More on feldspars & quartzHalides, sulfates, borates,

phosphatesEvaporites & Hydrothermal

More on feldspars & quartzHalides, sulfates, borates,

phosphatesEvaporites & Hydrothermal

JD PriceJD 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 aDd

aAa aB

b

General Mass ActionGeneral 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 2O1

aNaAlSi3O8

2 aH 2O1

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.

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k = pH 2O1

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.

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Because quartz and feldspar coprecipitate, this leads to some interesting intergrowths.

Graphic granite, Ontario

Granophyre K. Hollicher (Union)

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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 + MeltMsc + Qtz Ksp + Bt + Als + Melt

ASI ~1.3

Melt CompositionMelt Composition

Melt GeometryMelt Geometry

Faceted boundaries

Dry quartz junctions

Mount Scott GraniteMount Scott Granite

An intergrowth of quartz and feldspar

Likely result of too few nucleation sites

Undercooling

Viscosity contrasts

Rapid diffusion

An intergrowth of quartz and feldspar

Likely result of too few nucleation sites

Undercooling

Viscosity contrasts

Rapid diffusion

Eastern Wichita MountainsEastern Wichita Mountains

Anorthoclase-N Anorthoclase-K

Matrix Feldspar

Rims

5 Feldspars

5 Feldspars

Feldspar CompositionFeldspar Composition

An50

a H2O = 0.1a H2O = 1.0

Seck, 1971Yoder, Stewart, and Smith, 1957

An50

An50

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

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

Depth suggested by hornblende geobarometry

Depth suggested by hornblende geobarometry

Plagioclase sensitive to F contents

Plagioclase sensitive to F contents

Matrix crystallization in rhyolite pile

Matrix crystallization in rhyolite pile

Final coolingFinal cooling

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Halite NaCl

Sylvite KCl

Fluorite CaF2

HalidesHalides

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Halite - NaCl

Fluorite – CaF2

Gypsum CaSO4 2H2O

Anhydrite CaSO4

Barite BaSO4

Epsomite MgSO4 7H2O

Hexahydrite MgSO4 6H2O

Image from mineral.galleries.com

Sulfates

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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.

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Single epsomite crystals. Large crystals often show growth planes.

Are yours large enough to index?

Which symmetry system is this?

AnhydriteAnhydriteCaSO4

Anhydrite has tetrahedrally coordinated sulfur and cubically coordinated Ca.

As the name indicates no H2O

Image from Klein and Hurlbut, 1985

TungstatesWolframite (Fe,Mn)WO4

Scheelite CaWO4

MolybatesWulfenite PbMoO4

BoratesUlexite 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 5H2OTurquoise CuAl6(PO4)4(OH)8 4H2O

Phosphates

Image from Klein and Hurlbut, 1985

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

Chemogenic SedimentsChemogenic Sediments

Sea water - crystallizationSea water - crystallization

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Sabkha

Arid near-marine environments may host anhydrite and gypsum deposits.

The mineral precipitated is largely a function of proximity to water

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Salt FlatSalt FlatShallow seas, lakes in closed basins where evaporation outpaces input.

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are needed to see this picture.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 SaltNY Salt

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Salt domesSalt 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 depositsHydrothermal depositsHot 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.

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

-170.01 0.02 0.03 0.04 0.050.00

fluid fraction, f

quartzite

marble

measuredmeasured

War

k a

nd W

atso

n (1

998)

; Li

ang

et

al.

(200

1)

Variable grain size, pore distribution and , but monomineralic