More on feldspars & quartz Halides, sulfates, borates, phosphates Evaporites & Hydrothermal JD Price
General Mass Action aA + bB = cC + dD a,b,c,d are concentrations of A,B,C,D At equilibrium, we may define a constant k = � a products / � a reactants c a D d k = a C a a B b a A For an ideal gas, PV = nRT � Gº TP - � Gº = RT ln K = 2.303 RT log K At s.s. � Gº TP =0 with T=298K, log kº = -0.1750 � Gº
2NaAlSi 3 O 8 ab + H 2 O fl = Na 2 O melt + Al 2 O 3 melt + 6SiO 2 melt + H 2 O melt 1 1 6 1 a Na 2 O a Al 2 O 3 a SiO 2 p H 2 O k = 2 1 a NaAlSi 3 O 8 a H 2 O P is used to indicate that H 2 O has a partial pressure P < 1 when melt in s H 2 O undersaturated P = 1 when melt is H 2 O saturated
The a ’s for Ab and fluid are 1 Let’s assume a ’s for the other components in melt are also 1. 1 k = p H 2 O With the appropriate thermodynamic data, we could determine � Gº And find log pº H2O = -0.1750 � Gº For some other T,P log p H2O = � Gº TP - � Gº / (2.303 RT)
H 2 O and melting (after Burnham and Davis, 1974)
A ternary system of minerals A, B, and C. We may project the melting surfaces of each on to the ternary diagram as isothermal contours (Barker, 1982)
So what happens to a melt of the star composition. 1. At 1450, it encounters the A surface. 2. With continued cooling, the liquid increases in B & C, following the contours 3. At 1250ºC, it encounters the cotectic. Crystals of B begin to precipitate. 4. Composition of liquid follows cotectic to the eutectic (E). 5. At 1000ºC, the C precipitates. Liquid composition is fixed until completely solidified.
Because quartz and feldspar coprecipitate, this leads to some interesting intergrowths. Graphic granite, Ontario Granophyre K. Hollicher (Union)
Feldspar growth can be zoned - dramatic zoning such as this, where plagioclase mantles some of the alkali feldspar, is known as rapakivi texture.
Msc + Qtz Ksp + Bt + Als + Melt Melt Composition ASI ~1.3 ASI ~1.3
Melt Geometry Faceted boundaries Faceted boundaries Dry Dry quartz uartz junctions unctions
Mount Scott Granite An intergrowth of quartz and feldspar Likely result of too few nucleation sites Undercooling Viscosity contrasts Rapid diffusion
Eastern Wichita Mountains
Anorthoclase-K Anorthoclase-N 5 Feldspars Rims Matrix Feldspar
Feldspar Composition An 50
An 50 a H 2 O = 0.1 a H 2 O = 1.0 An 50 Solvus pairs: calculated temperature for coexisting anorthoclase- N and -K based on the solvus at 200 MPa Seck, 1971 Yoder, Stewart, and Smith, 1957
Final cooling Matrix crystallization in rhyolite pile Plagioclase sensitive to F contents Depth suggested by hornblende geobarometry
Halides Halite NaCl Sylvite KCl Fluorite CaF 2
Halite - NaCl Fluorite – CaF 2
Sulfates Gypsum CaSO 4 2H 2 O Anhydrite CaSO 4 Barite BaSO 4 Epsomite MgSO 4 7H 2 O Hexahydrite MgSO 4 6H 2 O Image from mineral.galleries.com
Cave of Crystals in the Naica mine, Chihuahua, Mexico. The giant faceted and transparent single crystals of gypsum measure up to 11 m in length. Garcia-Ruiz et al. propose that these crystals derived from a self-feeding mechanism driven by a solution-mediated, anhydrite-gypsum phase transition. Geology � Volume 35, Number 4
Single epsomite crystals. Large crystals often show growth planes. Are yours large enough to index? Which symmetry system is this?
Anhydrite CaSO 4 Anhydrite has tetrahedrally coordinated sulfur and cubically coordinated Ca. As the name indicates no H 2 O
Image from Klein and Hurlbut, 1985
Tungstates Wolframite (Fe,Mn)WO 4 Scheelite CaWO 4 Molybates Wulfenite PbMoO 4 Borates Ulexite NaCaB 5 O 6 (OH) 6 5H 2 O www.gc.maricopa.edu/.../ Ulexite%20top651.jpg
Phosphates Apatite Ca 5 (PO 4 ) 3 (F, Cl, OH) Monazite (Ce, La, Y, Th)PO 4 Wavelite Al 3 (PO 4 ) 2 (OH) 3 5H 2 O Turquoise CuAl 6 (PO 4 ) 4 (OH) 8 4H 2 O
Image from Klein and Hurlbut, 1985
Chemogenic Sediments Minerals precipitate due to oversaturation of an evaporating fluid (just like your epsomite). Some form in closed bodies of water, with significant evaporation. Gypsum, Anhydrite, Halite, Sylvite
Sea water - crystallization
Sabkha Arid near-marine environments may host anhydrite and gypsum deposits. The mineral precipitated is largely a function of proximity to water
Salt Flat Shallow seas, lakes in closed basins where evaporation outpaces input. Shallow mesozoic seas covered modern-day Colorado, leaving thick deposits.
NY Salt Lansing mine, 6 miles of room and pillar 2,300 feet below Lake Cayuga. 300 Ma deposit made from a shallow sea, now buried deep. Daily Ithican Online
Salt domes Salt is less dense and more fluid than surrounding rocks. May move upwards as diapirs. Classic examples are found along Gulf Coast - Louann Salt American Scientist, Sept.-Oct. 1991, p.426
Gypsum expansion Anhydrite becomes rehydrated - forming gypsum. The expansion produces bowing of layers in Triassic rocks in Caprock Canyon, Texas
Hydrothermal deposits Hot water movement in upper crust can deposit many, many minerals. Heat may be provided by magmatism or ovethrust. Fluid sources may be magmatic, from dewatering reactions, or meteoric. Movement is contingent on nature of porosity/ permeability.
Permeability and � k � d 2 � 2 / 1600 k in m 2 d = 1 mm from von Bargen & Waff, 1986
Measured permeabilities are slightly lower than predicted by models -11 Wark and Watson (1998); Liang et al. (2001) quartzite -12 -13 -14 measured measured -15 -16 marble -17 0.00 0.01 0.02 0.03 0.04 0.05 fluid fraction, � Variable grain size, pore distribution and � , but monomineralic
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