Minerals In Thin Section

Front 1

microcline

Back 1

KAlSi3O8
*scotch plaid
*lamellar intergrowth
*one/two cleavages

Hint 1

-triclinic
-biaxial
-2V = 77 - 84

Front 2

plagioclase

Back 2

CaAl2Si2O8 - NaAlSi3O8
*polysynthetic twinning
*clear color

Hint 2

-triclinic
-white to pale yellow interference colors

Front 3

quartz

Back 3

SiO2
*colorless
*lack of cleavage/alteration/twinning
*undulatory extinction!

Hint 3

-hexagonal
-could show fracture
-very low relief
- (+)

Front 4

beryl

Back 4

Be3Al2Si6O18
*hexagonal basal sections
*often contains fluid inclusions

Hint 4

-colorless, maybe pale yellow/green
-moderate relief
-hexagonal

Front 5

cordierite

Back 5

(Mg,Fe)2Al4Si5018
*alters!
*not perfect twins
*pseudohexagonal basal sections

Hint 5

-orthorhombic
-biaxial
- (-)

Front 6

Talc

Back 6

Mg3Si4O10(OH)2
*high interference colors
*platy masses
*fibrous aggregates

Hint 6

-talc + quartz = RARE!
-could be pryophyllite
-monoclinic
-one perfect cleavage

Front 7

Biotite

Back 7

*brown to yellow,red or green mica
*one excellent cleavage
*shows bird's eye extinction

Hint 7

-pleochroic
-parallel extinction

Front 8

Muscovite

Back 8

*clear mica
*2nd order interference colors
*bird's eye extinction

Hint 8

-pale green ? slightly pleochroic
-one excellent cleavage
-monoclinic

Front 9

Chlorite

Back 9

In general, keys to identifying chlorite are its lack of pronounced features, habit, (normally) colorless or greenish color, low order or anomalous interference colors. Chlorite's features generally make identification straightforward.

Hint 9

Chlorite is common as a secondary mineral, forming after mafic minerals, in rocks of many types. It may also be a primary mineral in low- to medium-grade metamorphic rocks.

Front 10

Stilpnomelane

Back 10

*dark brown
*pale yellow
* or green mica- like
*strong pleochroism
*shows 2 cleavages

Hint 10

low-grade regional metamorhpic rocks
-associated with chlroite, muscovite, garnet, actinolite, calcite or epidote

Front 11

Chloritoid

Back 11

Keys to identifying chloritoid are its relatively high relief, single cleavage, greenish color, and low-order or anomalous interference colors. If visible, an "hourglass" structure, due to zoning, is diagnostic.

Hint 11

Chloritoid is found in low- to meidum-grade metasedimentary rocks. It has general compostion (Fe,Mg,Mn)Al2SiO5(OH)2. It is a layered silicated, similar to chorite and to micas in some ways.

Front 12

Staurolite

Back 12

Keys to identification are staurolite's yellow color and pleochrosim, and its occurrence in medium grade pelitic rocks.

Hint 12

·Appearance and habit - Staurolite has diagnostic pleochroic yellow color. It often contains many quartz inclusions, sometimes giving a "swiss cheese" appearance.

Front 13

Kyanite

Back 13

Keys to identification are high relief, bladed habit, good cleavage and first order interference colors.

Hint 13

·Color - Typically colorless but sometimes has a pale blue color. May be slightly pleochroic.

Front 14

Cordierite

Back 14

Cordierite can be difficult to identify if untwinned, inclusion free, and unaltered. It can be clear with birefringence equivalent to quartz and plagioclase. When twinned, the twins may be distinctive truncating polysyntheitc twins, but may also appear very much like typical plagioclase twins. Cordierite often alters partially or whooly to a fine grained sericite, termed pinite. Inclusions of zircons may give rise to burn marks around included grains, appearing sometimes as brown rings or pleochroic halos.

Hint 14

·Twinning common - Polysynthetic twins may look like plagioclase but frequently pinch out in one direction.

Front 15

Andalusite

Back 15

Keys to identification are high relief, low birefringence and parallel extinction.

Hint 15

·Appearance and habit - often euhedral crystals with square outlines or coarse columnar aggergates

Front 16

Silimanite

Back 16

Keys to identification are high relief, needle-like, fibrous or bladed habit, characteristic square cross sections with one diagonal cleavage. It is clear and shows upper 2nd order interference colors.

Hint 16

·Appearance and habit - Classic sillimanite forms needles with square cross sections that show one diagonal cleavage. It may also appear as blades or as fine fibrous mats (called fibrolite).

Front 17

Garnet

Back 17

Garnet is one of the few common isotropic minerals. It is generally colorless or has a pale tinge, often irregularly fractured, and has high relief.

Hint 17

·Appearance and habit - Euhedral crystals may have 6 or 8 sides. More typically garnet is subhedral equant or anhedral.

Front 18

Hornblende (actinolite)

Back 18

Strong brown or green coloration and pleochroism, and 60o-120o cleavage angles identify hornblende. Actinolite is similar, but its color ranges from light green (Fe-poor varieties) to dark green (Fe-rich varieties).

Hint 18

·Color - Hornblende is generally pleochroic in shades of brown and green, less commonly yellow or red-brown. Concentric or patchy color zonation may be present. Actinolite is similar but colors are restricted to greens.

