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 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)
Same as previous photo (xpl, x80)
Stubby prisms of Apatite included in Hornblende in a mafic rocks (ppl, x20)
Same as previous photo (xpl, 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
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