Download K-Fsp can be classified as Sanidine on the basis of low

EOSC 321 Labs
Page 1
Instructor: Kopylova
Igneous petrology EOSC 321
Laboratory 9:
Exotic volatile-rich rocks Kimberlites, lamprophyres, carbonatites
Material Needed: a) Microscope, b) a Manual on Optical Mineralogy (i.e. Minerals in
Thin Section by Perkins and Henke).
Lab Organization: You will start with a 1h test on alkaline rocks. Descibe a thin section
and write a short explanation what petrographic observations made you think the rock has
an alkaline affinity. In the next two hours of the lab period you will examine reference
thin sections of exotic, rare magmatic rocks rich in volatiles. Each thin section has a brief
petrographic description to assist you in the identification of minerals, textures and rock
classification. Please make sure you understand the reference petrographic descriptions
and can find all of the minerals mentioned. You should also be able to understand why a
rock is given a particular name. Your knowledge will be tested in the next Lab, which
starts with an independent assignment on a thin section of a volatile-rich rock.
Introduction: This lab introduces exotic, rare magmatic rocks rich in volatiles.
Kimberlites are potassic ultramafic, CO2-rich rocks with a characteristic inequigranular
texture, where larger crystals set in a fine-grained groundmass. Because kimberlites are
hybrid rocks and contain a lot of large xenocrysts, we describe them using terms with no
genetic connotations. Thus, we cannot call kimberlitic textures porphyritic, as it implies a
cognate origin of phenocrysts. Instead, we refer to kimberlitic texture as macrocrystal.
Macrocrystal texture is inequigranular, where larger crystals are set in a fine-grained
groundmass. It is analogous to porphyritic texture in common igneous rocks. Large grains
of unknown origin present in kimberlites are called macrocrysts or megacrysts.
Macrocryst is a crystal of unknown origin 0.5-10 mm in size set in a fine-grained matrix
of volcanic rocks. In kimberlites, olivine, phlogopite, Cr-diopside, chromite, ilmenite, and
garnet are common macrocrysts. Macrocrysts in kimberlite have dual origin: they could
have crystallized from the host magma, or they could be xenocrysts. Megacryst is a big
(1-10 cm) crystal of unknown origin in volcanic rocks. In kimberlites they are usually
olivine, phlogopite, pyroxenes, ilmenite, or garnet. Two generations of olivine are present
in kimberlites; in addition to macrocrystal olivine, kimberlite contains numerous smaller
(<0.5 mm) euhedral olivines that are usually termed microphenocrysts. All smaller
olivine crystallizes from the host magma.
In contrast to most igneous rocks, serpentine and carbonate in kimbelites are
primary, having crystallized in fine intergrown crystals on late stages of groundmass
crystallization.
Lamprophyres are defined as a group of rocks which are strongly porphyritic in mafic
minerals, typically biotite, amphiboles and pyroxenes, with any feldspar being confined to
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EOSC 321 Labs
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Instructor: Kopylova
the groundmass. Lamprophyres could be further classified into different rock types (such
as spessartine, Kersantite etc.), but because lamprophyres are relatively rare, we do not
require you to memorize all names of lamprophyric rocks. Lamprophyres encompass a
wide range of compositions, from ultramafic to silisic, but they all have high H2O content
and therefore crystallize abundant-mica-amphibole phenocrysts. Ultramafic lamprophyres
mineralogically resemble kimberlites and their classification is highly ambiguous.
Carbonatites are the most unusual type of igneous rocks. More than 50 modal % of the
rock is primary magmatic carbonate (calcite, dolomite or ankerite). It is impractical to
distinguish between these carbonate species in thin sections without special staining.
Carbonatites commonly has varied amounts of clinopyroxene, alkali amphibole, biotite,
magnetite and apatite. For carbonate-bearing rocks with 10 to 50% carbonates, we should
use modifying terms "calcitic' or "dolomitic" before the igneous rock name based on the
remaining silicate assemblage. Over 280 minerals are known to occur in various
cabonatites, reflecting the diverse and exotic carbonatite chemistry. We have some of
those exotic minerals in the Reference Thin Sections: pyrochlore (T/s 58) and
monticellite (T/s 262). You will not be asked to identify these minerals in Assignments.
