Chapter 7 basic mineralogy

Mineral classesBorates Vario us elements in combination with boron Borax [Na 2 B 4 O 7  10H 2 O] Carb onates Metals in combination with carb onate CO32 Calcite [CaCO 3 ] Cerrusite [P

Chapter 7 Basic Mineralogy

Ta ble 7-1 Mineral classes

Borates Vario us elements in combination with boron Borax [Na 2 B 4 O 7  10H 2 O]

Carb onates Metals in combination with carb onate

( CO32 )

Calcite [CaCO 3 ] Cerrusite [PbCO 3 ] Halides Alkali metals or alkaline earths in

co mbination with halogens (F, Cl, Br, I)

Halite [NaCl]

Fluorite [CaF 2 ] Hydro xides Metals in combination with hydro xyls (OH - ) Brucite [Mg(OH) 2 ] Native elements Pure co mpound of a metallic or nonmetallic

Vario us elements in combination with the

ZO 4 radical where Z = P, As, V, Cr, W , Mo

Apatite [Ca 5 (PO 4 ) 3 (F,Cl,OH)] Carnotite [K 2 (UO 2 (VO 4 ) 2  3H 2 O] Scheelite [CaWO 4 ]

Silicates Metals in combination with silica tetrahedra

( SiO44 ) for ming three dimensional networks, sheets, chains and isolated tetrahedra

Quartz [SiO 2 ] Forsterite [MgSiO 4 ] Orthoclase [KAlSi 3 O 8 ]

Sulfates Alkaline earths or metals in co mbination with

sulfate ( SO42)

Barite [BaSO 4 ] Epso mite [MgSO 4  7H 2 O]

Sulfides One or more metals in co mbination with

reduced sulfur or chemically similar elements (As, Se, Te)

Pyrite [FeS 2 ] Galena [PbS]

Skutterudite [CoAs 3 ]

Metals in combination with carb onate ( CO32 )

Alkali metals or alkaline earths in

Metals in combination with silica tetrahedra ( SiO44 ) for ming three dimensional

networks, sheets, chains and isolated

Alkaline earths or metals in co mbination with sulfate ( SO42)

One or more metals in co mbination with

Ionization potential: a measure of the energy required to

remove an electron from an atom and place it at an infinite distance from the nucleus

Electronegativity: a measure of the ability of an atom to

attract electrons (The smaller the electronegativity, the less likely the atom will attract electrons—it will most likely donate them instead.)

A Measure of electronegativity of elements as seen

in the periodic table.

Ta ble 7-2 Electroneg ativities

Z Ion

negativity Z Ion

negativity Z Ion

negativity

Ta ble 7-3 Percent ionic character of a single chemical bon d

Difference in

electronegativity

Ionic character, %

Difference in electronegativity

Io nic character, %

Example 7-1

Calculate the ionic character of the bond between Ca-F

From Table 7-2, the difference in electronegativity

From table 7-3, the bond is ~89% ionic

http://skywalker.cochise.edu/wellerr/mineral/fluorite/fluoriteL.htm

Coordination number: the number of

anions that surround a cation in an

ionic crystal

Radius ratio: the radius of the cation

divided by the radius of the anion

http://web.arc.losrios.edu/~borougt/MineralogyDiagrams.htm

So, we seem to think that silica (SiO44-) has a coordination

number of 4 Let’s test this

corresponding radius ratios from figure 7-2, we would see that it

fits nicely in the tetrahedral arrangement with a coordination

number of 4 Of course, we already knew that one!

http://www.minerals.net/Image/5/97/Olivine.asp

x

The Unit cell is the basic building block for a crystal In order to understand this concept, think of the unit cell as being like a brick in a wall (if the wall is built by stacking bricks directly

upon one another)

X-ray Crystallography: the science of determining the

arrangement of atoms within a crystal from the manner in which

a beam of X-rays is scattered from the electrons within the

crystal The method produces a three-dimensional picture of the density of electrons within the crystal, from which the mean

atomic positions, their chemical bonds, their disorder and sundry other information can be derived

Bragg’s Law describes

the relationship between the angle of the incident monochromatic x-ray

beam and the diffracted ray as a result of the

crystalline structure and interplanar spacing

nλ = 2dsinθ

A-C is the interplanar

spacing and is equal to d

λ is the wavelength of the x-ray and θ is the angle of incidence and diffraction

O

O

O O

Silica

(SiO4)

Examples of silicate minerals

olivine

epidote

beryl augite

hornblende

muscovite quartz

Mineral pictures from: mindat.org

Pink (Rose) : due to traces of iron, manganese or titanium

Amethyst : Maybe be manganese but some believe it could be

organic, iron or even aluminum.

