thin sheets along foliation • Phyllite - Fine-grained - Gradational between slate and schist - Glossy reﬂective sheen due to platy mineral grains - Wavy surfaces - Also exhibits rock cleavage • Schist - Medium-to coarse-grained - Conspicuous platy mineral grains - Exhibits irregular planar foliation of visible mica ﬂakes called schistocity - Composition indicated by using mineral names (ex:biotite schist) • Gneiss - Medium- to coarse-grained - High-grade metamorphism - Exhibits gneissic banding: conspicuous color segregation due to compositional layering • Migmatites - heterogeneous rocks with both igneous and metamorphic properties - Dark layers experienced meta changes, but did not melt - Lighter portions may have crystallized form a partial melts of the precursor rock November 14, 2014 Metamorphic Rocks continued • Metamorphic grade- somewhat informal way to indicate intensity of metamorphism • Progresses from subtle low grade (~250-400 deg. C) to high grade (over ~600 deg. C) • With increasing grade - grain size coarsens - yield different mineral assemblages, stable higher T/P Meta Mineralogy • Number of minerals in metamorphic rocks limited, even with different rock types • Same protolith always produces same minerals when subjected to same T and P conditions Look up Barrovian sequence Index minerals Example: clay-rich rocks • Characteristic minerals showing zones of increasing metamorphism in mudstone • Each zone characterized by new mineral not present in previous zone - Lowest grade to highest (Chlorite, Biotite, Garnet, Staurolite, Kyanite, Sillimanite) Metamorphic grade • Isograd: - Line on map along which an index mineral ﬁrst appears - Points along isograd have ~ same meta grade Metamorphic zones: • - regions between 2 isograds - represent different metamorphic intensities (T/P) • Problem with Barrovian metamorphic zones: - They were deﬁned based on single index minerals that form from only one kind of protolith • Metamorphic facies - Group of meta mineral assemblages that develop under a speciﬁed range of T and P conditions - a more complete indication of metamorphic intensity - address different protolith compositions - Identify the mineral assemblages that approach thermodynamic equilibrium - Page 400 • Facies are named after characteristic rocks found in them, but may contain many more than the single rock type • Facies correspond to different plate tectonic settings and geothermal gradients Hornfels Facies • high temp-low pressure • corresponds with contact metamorphism • most common in the upper crust where the country rocks are relatively “cold” compared to the intruding magma Zeolite Facies • low temperature- low pressure • corresponds with burial meta zeolites commonly form via hydrothermal metamorphism of volcanic rocks • Increasing T and P facies • Green-schist through granulite facies correspond approximately to Barrovian regional metamorphic zones - regional meta in continental collision zones - most common facies - corresponds approx. to the Barrovian chlorite, biotite, and garnet zones - key minerals: epidote, chlorite, actinolite (green amphibole) Amphibole Facies • Regional meta • corresponds approximately to the Barrovian upper garnet through sillimanite facies Granulite Facies • regional meta • Highest-grade metamorphism experienced by continental rocks • Hydrous minerals absent Common in Precambrian rocks • • May form migmatites at upper end November 17, 2014 Metamorphic Rocks continued Blue-schist and Eclogite Facies • High pressure facies • correspond to subduction zone metamorphism experienced by a subjecting slab (sinking crust remains cool, well down into mantle) and very deep lower crustal and mantle conditions • Blue-schist facies - low temperature-high pressure - often contains the blue amphibole, glaucophane • Eclogite facies- relatively rare, dense rocks that may represent crust- mantle boundary • Learn what Geothermal Gradient means Exam: Blackboard exam info, and selected slides Material in text: Sedimentary Rocks Chapters 10, 11, and 12 Metamorphic Rocks: Chapter 13, 14 Sedimentary Rocks: Formation, common/abundant sed rocks, weathering rates and products Order of things going on; weathering, transport, deposition, lithiﬁcation, diagnesis Questions: Most abundant siliclastic rock? shale Most abundant chemical/biogenic? Minerals found in maﬁc igneous rocks tend to LESS stable in a weathering environment Sediment formed form weathering of rock • detritus • grain size, sorting, sphericity, roundness • textural maturity concept • glacial sediment Immature? angular, poorly sorted (many different grain sized), clay Super mature? well rounded, well sorted Sediment transport: • laminar vs. turbulent ﬂow • rolling, saltation, suspension • bedload vs. suspended load • turbidity currents • debris ﬂows Would a ﬂuid with a low viscosity more likely exhibit laminar or turbulent ﬂow? TURBULENT Laminar would be like a glacier Sedimentary structures • stratiﬁcation • graded bedding • cross bedding ripple marks • • mudcracks • trace fossils Depositional environment • Marin/continental/transitional Siliciclastic rocks • diagnesis • compaction • cementation • types of cements pressure solution • Mudrocks • importance, types, characteristics Sandstones • Compositions • quartz arenites • feldspathic arenites (arkos) • lithic amenities • wackes Biogenic and Chemical Rocks Carbonate minerals • calcite group • aragonite group • dolomite group Carbonate rocks • Limestone Limstone textures • allochems carbonate sediment • • fossils ooids • peloids • limeclasts • mud, micrite, spar Carbonate deposition • role of CO2 • Limestone diagenesis • stylolites • dolomitization • carbonate