More on feldspars & quartz Halides, sulfates, borates, - - PowerPoint PPT Presentation

More on feldspars & quartz Halides, sulfates, borates, phosphates Evaporites & Hydrothermal

JD Price

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

k = aC

c aD d

aA

aaB b

General Mass Action

For an ideal gas, PV = nRT GºTP - Gº = RT ln K = 2.303 RT log K At s.s. GºTP =0 with T=298K, log kº = -0.1750 Gº

2NaAlSi3O8 ab + H2O fl = Na2O melt + Al2O3 melt + 6SiO2 melt + H2O melt

k = aNa2O

1

aAl2O3

1

aSiO2

6

pH 2O

1

aNaAlSi3O8

2

aH 2O

1

P is used to indicate that H2O has a partial pressure P < 1 when melt in s H2O undersaturated P = 1 when melt is H2O saturated

The a’s for Ab and fluid are 1 Let’s assume a’s for the other components in melt are also 1.

k = pH 2O

1

With the appropriate thermodynamic data, we could determine Gº And find log pºH2O = -0.1750 Gº For some other T,P log pH2O = GºTP - Gº / (2.303 RT)

(after Burnham and Davis, 1974)

H2O and melting

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

ASI ~1.3 ASI ~1.3

Melt Composition

Melt Geometry

Faceted boundaries Faceted boundaries Dry Dry quartz uartz junctions unctions

Mount Scott Granite

An intergrowth

• f quartz and

feldspar

Likely result of too few nucleation sites Undercooling Viscosity contrasts Rapid diffusion

Eastern Wichita Mountains

Anorthoclase-N Anorthoclase-K Matrix Feldspar Rims

5 Feldspars

Feldspar Composition

An50

a H2O = 0.1

a H2O = 1.0

Seck, 1971 Yoder, Stewart, and Smith, 1957

An50 An50

Solvus pairs: calculated temperature for coexisting anorthoclase- N and -K based on the solvus at 200 MPa

Depth suggested by hornblende geobarometry Plagioclase sensitive to F contents Matrix crystallization in rhyolite pile Final cooling

Halite NaCl Sylvite KCl Fluorite CaF2

Halides

Halite - NaCl Fluorite – CaF2

Gypsum CaSO4 2H2O Anhydrite CaSO4 Barite BaSO4 Epsomite MgSO4 7H2O Hexahydrite MgSO4 6H2O

Image from mineral.galleries.com

Sulfates

GeologyVolume 35, Number 4

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.

Single epsomite crystals. Large crystals often show growth planes. Are yours large enough to index? Which symmetry system is this?

Anhydrite

CaSO4 Anhydrite has tetrahedrally coordinated sulfur and cubically coordinated Ca. As the name indicates no H2O

Image from Klein and Hurlbut, 1985

Tungstates

Wolframite (Fe,Mn)WO4 Scheelite CaWO4

Molybates

Wulfenite PbMoO4

Borates

Ulexite NaCaB5O6(OH)6 5H2O

www.gc.maricopa.edu/.../ Ulexite%20top651.jpg

Apatite Ca5(PO4)3(F, Cl, OH) Monazite (Ce, La, Y, Th)PO4 Wavelite Al3(PO4)2(OH)3 5H2O Turquoise CuAl6(PO4)4(OH)8 4H2O

Phosphates

Image from Klein and Hurlbut, 1985

Minerals precipitate due to

• versaturation of an evaporating fluid

(just like your epsomite). Some form in closed bodies of water, with significant evaporation. Gypsum, Anhydrite, Halite, Sylvite Chemogenic Sediments

Sea water - crystallization

Sabkha

Arid near-marine environments may host anhydrite and gypsum deposits. The mineral precipitated is largely a function

• f proximity to

water

Salt Flat

Shallow seas, lakes in closed basins where evaporation

• utpaces input.

Shallow mesozoic seas covered modern-day Colorado, leaving thick deposits.

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

NY Salt

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

• vethrust. Fluid

sources may be magmatic, from dewatering reactions,

• r meteoric.

Movement is contingent on nature of porosity/ permeability.

k d 2 2 / 1600

k in m2 d = 1 mm

Permeability and

from von Bargen & Waff, 1986

Measured permeabilities are slightly lower than predicted by models

• 11
• 12
• 13
• 14
• 15
• 16
• 17

0.01 0.02 0.03 0.04 0.05 0.00

fluid fraction,

quartzite marble

measured measured

Wark and Watson (1998); Liang et al. (2001)

Variable grain size, pore distribution and , but monomineralic