Exotic rocks we see today in the lab host many minerals that are new to us. Phlogopite is
a mafic Mg mica. Petrographers call any mica that exhibits brown or yellow to colourless
pleochroism a phlogopite. Monticellite is Ca olivine (CaMgSiO4) and it resembles olivine
in thin sections. It has a high relief, parallel extinction, high birefringence, and can be
distinguished from olivine only in large grains by slightly different grain habits (see T/s
262) and the absence of serpentinization. Monticellite is a common mineral in the
kimberlite groundmass, where it forms tiny fresh crystals atypical of olivine (T/s
BAK39). Perovskite (T/s KL05) is found in kimberlites and lamprophyres where it is
identified easily due to a dark brown colour and its isotropy. Zeolites (T/s 1229) are a
group of minerals (hydrated aluminosilicates) that encompasses more than 40 species. All
zeolites have low indices of refraction and low birefringence. They commonly fill
vesicles, veins and voids in volcanic rocks. It may be difficult to distinguish zeolite
species based solely on their optical properties.
The Reference collection for the Lab is very large, but don’t feel intimidated. During the
Lab, you need to thoroughly examine just one kimberlite, one lamprophyre and one
carbonatite, and make sure you see the following important rock characteristics in the
Reference Collection:
1. Mineralogical differences between kimberlites (no Plag) and mafic lamprophyres
(Plag in the groundmass)
2. Primary carbonate in all rock types
3. Differences between primary and secondary serpentine in lamprophyres and
kimberlite
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EOSC 321 Labs
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Instructor: Kopylova
Reference collection (in two boxes)
Exotic volatile-rich rocks Kimberlites, lamprophyres, carbonatites
Thin Section 98KL 20B, 98Kl- no name, KL07, 98KL-19A, 98KL18, 98KL16,
98KL23P-3, 98KL-23P-2, 98KL-23P-1
Sample Number
KL
Rock Name:
Ultramafic lamprophyre
Location: West Greenland
Thin Section Description:
Texture:
Porphyritic, with poikilitic groundmass
0-15% Phenocrysts:
Olivine, subhedral, rounded
85-100%
Groundmass:
~45% Olivine microphenocrysts, subhedral, rounded
~40% Carbonate in large poikilitic grains, with pearl interference colours
5%
Phlogopite, in small laths, pleochroic from colourless to light-yellow. Some grains
are bigger, anhedral, strongly zoned with orange rims
5%
Opaque mineral, in euhedral isometric microlites and in large microphenocrysts
5%
Serpentine, anhedral grains, light-yellow, with grey interference colours
1%
Perovskite, in small euhedral rhombic crystals, dark brown, isotropic, with high
relief.
Comment: The rock is classified as lamprophyre based on major element chemistry and
composition of minerals. In this case optical mineralogy alone cannot determine if the
rock is kimberlite or ultramafic lamprophyre.
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EOSC 321 Labs
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Instructor: Kopylova
Thin Section KL 056 - 3B
Sample Number
KL 056
Rock Name:
Ultramafic lamprophyre
Location: West Greenland
Thin Section Description:
Texture:
Porphyritic, with poikilitic groundmass
5%
Phenocrysts:
Olivine, subhedral, rounded, 70% replaced by serpentine and a black powder of Fe oxides
95%
45%
24%
10%
10%
5%
1%
Groundmass:
Olivine microphenocrysts, subhedral, rounded, 70% replaced by serpentine and a
black powder of Fe oxides
Carbonate in large poikilitic grains, with pearl interference colours
Phlogopite, in long laths, pleochroic from colourless to light-yellow. Some grains
are anhedral, interstitial, stongly zoned with orange rims
Opaque mineral, in euhedral microlites
Serpentine, anhedral grains, light-yellow, with grey interference colours
Perovskite, in small euhedral rhombic crystals, dark brown, isotropic, with high
relief.
Secondary Minerals:
30% Seprentine + Opaque Fe oxides after olivine
Comment: The rock is classified as lamprophyre based on major element chemistry and
composition of minerals. In this case optical mineralogy alone cannot determine if the
rock is kimberlite or ultramafic lamprophyre.
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EOSC 321 Labs
Thin Section 1228
Sample Number
Rock Name:
Location: ?
Page 5
Instructor: Kopylova
P757
Augite Spessartite - a subtype of lamprophyre
Thin Section Description:
Texture:
Porphyritic, with poikilitic groundmass
7%
Phenocrysts:
Hornblende, up to 2 cm long, euhedral, pleochroic from yellow-green to green, Often
with inclusions of Cpx, and partly replaced by low relief chlorite.