depositional sites carbonate compensation depth • Metamorphic Rocks • metamorphism • protolith agents of metamorphism sources of heat • • prograde/retrograde • conﬁning pressure, differential stress • hydrothermal ﬂuids • metasomatism Different metamorphic environments foliation • slaty cleavage • schistocity • gnessic banding • slate, phyllite, schist, gneiss • Migmatite • Mylonite November 21, 2014 Mineral Resources Minerals are non-renewable resources • - always searching for new sources • Sources of new materials - New deposits - Recycling materials - Substitution of other materials Economic Geology Field of study for ﬁnding and producing mineral products • • Can involve: - Exploration of new deposits - Feasibility studies for proposed deposits - Mapping ore-bodies - Extraction methodology • Crustal abundance: average concentration in crust • Certain geologic processes will segregate and concentrate materials • Higher the concentration, easier it is to recover • Ore: naturally occurring concentration that can be mined, processed, and sold at a proﬁt • Orebody (ore deposit): contains enough material to extract economically • Ore grade: describes relative abundance of economic material in the deposit Concentration Factor • concentration factor: ratio of elemental abundance in ore deposit to average crustal abundance • Examples: - Low: Al 4X Fe 5X - Moderate: Zn 300X Ag 1,200X - High: Au 4,000X Hg 100,000X • Typical concentrations to be considered an ore: Iron mine: 30% FE • • Copper mine: 0.5%Cu Gold mine: 0.1 ounce per ton • ******Reserves vs. Resources*****test question • Resources include - Reserves - Discovered deposits that are not currently economic - Undiscovered • New discoveries are not automatically reserves Must be evaluated and be economically feasible to mine, process, and sell • Recycling • Very price-driven business - Typically restricted to metallic commodities • amount of recycling heavily dependent on how material is being used • Examples: - Some materials tied up for long time periods Industrial Minerals • Materials used mostly for their own speciﬁc physical or chemical properties • Include majority of nonmetallic minerals • Often of low unit value - Transportation is major cost factor - Results in high place value - Often more sensitive to market ﬂuctuations December 1, 2014 Geology of Mineral Deposits • Processes that concentrate minerals in crust vary • Typically involve some type of ﬂuid movement - May be magmas, hydrothermal ﬂuids groundwater, etc. Circulation of hydrothermal ﬂuids • Fluids expelled from or heated by magmas can interact with surrounding host rock • Metallic minerals with elements like Au, Ag, Cu, Pb, and Zn may concentrate this way • Veins: high concentrations along relatively narrow zones, usually faults or fractures • Disseminated deposits: ore minerals scattered throughout host rock Seaﬂoor Hot Springs • Spreading centers supply heat to create hot springs on ocean ﬂoor • Waters are heavily enriched with Mn, Cu Pb, and Zn • Materials deposited around springs, accumulating in layers • Massive sulﬁde deposits: large masses of various sulﬁde minerals Weathering and Ore Deposits • Surﬁcial processes can also result in ore deposits Transported deposits • Flowing water will cause certain resistant minerals to segregate and accumulate into placer deposits • Typically found in river banks and along beaches • Deposits of gold, tin, and diamonds commonly form this way In-place deposits (not transported) • Intense weathering will remove all but the least soluble compounds Called residual deposits • • Ex: laterite- ancient A-horizon tropic soils Magmatic segregation processes Occur during crystallization of magma • • Several metallic minerals (chromite, magnetite, platinum) form this way • Certain minerals have higher speciﬁc gravities, allowing them to sink • Results in layers within intrusive body • Pegmatites - another example of segregation during crystallization - very coarse-grained intrusives - usually bulk composition of a granite - contain huge XLs of quartz, feldspars, and micas - often have collection of rarer minerals - commonly dike-like or lens-shaped - Most are relatively small - Range from X m to X00 m long, 1 cm to 200 m wide - Classiﬁed based on presence of absence of internal zoning Pegmatite origins Represent ﬁnal water-rich material of granitic magmas • • Formed in deep, high-pressure environments • Majority found near borders of granite plutons How to pegmatites grow? • Grain size misleading - Probably formed as rapidly as other crystals in magma Differences in conditions of formation: 1. Nucleation sites are few 2. Diffusion of materials in supercritical water is fairly rapid • Allows very large crystals to grow at normal rate Economic Value of Oxides • Titanium - major use of TiO2 whitening agent in paints - used in welding rods - Sources are heavy mineral accumulations of rutile and ilmenite Manganese • - used in making steel, dry cell batteries, paints Hydroxide Minerals • Contain either hydroxyl within structure - Results in lower hardness and lower speciﬁc gravity than oxide minerals • Commonly found as alteration product, generally from weathering - Fe hydroxides often red to yellow on rocks and in souls Bauxite Rock name for primary aluminum ore • • Mixture of - diaspore and gibbsite • Typical ore grade material >45% Al2O3 • Requires: 1. Aluminum-rich parent rocks 2. Warm climate 3. Abundant rainfall with wet/dry conditions 4. Good drainage Bauxite continued • Main use is making aluminum metal • Recycling accounts for about 36% of all aluminum usage in US • 1/2 is from “old” scrap
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