Citrine : iron

Aventurine : inclusion of green mica (fushite)

Tiger's eye : inclusion of fiber of silicified crocidolite (variety of

asbestos)

Prasiolite : Iron or copper

Milk quartz : gas and liquid inclusions

Smoky : Radioactivity on quartz containing aluminium

Blue : pressure.

Chalcedony is a variety of quartz with micro-crystals Agate is a

multicolor variety of chalcedony and onyx is a variety of agate with parallel strips of various nuances of black.

Quartz Varieties

Ionic Substitutions

When minerals crystallize, certain minor or trace elements that

are present in the environment can enter the structure of the crystallizing mineral There are four rules that predict,

with many exceptions, the uptake of trace elements by

crystallizing minerals

1 Ions of one element can substitute for those of another in a crystal

structure if their radii differ by less than ~15%.

2 Ions that differ by one charge unit substitute readily for each other as

long as charge neutrality is maintained.

3 When two ions occupy the same site in a crystal structure, the ion with

the higher ionic potential preferentially enters the site.

4 Even if the size and charge of the minor and major ion are similar,

substitution may be limited for the minor ion if it has a very different

electronegativity and forms a bond of very different character from that

of the major ion.

Clay Minerals and Surface Ion Exchange

Clay mineral – fine-grained hydrous silicate composed of layers of

tetrahedrally and octahedrally coordinated cations

Figure 7-5 Structure of the octahedral and tetrahedral layer

Mg 2+ in the octahedral layer = brucite Al 3+ in the octahedral

layer = gibbsite Al 3+ can substitute for Si 4+ in the tetrahedral

layer.

Clays – any particle less than 2 microns in size May or may not be clay mineral

General clay types

Kaolinite, illites, smectites, vermiculite

Kaolinite – 1 tetrahedral and 1 octachedral layer (1:1)

-net surface charge minimal, negligible CEC

Illite – 2 tetrahedral and 1 octachedral layer (2:1) ….the octahedral sandwich

-Al substitution for Si in tetrahedral layer -marginal net surface charge minimal, low CEC

Smectites – also a 2:1 clay

-lots of Fe and Mg substitutions for Al in octahedral layer -lots of Al substitution for Si in the tetrahedral layer

-swelling clay -significant net surface charge, high CEC

Vermiculites – also 2:1 clay

-higher net surface charge -high CEC

1:1 Clays: consist of tetrahedral layer and an octahedral layer; substitutions are limited and the net charge is minimal (have a low CEC.)

kaolinite

2:1 clays: consists of two tetrahedral layers with an intervening octahedral layer The octahedral layer can be either di- or tri-octahedral and a large variety of substitutions are possible 2:1 clays have a greater variation with net charge possibilities and generally have a greater C.E.C.

montmorillonite

The octahedral and tetrahedral layers are arranged in different ways with different amounts of elemental substitutions to produce different clay

minerals.

Table 7-5 Summary of the principal characteristics of the layered clay mineral groups*

Ka olinites Illites S mectites Ve rmiculites Structure

M ostly octa hedral Inte rlayer c ations Nil K Ca , Na Mg

tri-Inte rlayer water Only in ha lloysite So me in

hydro muscovite

Ca , two la yers

Na, o ne to many layers

Ca , two la yers

K, one la yer to nil

Basal spacing 7.1  10  Va ria ble

most ~15 

Va ria ble 14.4  whe n fully hydrated

Ethylene glycol Only taken up by

Exa mples Ka olinite, dickite ,

nac rite , ha lloysite

Illite , hydrous micas , phengite , bra mmallite, glauc onite,

ce ladonite

M ontmo rillonite , beide llite,

nontronite, hec torite , saponite, sauconite

Ve rmiculite

*M odified fro m Dee r et al (1992)

Clays will have negative net surface charge caused by:

2) Imperfections in crystal structure (e.g missing cations)

For the 2:1 clays surface charge arises mostly from substitutions and imperfections For 1:1 clays surface charge arises mostly from broken bonds at crystal edges

Table 7-7 Per manent negative surface charge of 2:1 clay minerals11

Mineral group Charge ( mol sites kg -1 ) 2

Kaolinite 0.02 - 0.2 Illites 0.1 - 0.9

S mectites 0.7 - 1.7 Vermiculites 1.6 - 2.5

1 Data fro m Sposito (1989), Langmuir (1997)

2 Charge in moles of monovalent sites per kg of clay

What is Cation Exchange Capacity (CEC) and why is it important?