93%
60%
5%
Groundmass, relatively coarse-grained (grains up to 0.3 mm):
Augite, euhedral, with good cleavage, grey-greenish
Hornblende, euhedral, pleochroic from yellow-green to green, with perfect
cleavage at 120o, and partly replaced by a low relief fibrous mineral
28% Plagioclase, in interstitial anhedral grains, poikilitic, encloses Aug.
Polysynthetically twinned. Partly replaced by sericite and a sericite-bearing
aggregate.
Sphene - yellow- grey grain with high relief
Secondary Minerals:
5% - An aggregate of secondary minerals with high relief and first order
interference colours. The aggregate may contain fine-grained sphene (?) or epidote
(?) that give it a high relief, and sericite that accounts for the birefringence. The
aggregate replaces plagioclase and amphibole.
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EOSC 321 Labs
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Instructor: Kopylova
Thin Section 1229
Sample Number
P758
Rock Name:
Olivine Spessartite - a subtype of lamprophyre
Location: near Boulder Dam, Arizona.
Thin Section Description:
Texture:
Porphyritic, amygdaloidal
5%
Phenocrysts:
Hornblende, reaching 3 cm in length, euhedral, pleochroic from yellow to light brown.
Some rims show recrystallization into microlites of Bi.
One grain of plagioclase with a faint polysynthetic twinning
10% Vesicles of irregular shapes and veins. Infilled with secondary coarse-grained
carbonate and zeolite.
85%
10%
Groundmass:
Olivine, in euhedral microphenocrysts, almost totally replaced by reddish-brown
iddingsite. Only in larger crystals grain cores remain unaltered.
10% Opaque mineral in tiny euhedral crystals
5%
Carbonate, forms occasional segregations (?) devoid of all other groundmass
minerals
60% Plagioclase, in needle-like long crystals or skeletal crystals with simple Albite
twinning. Calcic andesine.
Occasional grains of euhedral apatite, colourless garnet and anhedral quartz. The latter
two may be xenocrysts.
Secondary Minerals:
7% - Carbonate in amygdales and veins
3%
Zeolite, with very low birefringence, dark-grey, almost isotropic, with
polysynthetic twinning and a low negative relief.
10% Iddingsite on olivine rims
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EOSC 321 Labs
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Instructor: Kopylova
Thin Section 98KL076, 98KL13-2, 98KL13-1, 98Kl-12B, 98KL-21C, KL05-02A515A,
98KL-19B, KL05-5B2
Sample Number
KL
Rock Name:
Ultramafic lamprophyre
Location: West Greenland
Thin Section Description:
Texture:
Porphyritic
0-10% Phenocrysts:
Olivine, subhedral, rounded
90-100%
Groundmass:
~40% Olivine microphenocrysts, subhedral, rounded
~20% Phlogopite is present in large interstitial grains and in smaller euhedral zoned
crystals with orange rims. Pleochroic from colourless to light-yellow.
~15% Carbonate in large poikilitic grains, with pearl interference colours
~10% Serpentine, forms fine-grained aggregates of light-green colour.
3%
Perovskite, in small euhedral rhombic crystals, dark brown, isotropic, with high
relief.
3%
Opaque mineral, in euhedral isometric microlites, concentrate around Ol
microphenocrysts
1%
Opaque mineral in long rods, most likely Ilmenite.
Comment: The rock is classified as lamprophyre based on major element chemistry and
composition of minerals. In this case optical mineralogy alone cannot determine if the
rock is kimberlite or ultramafic lamprophyre.
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EOSC 321 Labs
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Instructor: Kopylova
Thin Section BAK39 270m
Sample Number
BAK39 270m (4 thin sections)
Rock Name:
Phlogopite kimberlite
Location: Kennady Lake kimberlite, NWT
Thin Section Description:
Texture:
Macrocrystal
15%
Macrocrysts:
Olivine, subhedral, rounded, partly (5%) replaced by serpentine
85%
10%
35%
Groundmass:
Microphenocrysts of olivine in euhedral grains
Phlogopite, in interstitial anhedral grains poikilitically enclosing opaques.
Pleochroic from light yellow to orange. Zoned.
Fine-grained aggregate of groundmass serpentine, bluish-green, anisotropic. The
aggregate is present in interstities between an opaque mineral, phlogopite and
microphenocrysts.
