Framework Silicates & Building the Crust JD Price
Silicate Structure
Silicate Structure
(SiO2)
Zeolites Zeolites have cage-like structures that store water (or other large molecules). Typically found in amygdales within basaltic rocks. Natrolite Na 2 [Al 2 Si 3 O 10 ] 2(H 2 O) Thomsonite NaCa 2 Al 5 Si 5 O 20 6(H 2 O) Chabazite CaAl 2 Si 4 O 12 6H 2 O Natrolite - mineralien-verkauf.de
Natrolite Water is located in open structures within the lattice. But note the framework of Si-O tetrahedra.
mineral.galleries.com SiO 2 Quartz Coesite Stishovite De minuscules cristaux de quartz ont une disposition radiale autour de la coesite : ceci montre que le quartz se forme au d riment de la coesite (LPA) High birefringence Stishovite in coesite, synthetically grown by J. Mosenfelder, CalTech.
SiO 2 Cristobalite nodules within a vitrophere - Snowflake obsidian Rockhoundblog.com Tridymite crystal, from deposit associated with Mono Lakes volcano. gemandmineral.com
Image from Blackburn and Dennen, 1988
Silica Polymorphs Beta quartz form from Bishop Tuff, CA. Note that crystal is actually alpha quartz CalTech
SiO 2 Agate Jasper Chert Flint Chalcedony Agate is name applied banded rocks made of microcrystalline quartz, typically made of fibrous quartz, called chalcedony. Colors result from impurities within the crystals Chert and flint are homogenous chalcedony - often related to fossilization Lace agate / www.lhconklin.com
Silica saturation in water is quite low at ambient conditions, ph=7, but increases rapidly with T. Note that amorphous silica has a higher solubility than quartz, Rimstidt & Cole 1983.
In the absence of organic ligands or fluoride, quartz solubility is relatively low in natural waters. Below ph 9, the dissolution reaction is: SiO 2 (quartz) + 2H 2 O(l)! H 4 SiO 4 0 Quartz is frequently supersaturated in natural waters because quartz precipitation kinetics are slow. Quartz saturation does not usually control the concentration of silica in low-temperature natural waters.
Opals are made of spheres of silica and water - not exactly a mineral. opal Image from Klein and Hurlbut, 1985
Feldspars Plagioclase Feldspar Anorthite (CaAl 2 Si 2 O 8 ) Alkali Feldspar Albite (NaAlSi 3 O 8 ) Orthoclase (KAlSi 3 O 8 ) Images from mineral.galleries.com
Plagioclase Solid Solution Liquid Liquid + Plag Plag
M T T
Common Feldspar Ternary
Feldspar Twinning Image from Blackburn and Dennen, 1988
Feldspathoids Leucite (KAlSi 2 O 6 ) Nepheline (NaAlSiO 4 ) Sodalite (Na 4 Al 3 Si 3 O 12 Cl) dkimages Image from mineral.galleries.com
Foids Nephaline vs. Albite NaAlSiO 4 NaAlSi 3 O 8 1 Si per NaAl 3 Si per NaAl Leucite vs. Orthoclase KAlSi 2 O 6 KAlSi 3 O 8 2 Si per NaAl 3 Si per NaAl Lower Si than feldspar counterparts. Arise from melts that are low in SiO 2 undersaturated.
Igneous nomenclature archaic texture discrimination boundary at 1mm. An alkali feldspar and quartz dominant rock with an average grainsize >1 mm is a granite, <1 mm is a rhyolite. > 1mm Phaneritic < 1 mm Aphanitic Glassy - vitrophyric
Rock names Diorite Dacite Granite Tonalite Gabbro Rhyolite Andesite Basalt Anorthosite Foid Syenite Phonolite Mean grain size > 1mm Mean grain size < 1mm After Strekeisen, 1978
Igneous rocks are the end products of partial melting in the earth, so their compositions are determined by the chemical processes involved in melting earth materials Source, conditions, and degree of melting
Partial melting in S.W. Maine
Tectonic thinning of the crust
Tectonic increase of crustal thickness
H 2 O and melting (after Burnham and Davis, 1974)
H 2 O and melting (after Burnham and Davis, 1974)
IUGS Phaneritic Phaneritic rock classification - note that the scheme precludes the coexistence of foids and SiO 2.
IUGS Aphanitic Aphanitic rock classification - compare with the previous slide. This is a nice diagram, but the fine-grained nature of these rocks can impede its use.
IUGS TAS If you really want to classify a volcanic rock - powder it and analyze its composition. How does this relate to the previous slide? What of plutonic rocks?
Again our understanding of the earth falls off with depth.
Mafic (Mg-Fe rich) Rocks Coarse: Gabbro, Anorthosite Fine: Basalt Minerals: Largely olivine (isolated), pyroxene (singlechain), and feldspar (framework). Origin: Partial melting of the mantle Oceanic crust, oceanic islands, the moon. Anorthosite Univ. North Dakota, Plutonic Images Univ. North Carolina, Atlas of rocks, minerals, and textures website
Basalt Gabbro Univ. North Carolina, Atlas of rocks, minerals, and textures website
Plume theory Earth is hottest at its core, the release of heat may be convective - mass transferred. Some masses may move as buoyant ductile bodies. Diagram by J. Tarney
End of the ride if such plumes exist, they would become more or less stalled at the transition to brittle mantle. Diagram by J. Tarney
If most of the heat needed for melting is deep within the earth, why are there volcanoes? Magma is driven upwards. Parameters - density, viscosity P d = P h + P o - P vis - S h
Ascent of Magmas Hogan et al., 1998
Kilauea, Hawaii March, 1996
A cross section of a shield volcano Generally low Si Low volatile High rate Figure 20.19 20-406
Mauna Kea, Hawaii
Pu u Hulu Mauna Loa, Hawaii Pahoehoe
Oceanic rifting
Continental Rift http://jsc.nasa.gov STS-32 Example: the East African Rift
Rifting - Birth of an ocean Diagram by J. Tarney
Cartoon to left shows some general attributes of rift formation Presumably, the tension that drives rifting is not well understood. Diagram by J. Tarney
Intermediate rocks (less Fe-Mg, more Na-K-Si-Al) Coarse: Granodiorite, Tonalite Fine: Andesite, Dacite Minerals: Feldspar (framework), pyroxene (single chain), amphibole (double chain), and quartz (framework) Origin: liquid fraction of crystallizing mafic liquids. Location: Convergent tectonic settings Tonalite
Subduction: Island Arcs Example: the Aleutians
Subduction: Continental Arcs Example: the Cascades
Mount Saint Helens: May 18, 1980 0832
Q: What is a volcanic dome? View from South Rim, June 1991
Crater Lake Oregon
Felsic (K, Na, and Si rich) rocks Coarse: Granite Fine: Rhyolite Minerals: Feldspar and quartz (framework), with small amounts of amphibole (double chain) and mica (sheet). Origins: Partial melting of crustal rocks and liquid fraction of crystallizing intermediate liquids. Location: continental magmatism Granite
Felsic rocks Rhyolite Univ. North Carolina, Atlas of rocks, minerals, and textures website Pam Gore, GPC Vitrophyre
Barker, 1983 after Bowen, 1928
Fractional Crystallization
Pegmatite Pegmatites are typically coarsely crystalline - may contain rare minerals. Most result from extreme fractionation. Slow cooling Low nucleation Lots of H 2 O