Front 19

Epidote

Back 19

These minerals may be hard to identify if present as small accessory grains. When larger, identification is simpler. High relief, anomalous interference colors or (for epidote) grains showing many interference colors, are keys to identification. Relief and birefringence increase with increasing Fe-content.

Hint 19

·Extinction - Extinction is parallel in elongate grains (not in grains in other orientations).

Front 20

Tremolite

Back 20

Tremolite is identified by its bladed habit, amphibole cleavage, clear to pale green color and upper-first to low-second order interference colors.

Hint 20

·Color - Tremolite is clear to light green, may be slightly pleochroic. The color and pleochroism are more pronounced with greater Fe content.

Front 21

Olivine

Back 21

Olivine is in many ways similar to clinopyroxene. The keys to identifying olivine are its high birefringence, lack of cleavage (but often having fractures), and alteration.

Hint 21

Olivine is typically found in mafic to ultramafic igneous rocks and in their metamorphic equivalents. Less commonly it is found in marbles and a few other metamorphic rock types. Olivine has general formula (Mg,Fe)2SiO4. Most natural olivines contain only minor other components.

Front 22

Spinel

Back 22

·Appearance and habit - Spinels are isotropic, have high relief, and show no cleavage. They may form octahedra but, more commonly, are anhedral.

Hint 22

·Color - Color is highly variable. Common spinels may be colorless, green, red, brown, blue, or black.

Front 23

Clinopyroxene

Back 23

The keys to identifying calcic clinopyroxene are normally its high relief, pale green (sometimes clear or light brown) color, middle second-order interference colors, and near 90o cleavage seen in some views. Distinguishing the different pyroxenes and olivine can sometimes be difficult.

Hint 23

·Color - Common augite is generally light green; Fe-free clinopyroxene such as diopside may be clear; other varieties may be light brown or yellow. Darker color and weak pleochroism are characteristic of more Fe-rich varieties.

Front 24

Orthopyroxene

Back 24

Orthopyroxene is in many ways similar to clinopyroxene. The keys to identifying orthopyroxene are its relief, usually pale green (sometimes pleochroic to pink) color, low-order interference colors, and near 90o cleavage seen in some views.

Hint 24

Well developed end sections may appear as blocky four- or eight-sided crystals; more typically orthopyroxene forms stubby prisms, rectangular lathes, or anhedral crystals and masses. Basal sections show cleavage angle near 90o. Longitudinal sections show one cleavage. Extinction is parallel to the cleavage.

Front 25

Sphene

Back 25

Titanite is one a few minerals with very high relief and very high order interference colors. When visible, its sphenoid or wedge-shaped crystals are diagnostic.

Hint 25

·Appearance and habit - The characteristic euhedral crystals, if present, are shaped like diamonds, wedges, or distorted hexagons. Typically appears as small accessory grains that stand out from other minerals.

Front 26

Rutile

Back 26

Keys to identifying rutile are its strong yellowish to reddish brown color, high relief, and extreme birefringence.

Hint 26

Rutile is a common accessory mineral in intermediate to mafic igneous rocks, and in many metamorphic rocks. It has composition TiO2.

Front 27

Graphite

Back 27

Graphite is one of several opaque minerals that can be difficult to distinguish. In reflected light it appears silvery and metallic, occasionally brownish gray. It may appear similar to magnitite or other silver-colored oxides, but generally has lower reflectance. Perhaps the best way to distinguish it from other opaques is that is sometimes has a sheety habit and cleavage.

Hint 27

Graphite is a native element, having composition very close to 100% carbon. It is a widespread accessory mineral in schists, marbles, gneisses, and other metamorphic rocks. It is also, much less commonly, found in some igneous rocks.

Front 28

Calcite

Back 28

These two carbonates are generally colorless, have extreme birefringence, and show one or two sets of polysynthetic twins that may show in both PP and XP views. They are typically told apart by using a chemical stain (alizarin red) that turns calcite pink.

Hint 28

·Twinning common - depending on grain orientation, one or two sets of parallel twins may show. The carbonates are the only common minerals that typically show twinning in both PP and XP views.

Front 29

Zircon

Back 29

small prismatic crystals with high relief and exteme birefringence, often surrounded by pleochroic halos when present as inclusions in other minerals

Hint 29

tetragonal, crystlas may appear as small prisms with pyramidal terminations or small slender rounded grains

Front 30

apatite

Back 30

colorless, moderate relief, white-to-gray interference colors; often small lathlike prismatic crystlas with a hexagonal cross section; uniaxial

Hint 30

hexagonal, sometimes course anhedral crystals, more typicall small subhedral to euhedral hexagonal crystals or prisms

Front 31

Monazite

Back 31

found in sandstones and siltstones

Hint 31

twinning common
biaxial
positive

Front 32

Sericite

Back 32

.

Hint 32

.

Front 33

Leucoxene

Back 33

.

Hint 33

.

Front 34

Hematite

Back 34

normally opaque, but thin crystals may show some red color at their edges, red, black or steel blue in reflected light

Hint 34

very high relief, common imineral in many rocks

Front 35

Zeolite

Back 35

colorless, low to moderate relif, very fine grained. colorless

Hint 35

heulandite, stilbite, natrolite, chabazite

Front 36

Magnetite

Back 36

opaque, steel blue-black in thin sections, euhedral crystals are rhombohedral cross sections of octahedra and are magnetic

Hint 36

massive or granular aggregates are also common, contact or lamellar twis are commone