Monticellite, in euhedral high-relief grains unevenly distributed throughout the
groundmass. Light green to colourless under 1 polar, interference colours up to
2nd order yellow. Readily visible on thin section margins.
Opaque minerals (most common spinel perovskitemagnetiteilmenite) in
euhedral microlitic grains
Carbonate, interstitial
Chlorite, in radiating fibres
20%
15%
15%
2%
5%
Megacrysts:
Olivine
Secondary Minerals: Chlorite
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EOSC 321 Labs
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Instructor: Kopylova
Thin Section JD30 150’5”, JD35 (11+ thin sections)
Sample Number
Rock Name:
Serpentine carbonate kimberlite
Location: Jericho pipe, NWT
Thin Section Description:
Texture:
Macrocrystal, with segregational groundmass
20%
Macrocrysts:
Olivine, subhedral, rounded, 10-100% replaced by seprentinebrown minerals of
Fe oxides and hydroxides, mainly on rims and in fractures;
Garnet with kelyphitic rims
80%
37%
Groundmass:
Microphenocrysts of olivine and opaque mineral (most likely spinel) in euhedral
grains
Fine-grained aggregate of groundmass serpentine and carbonate. Serpentine is
light-green, fine-grained and almost isotropic. Carbonate is not recognizable under
an optical microscope and was determined under a Scanning Electron Microscope
(SEM). The aggregate is present in interstities between an opaque mineral and
microphenocrysts.
Opaque minerals (most common spinel perovskitemagnetiteilmenite) in
euhedral microlitic grains
Segregations of coarse-grained serpentine and carbonate
25%
15%
3%
Megacrysts:
Olivine,
Clinopyroxene with exsolution lamellae of Opx. Cpx is rimmed by a
brown reaction rim
Xenoliths:
Country Rocks and mantle peridotites
Secondary Minerals: Serpentine after olivine, light green, isotropic or anisotropic in
fine-grained aggregates
Note: There are 2 variety of serpentine in this rock: primary and secondary. Primary is
present in the groundmass, the secondary replaces primary olivine. Their optical
properties are similar.
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Instructor: Kopylova
Thin Section SI-1
Sample Number
SI-1
Rock Name:
Carbonate kimberlite
Location: Somerset Island, NWT
Thin Section Description:
Texture:
Macrocrystal
7%
Macrocrysts:
5%
Olivine, subhedral, rounded. 20-50% replaced by a black opaque mineral
2%
Phlogopite, subhedral, rounded. Pleochroic from colourless to light brown.
Often zoned, with darker brown rims, rarely recrystallized into fine-grained
opaque-rich aggregate on rims.
93%
10%
Groundmass:
Microphenocrysts of olivine, phlogopite and opaque mineral (most likely spinel)
in euhedral grains
Carbonate, in elongate euhedral rectangular laths, with high pearl interference
colour.
Fine-grained aggregate of groundmass carbonate and serpentine (?). Carbonate has
high pearl interference colour. The aggregate is present in interstities between
opaque mineral, carbonate laths and microphenocrysts.
Opaque minerals (most common spinel perovskitemagnetiteilmenite) in
euhedral microlitic grains
30%
30%
23%
Xenoliths:
3%
Country Rocks
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Instructor: Kopylova
Thin Section 989
(Caution! Thicker than normal thin section! )
Sample Number
P121
Rock Name:
Augite Kersantite - a subtype of lamprophyre
Location: Franklin Furnace
Thin Section Description:
Texture:
Porphyritic
25%
20%
5%
75%
20%
20%
10%
10%
5%
Apatite
Phenocrysts:
Augite, euhedral, tan colour. Shows strong dispersion, i.e. does not extinct
entirely, but instead becomes dark-grey-blue.
Phlogopite, in euhedral long laths, pleochroic from colourless to dark brown.
Groundmass:
Plagioclase, low (+) relief, in interstitial anhedral grains. Forms wavy radiate
aggregates. Probably largely altered - some grains of secondary calcite and quartz
seen.
Augite, euhedral, tan colour. Shows strong dispersion, i.e. does not extinct
entirely, but instead becomes dark -grey-blue.
Phlogopite, in euhedral long laths, pleochroic from colourless to dark brown.
Grey fine-grained mineral, most likely secondary epidote replacing laths of mica
and Aug grains
Opaque mineral, euhedral to anhedral, often rimmed by secondary epidote.
