rocks Flashcards
What are the most common and important minerals?
Quarts, feldspar (plagioclase, alkali feldspar)
Micas (Ms, Bt, Chl)
Amphiboles
Pyroxenes
Calcite and dolomite
Garnet
Olivine
Excluding the most common minerals, what important minerals are also found in igneous rocks?
zircon, apatite, titanite, rutile (accessories)
cordierite (mostly granites)
leucite, nepheline (alkali igneous; feldspathoids)
tremolite, actinolite, epidote, serpentine (alterations)
Excluding the most common minerals, what important minerals are also found in metamorphic rocks rocks?
staurolite, andalucite, kyanite, sillimanite, cordierite,
epidote, glaucophane, rutile, titanite, diopsite, tremolite, actinolite, serpentine (+serpentinites), phlogopite (marbles)
Excluding the most common minerals, what important minerals are also found in sedimentary rocks?
zircon, apatite, titanite, rutile, gypsum, anhydrite, halite, glaucophane, hematite, calcite
What minerals characterise acid igneous rocks and what is the word used to describe the colour of these minerals?
Quartz and feldspar (leucocratic)
What minerals characterise intermediate igneous rocks and what word is used to describe the colour of these minerals?
Plag = mafics (50:50)
Hornblende common but can be pyroxene (mesocratic)
What minerals characterise basic igneous rocks and what word is used to classify these minerals?
Plag + Pyroxene (melanocratic)
What minerals characterise ultrabasic igneous rocks?
Olivine + mafics (melanocratic) (ultrabasic often green HS due to abundant olivine)
What are the main minerals to look out for in a hornfels?
Cordierite, Andalusite
What index minerals characterize the Greenschist facies?
Garnet, Chlorite and Biotite
What index minerals characterize the Amphibolite facies?
Staurolite, Kyanite, Sillimanite
What minerals are indicative of a Blueschist facies rock?
Glaucophane and lawsonite
What minerals are indicative of a Eclogite facies rock?
Garnet and omphacite
What’s the field of view on x4 magnification?
4.4mm
What’s the field of view on x10 magnification?
1.8mm
What’s the field of view on x40 magnification
0.4mm
What grain size is a medium grained igneous rock?
1mm-3mm
What grain size is a coarse grained igneous rock?
3mm-3cm
What grain size is a very coarse grained igneous rock?
>3cm
What mineral? Cleavage: Two: perfect [001], good on [010] PPL: Colourless, No pleochroism, Low relief XP: First order δ (greys), commonly exhibits simple Carlsbad twins
Sanidine
What are the optical properties of sanidine?
Cleavage: Two: perfect [001], good on [010] PPL: Colourless, No pleochroism, Low relief XP: First order δ (greys), commonly exhibits simple Carlsbad twins
What is the difference between cpx and opx?
Cpx has inclined extinction while Opx has straight extinction Cpx max 2nd order birefringence while Opx max first order
What is the difference between an amphibole and a px?
Amph has 2 intersecting at 124°/56° (in basal sections) and px has 2 at 90° (only seen in basal sections) Amph often has strong body colour that masks its 1st to low 3rd order birefringence.
What is the distinguishing feature of titanite?
Lozenge shaped
What is the distinguishing feature of zircon?
Forms pleochroic haloes in biotite
Chiastolite is a type of what?
Andalusite
What are the distinguishing features of Andalusite?
highly symmetrical parallelogram shaped basal sections chiastolite has black cross in basal section
What are the distinguishing features of Kyanite?
Two well-developed ≠ 90° (but close to 90 so can be confused with pyroxene) Often bladed, elongate laths PPL: Characteristically pale blue, moderate-high relief XP: Inclined extinction, low 1st order δ
In what rock is sanidine most likely to be found?
Sanidine is a high temperature, potassium-rich alkali feldspar, Typically forms tabular laths,
Occurs in silicic lavas (rapidly cooled igneous rocks)
What are the distinguishing features of olivine?
PPL: High relief, curved fractures, weathering (serpentine, iddingsite)
XP: 2nd – 3rd order δ
What are the distinguishing features of Sillimanite?
PPL: Fibrous masses or acicular, moderate-high relief, colourless to yellow brown
What are the distinguishing features of Garnet?
PPL: High relief, colourless to pale pink
XP: Isotropic (because cubic), often poikilitic
What are the distinguishing features of cordierite?
PPL: Low to negative relief, usually colourless, often filled with many tiny inclusions
XP: Low 1st order δ, weathers easily to mixture of clay minerals + muscovite called pinite – alters centre of crystal
Common in LP/HT contact metamorphism of mudstones, LP equivalent to garnet, form large crystals (up to 0.5cm)
What are the distinguishing features of staurolite?
PPL: Almost always has honey yellow colour, strong pleochroism
XP: Low order δ colours masked by body colour
What are the optical properties of lecite?
cleavage: none: very poor
PPL Colour: colourless
Pleochroism: none
Relief: Negative, Low
XP: characteristic lamellar twinning, very low, almost isotopic, birefringence
What is this mineral?
Leucite
What are the optical properties of nepheline?
Cleavage: Two: fair [100] and [001]
PPL Colour: none
Pleochroism: none
Relief: None
cloudy appearance
hexagonal habit where euhedral
XP: Low 1st order δ, lack of twinning, night sky appearance due to abundant small inclusions,
What is leucite and where is it found?
Leucite occurs in alkaline igneous rocks such as syenite, phonolite, basanite, tephrite. It can occur with nepheline and sodalite. It is the principal phase within leucitites
What is nepheline and where is it found?
A sodium potassium feldspathoid, the most common feldspathoid in alkaline igneous rocks such as nephelinite, nepheline syenite and phonolite; distinguished from leucite by the lower birefringence
What is the grain size of a fine grained igneous rock?
<1mm
Where are alkaline igneous rocks formed and how do they originate?
Alkaline rocks are found in a wide range of tectonic environments but are particularly common in ocean islands and continental rifts
Alkaline parental magmas generally originate by melting of mantle rocks at smaller degrees of partial melting or greater depth than sub-alkaline magmas
When explaining your classification for a rock, what should you include?
The explanation should justify the classification in terms of rock group, phase abundance and texture.
When writing a petrogenesis for a rock, what should you specifaically reference?
Minerals and textures observed in the rock
What tectonic enironments are syenites associated with?
intraplate continental magmatism
What tectonic environments are monzogranites associated with?
shallow level, subvolcanic acid intrusions
