Jan 25 Optical Mineralogy 7

Tintina

IM

MT-CR

IS

NRMT

AA

KHO

NAO

(a)

(b)

YT

D

R

S

DA

DA

DA

O

O

O

P

P

P

B

B

B

K

K

K

N

5 0 0 k m

NS

E

E

T NS

NA

SB

PCYT

SM

SM

WYT

CA

NA

T

T

NF

NF DLM

H

RR

A

A

KY

KS

TGGN

KB

KBA

C

DS

Y

80 Ma 70 Ma 60 Ma 55 Ma 45 Ma

Optical Mineralogy VII: Optical mineral properties and other features of rock-forming

minerals (cont.)

William D. Nesse, Introduction to Optical Mineralogy, Chapter 8

Anthony R. Philpotts, Chapters 3

Zoning •  Many mineral grains are compositionally

heterogeneous; they appear zoned with areas of different compositions exhibiting slightly different optical properties.

•  Heterogeneity can be primary (due to

events that occur at initial crystallization) or secondary (as a result of alteration or metamorphism).

Compositional zoning in plagioclase in an andesite from Mt. St. Helen (xpl, x20)

Aegerine-Augite zoned crystals and zoned crystals of sodic hornblende. Notice high relief hexagonal

prisms of Apatite (ppl, field with 0.9mm)

Zoned Augite crystal. Augite is zoned outward through purple titanaugite and marginal bright

green Aegerine (ppl, field with 3.7mm)

Zoned garnet crystals (as seen under a microprobe)

(a) (b)

(c) (d)

(e) (f)

Fig. 5. Element distribution maps and BSE images of garnet used for thermobarometric estimates.

METAMORPHIC H ISTORY OF A SYN-CONVERGENT OROGEN-PARALLEL DETACHMENT 9

! 2010 Blackwell Publishing Ltd

Exsolution •  Chemical mixing may be increasingly

difficult in some minerals as they cool. – High temperatures minerals are commonly not

stable at low temperatures. – Accordingly, some minerals exsolve (unmix)

to form two distinct minerals. •  Sometimes exsolution lamellae form

instead of discrete grains (e.g. perthite).

Exsolution lamellae of Augite in Orthopyroxene (inverted pigeonite) (xpl, x20)

A large crystal of orthopyroxene, which inverted from pigeonite. Exsolution of augite from the original twinned pigeonite crystal (xpl, x20).

Orthoclase perthite formed by exsolution of albite-rich feldspar from potassium-rich feldspar (ppl x20)

Perthites- Same as previous image (xpl, x20)

Alteration •  Alteration is a common phenomenon in minerals due

to the presence of hydrothermal fluids during cooling, weathering or retrograde metamorphism

•  Alteration products are commonly referred as

secondary minerals –  If alteration results in complete replacement of the primary

mineralà pseudomorphs –  Partial replacement

•  Very useful in mineral identification

•  Examples: chloritization, seritization, saussuritazion –  olivineà serpentine/ talc or Iddingsite –  pyroxeneà amphibole/ biotite –  Cordierite à pinite

Talc pseudomorph after olivine that was first veined by serpentine (xpl, x8)

Alteration in late crystallizing Nepheline (ppl, x8)

Same as previous photo (xpl, x8)

Muscovite flecks in Nepheline under extinction. Same as previous photo (xpl, x20)

Saussuritization of Ca-rich plagioclase. Main alteration mineral is epidote (zoisite).

Augite being altered to calcite and epidote

Andalusite psuedomorphed by muscovite and quartz

Garnet altering to muscovite and quartz. Locally psuedomorphed. Retrograde metamorphism.

Inclusions •  Minerals may have inclusions of other minerals or

trapped fluid

•  Presence/absence of inclusions can be very useful in identification

•  Small inclusionsà cloudy appearance

•  Garnets typically contain inclusions •  Stauroliteà “swiss chess” appearance •  Cordieriteà sillimanite needles •  Biotite and Muscovite à zircon •  Quartz à rutile needles/tourmaline

Apatite and Zircon in late crystallizing Quartz (ppl, x80)

Stubby prisms of Apatite included in Hornblende in a mafic rocks (ppl, x20)

Staurolite with quartz inclustions

Cordierite with abundant inclusions (field of view is 2.5 mm across)

http://und.edu/instruct/mineral/320petrology/opticalmin/cordierite.htm

Syntectonic Garnet

Jan 25 Optical Mineralogy 7

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