Secondary Minerals: Epidote abundant in the groundmass, around opaque mineral and
replacing mica and/or Aug. Epidote is slightly pleochroic from colourless to
yellow, with the characteristic very bright interference colours of the 2nd order.
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Instructor: Kopylova
Thin Section 262
Caution! One part of the thin section is thicker
than normal!
Sample Number
P2605
Rock Name:
Carbonate-Monticellite rock
Location: ?
Thin Section Description:
Texture:
Hypidiomorphic
55%
13%
13%
8%
7%
3%
Monticellite, in large subhedral grains of rectangular shapes, birefringence up to
second order blue, biaxial negative, high relief, extinction is parallel to elongation
of the crystals. Optically resembles olivine. Shows brown alteration along
fractures.
Opaque mineral (Magnetite?) in large round anhedral grains
Carbonate in large subhedral grains with pearl interference colours and 2 sets of
perfect cleavage at 60o
Phlogopite, in euhedral crystals pleochroic from colourless to light-yellow. Often
is associated with opaques.
Apatite, euhedral, in long crystals with a low birefringence and a parallel
extinction, or in hexagonal and isometric cross-sections that look almost isotropic.
Often found in radiate aggregates.
Perovskite in large dark-brown isotropic grains with high relief
Thin Section 837
Sample Number
P1717
Rock Name:
Carbonatite
Location: Ring Dyke, Southern Zimbabwe
Thin Section Description:
Texture:
Hypidiomorphic. Rock shows veins of brecciation and mylonitization.
85%
15%
1%
5%
1%
Carbonate in large subhedral grains with pearl interference colours and 2 sets of
perfct cleavage at 60o
Apatite, euhedral, in long crystals with a low birefringence and a parallel
extinction, or in hexagonal and isometric cross-sections that look almost isotropic.
Forms veins, the orientation of larger crystals parallel to veins marks crude
lineation of the rock. Distributed unevenly, in some zones apatite comprises up to
80%.
Phlogopite, in euhedral crystals pleochroic from colourless to light-yellow. Zoned.
Opaque mineral (Magnetite?) in large round anhedral grains
Quartz, in large poikilitic grains enclosing apatite in apatite-rich zones.
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EOSC 321 Labs
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Instructor: Kopylova
Thin Section
SWE/ALN/11
Sample Number
Rock Name:
Melilitite
Locality:
Alno, Sweden
Microscopic description:
The rock has porphyritic texture, with ~10% coarse phenocrysts set in a medium-grained
groundmass. The phenocrysts include:
3% Euhedral phlogopite, pleochroic from light yellow to brown, some grains are
deformed;
3% Euhedral serpentine, 99% replaced by bright yellow serpentine with 2nd order
interference colours;
1% clinopyroxene grains surrounded by a melilite-rich reaction rim,
Occasional rare grains of colourless resorbed amphibole, low grey interference
colours, perfect cleavage at 60o, high relief,
The phenocrysts are set in a a groundmass composed of laths of melilite completely
replaced by brown cebollite (30%), euhedral and poikilitic, sometimes deformed
phlogopite (20%), euhedral opaque mineral (most likely spinel, 20%), euhedral apatite
(20%), olivine replaced by serpentine (10%). Melilite is distinguished by 1) its
characteristic "pegs" separating simple twins along central lines parallel to the lath
elongation; 2) its characteristic brown alteration product with high birefringence
(cebollite).
Note: Two important things can be observed in this thin section: 1). A typical
morphology and intense alteration of melilite in volcanic rocks; 2) Strange colour (or
rather the absence of colour) of alkaline phenocrystal amphibole. This phenomena is
typical of alkaline rocks where familiar minerals acquire unfamiliar habits and colours.
Thin Section P2958
Sample Number
Rock Name:
Carbonatite
Location: Fen Complex
Thin Section Description:
Texture:
Hypidiomorphic
89%
10%
1%
Carbonate in large subhedral grains with pearl interference colours and 2 sets of
perfect cleavage at 60o
Apatite, euhedral, in long crystals with a low birefringence and a parallel
extinction, or in hexagonal and isometric cross-sections that look almost isotropic.
Forms veins; the orientation of larger crystals parallel to veins marks crude
lineation of the rock.
Phlogopite, in euhedral crystals pleochroic from colourless to light-yellow.
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Instructor: Kopylova
One grain of Pyrochlore CaTa2O6(OH)2, dark brown anisotropic mineral, partly altered to
black secondary mineral.
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