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Cation Exchange Capacity

The net negative surface charge will attract (adsorb) ambient dissolved cations

If other cations are introduced, these adsorbed cations will be replaced by the new cations to varying degrees ….cation exchange

The cation exchange capacity (CEC) will vary from clay to clay depending on clay structure, amount and type of substitution, pH, and particle surface area

when the clay is exposed to a 1 M ammonium acetate solution at pH 7.0

Units for CEC = meq / 100g

If surface has a net positive charge then the AEC (anion exchange capacity) is measured

Surface area effects

Table 7-8 Surface area per unit mass of illite with a density of 2600 kg m-3

Length of side ( m) Nu mber of cubes Surface area of cube ( m 2 ) Surface area (m 2 g -1 )

Example 7-6: Given a smectite with a negative surface charge of 0.8 mole sites kg-1 , what is the CEC?

Determining ion-exchange properties

Adsorption Isotherms

Represent partitioning of a particular species between an aqueous phase

and solid particles (sorbate)

Figure 7-8 Representation of a typical adsorption isotherm showing the distribution of a

species between an aqueous phase and a solid (sorbent) At very low concentrations, the

distribution behaves ideally and can be represented by a unique value, K d At higher

concentrations, the partitioning deviates from ideality If precipitation occurs, the

concentration of the species in solution will remain constant; i.e., the solution is saturated

with respect to the particular species.

Kd is the tangent to the isotherm found

at the origin

At high concentrations, precipitation keeps the aqueous concentration constant

Column Test Method

In this case, the sorbent is packed into a column and a volume of solution is passed through the column The concentration of the ion of interest in the original solution is compared to that in the

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http://www.cresp.org/cresp-projects/waste-processing-special-nuclear-Example 7-7

Ten grams of montmorillonite are placed in a column and 100ml

of solution are passed through the column The initial solution

the solution and the montmorillonite

Zeolites: a crystalline structure characterized by a framework

of linked tetrahedra, each consisting of four O atoms

surrounded by a cation This framework open cavities in the form of channels and cages These channels are usually

guest species Zeolites have relatively large CEC and are

useful for a variety of environmental remediation processes

Asbestos minerals: a group of silicate minerals that occur as

long, thin fibers They have high tensile strength, flexibility, and heat and chemical resistance Asbestos minerals can be

described by two different structures: chrysotile and amphibole

Chrysotile structure: consists

of a layer of silica tetrahedra bonded to a layer of

octahedrally coordinated Mg

by four hydroxyl molecules and two oxygens The distance

between the oxygens in the

octahedral layer is slightly

greater than the distance

between the oxygens in the

tetrahedral layer This results

in the octahedral layer curling around the tetrahedral layer

forming a scrolled tube

Amphibole structure: consists of a strip of octahedrally coordinated cations sandwiched between two double silica chains The chains extend for an infinite distance The cations can be Na, Li, Ca, Mn, Fe,

Mg, Al, and Ti

Health Effects of Asbestos Exposure

Asbestosis: a lung disease caused by asbestos particles

deposited in the lungs through inhalation Over time, the lung encapsulates these fibers and hardens leading to a decrease

Mesothelioma: a rare, diffuse malignant cancer of the lining of the lung and stomach It has a long latency period of 35 to 40 years

Lung cancer: usually linked to smoking, however, some cases have been attributed to radon, second-hand smoke, or

exposure to asbestos

Crystalline and Amorphous Silica

There are six polymorphs (same chemical composition, but different crystalline structure) of silica composition with a

Amorphous silica (opal, SiO2·nH2O) is found in siliceous oozes

in the seafloor sediments and on land as preserved deposits

of marine sediments or precipitated from geyer fluids that

contain high amounts of dissolved silica

Dissolution of Silica Minerals

For quartz:

For amorphous silica:

Where T is the temperature in Kelvin

When figuring the solubility in ppm, remember to multiply the

silica.

Example 7-8

Chapter 7 Problem set due November 26:

#s: 1, 9, 10, 14, 36, 49, 55, 57

dinner/