Origins of Color in Minerals

7/24/2019 Origins of Color in Minerals

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r g nsof Color

inMinerals

of Color

inMinerals

MeisamRasouli

MeisamRasouliPh.D. student ofeconomic geology

Department of EarthSciences,Shiraz University

Ph.D. student ofeconomic geology

Department of EarthSciences,Shiraz University


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crystal eld theory trap concept

molecular or!ital theory

" !and theory

# color caused !y physical opticse$ects

%auses

of%olor


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Energies associated 'ith electronictransitions


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)he crystal eld theory

ions should possess unpaired dor fshell electrons*+, %r, n, -e, %o, i, and %u, actinides, lanthanides/s22s22p0&s2&p0&d& > interaction 'ith visi!le

lightnot interacting 'ith visi!le light* %r01, %e(1, %u1

%r01* energy levels*ground state* not interacting 'ith lightecited states

energy levels a$ected !y*coordination ployhedron of charges 3ligand eld4

5 polyhedra distortionsstrength of crystal eld 3!ond strength4


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%rystal eld * Selection rules* parallel electron paths

7u!y 3%orundum4 8l29& %rsu!stitution for 8l&1


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7u!y*

-e impurity can ;uench red


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Emerald 3%orundum4 8l29& %rsu!stitution for 8l&1

=eand Sisu!stitution * ore open structure of =e&8l2Si09/>

compared to that of 7u!y5 %rystal eld of reduced strength

change in level %suppresses transmission in red andenhances !lue?greentransmission.

8leandrite3%r&1in 8l2=e9(4

3!alanced red and !lue?green transmission43green in daylight and red in incandescent light4 *8leandrite e$ect

Pleochroism* change in direction of the incident light


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%omplications*)he same ion in di$erent crystal sites*5 complications of a!sorption !ands5 charge transfer 5 molecular or!ital theory

)he crystal eld theorydescri!es*idiochromatic colors* maBor ingredients

3some transitions may not cause color, e.g. %u/14allochromatic colors* impurities

color centers


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%olor centers*possi!ility * unpaired electron may !e on a non?transition element impurity ion or on a crystal defectsuch as a missing ion.=oth can !e cause of color centers.

/. Celectron color center2. Chole color center


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%olor center*-?centeror -reneldefect of


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-reneldefect


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%olor center*Smoy ;uartz 3hole color center4*

Precursor* 8l&1 su!stitution for Si(1 * 3'ith some a1,F1near!y4

Grradiation'ith* H?rays, gamma rays, neutrons, etc.


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%olor center*Smoy ;uartz*

hole centerelectron center

=leachingtemp.(AA C


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%olor center*8methyst*

Gnvolving -e impurity instead of 8l*citrine 3yello'4, green 3greened amethyst4

Feating causes transition metal colorsI

even irradiation 'ith ultraviolet light can produce !andcenters that are not sta!le to light eposure.


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%olor center*

selection rule

=JEJ-*

5smoy colorground state* 5 colorless

light?sta!le* large 8?D large =?%


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%olor center*!leaching during a!sorption*Small =?5%* aie?type deep !lue irradiated !eryl!ro'n irradiated topaz

fading after days eposure to light

preservation for more than 0A years in darness


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%olor center*

3f4 hacmanite

elevated to D !y

ultraviolet light

and !leached!y eposure to

light


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Phosphorescence* very small =?% !arrier energy*enough thermal ecitations at room temperature*


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)he molecular or!ital theory

multi?centered or!italsmetal?metalmetal?nonmetalnonmetal?nonmetal

h l l !i l h


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metal?metal charge transfer

!lue sapphire* 8l29&'ith -e and )i impurities

state a4 -e211 )i(1 state !* high energy4 -e&11

)i&1

a!sorption of the deep red results in deep !lue color

dichroism* -e?)i distances 2.068 in the c direction and

2.:@8 in the perpendicular direction.

%4 -211 -e&1 D4 -e&11 -e21

8!osoription in the infrared region of the spectrum

)h l l !i l h


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metal?metal charge transfer

vivianite 3a hydrated iron phosphate mineral4*iron present in t'o di$erent sites and one of thoseoidizes preferentially. 3-e21-e&14

deep !lue color for light polarized along -e21?-e&1

direction

%ordierite* -e21?-e&1

)h l l !it l th


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metal?nonmetal charge transferElectrons !elong to the 'hole unitP!%r9(* closed electron shells for P!21, %r01, and 92?

%harge transfer* 9 %ra!sorption* !lue

transmittance* orange color

sheelite* 9 K in K9(2?units

)h l l !it l th


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electrons not on metal ions*o metals and no unpaired electronslazurite 3lapis lazuli4* 3a,%a4>38lSi4/292(3S2,S9(4

ecited states of S&?molecular units

Mraphiteo'es its properties to pi?electrons

)h ! d th


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)he !and theory

the !and theory

of metals*

/A2&electron per

cm&of metal

electrons aree;uivalent in

energy*degenerate

)h l l !it l th


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!and theory and semiconductors*covalent !onding* and eactly ( electrons per atom inthe !ond

diamond, moissanite 3Si%4, greenocite 3%dS4valence !and* eactly lled !y electrons

conduction !and* empty from electrons

)h l l !it l th


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)h l l !it l th


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!and theory and semiconductors*moderate 3e.g. prousite* 4 all colors are a!sor!edecept for red

greenocite* * !lue and violet are a!sor!ed and the

resulting color is yello'.

)h l l !it l th


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su!stituting for% in diamond34

donor electronsviolet a!sorption3Nello' color4

*% O/*/AA,AAA

)he molecular or!ital theor


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= su!stituting for% in diamond

acceptor levels

impurity !and*comple structure!lue color

8l J =



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shallo' level

am!ient thermalecitationholes in the

valence !andelectricla conductivty

es

ces


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7eferences7eferences

urt assau 3/@:>4 )he origins of color inminerals8mericam mineralogist, vol. 0&, pp. 2/@?22@


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Origins of Color in Minerals

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