Biotite

Chem: K(Fe,Mg)₃AlSi₃O₁₀(OH)₂
Type: Sheet / Phyllosiclicate
Structure: T-O-T+c structure
Comp. Anio: (Si₄O₁₀)^4-
Si-to-O: 2 ; 5
Cry. sys: Monoclinic
Hd: 2.5 / 3.0
Hs: shiny black
Col/pleo: tan/straw brown – dark brown or green – brown green
Relief: moderate/+ve
Cleve: perfect cleavage
Twin: none
Habit: massive/platy
Ext: parallel / pebbly
Int. col: 3rd – 4th
Other: vitrous

Calcite

Chem: CaCO₃
Type: rhombohedral / trigonal
Hd: 3
Hs: Colourless, white, grey colourless, maybe dirty
Relief: high/positive
Cleve: brittle / concoidal black with pastel stripes
Twin: wiggly twins?!?
Habit: crystalline/granular
Ext: twinkling extinction
Int. Col: VERY HIGH – 4th order
Other: reacts readily with HCl

Chromite (alt)

Chem: FeCr₂O₄
Hs: brown streak, metallic luster, black granular
Other: black under both xpl and ppl

Garnet

(Ca,Fe,Mg,Mn)3Al2Si3O12
Orthosilicate / Nesosilicate
T-O structure
Comp. An: SiO₄^4-
Si-to-O: 1 ; 4
Type: isometric / cubic
Hd: 6.5 – 7
Hs: red but variety too
Col/pleo: clear/dirty – none
Relief: high/positive
Twin: none/uneven
Habit: blocky/cubic
Ext: NA

Hornblende(type of amp)

Chem: Ca₂(Mg,Fe,Al)₅(Al,Si)₈O₂₂(OH)₂
Type: Double Chain / Insolcate
Structure: T-O-T structure
Comp. Anio: (Si₄O₁₁)^6-
Si-to-O: 4 ; 11
Cry. sys: Monoclinic
Hd: 5.0 – 6.0
Hs: shiny black – underlaying green?
Col/pleo: green-brown/light-darker
Relief: high/positive
Cleve: brittle – 2@60/120
Twin: simple twins (sometimes)
Habit: bladed / fibrous
Ext: inclined (14-25)
Int. col: 2nd low – 3rd high?
Other: dark opaque rim

K-Feldspar
(Orthoclase)

Chem: KAlSi3O8
Type: Framework / Tectosilicate
Comp. Anio: SiO₂
Si-to-O: 1;2
Cry: Monoclinic
Hd: 6
Col/pleo: perthitic texture in pink-white
Relief: very low/-ve
Clev: 2@90
Twin: tarten/carlsbad
Habit: tabular
Int. col: 1st – grey/white

Olivine

Chem: (Fe,Mg)SiO₄
Type: Orthosilicate / Nesosilicate
Comp. Anio: SiO₄^4-
Si-to-O: 1 ; 4
Cry. sys: orthorhombic
Hd: 6.5 – 7
Hs: yellow-green/olive green
Col/pleo: colourless-olive green/none
Relief: high/positive
Cleve: none/irregular
Habit: granular
Ext. parallel to crystal faces
Int. col: 2nd – 4th
Other: rarely with quartz

Orthopyroxene

Chem:(Fe,Mg)2Si2O6
Type: Single Chain / Inosilcate
TOT: T-O-T structure
Comp. An: (SiO3)2-
Si-O: 1 ; 3
orthorhombic
Hd: 5.0 – 7
shiny black – dull
weathered black pale green – pink
Relief: moderate/+ve
Cleve: 2@90
Habit: thin lamellar tabular parallel
Int. Col.: 2nd – brown/yellow

Plagioclase

Chem: (Ca,Na)AlSi₃O₈
Type: Framework / Tectosilicate
Comp. anio: SiO₂
Si-to-O: 1 ; 2
Cry. sys: Triclinic
Hd: 6 – 6.5
Hs: grey – white
Col/pleo: clear/dirty
Relief: low/-ve
Cleve: 2@90
Twin: albite/carlsbad/zone
Habit: tabular
Int. col: 1st – grey/white
Other: zoning – Na rich rim / Ca rich core
NOTE zoning in the last petrographic image.

Quartz

Chem: SiO₂
Type: Framework / Tectosilicate
Comp. Anio: SiO₂
Si-to-O: 1 ; 2
Cry. sys: rhombohedral / trigonal
Hd: 7
Hs: many colours
Col/pleo: translucide clear/none
Relief: low/negative
Cleve: none/concoidal
Twin: none
Habit: massive/colmnar
Ext. wavy
Int. col: 1st – grey/white

Tourmaline

Actinolite

Chem: Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂
Inosilicate
Monoclinic
moderate/+ve
green under PPL

Andalusite

Chem: Al₂SiO₅
Orthosilicate / Nesosilicate
orthorhombic
7-7.5
red? Dark cross-like in x-section = chiasolite
Col/pleo: clear/dirty – darker than Cordierite in ppl
Relief: moderate/+ve
Ext: parallel
Int. Col: 1st – grey
Other: looks like a dirtier Cordierite, higher relief LOW PRESSURE POLYMORPH

Bedding vs Cleavage

To be updated…

Blueschist

This rock is formed deep underground about ~15 to 30 km of depth with between ~200 to 500 degrees Celsius. Blue colour is caused by Glaucophane mineral in the rock, which is a type of amphibole.

Chert

Chem: SiO₂
Hd: 7
Hs: black, grey, white
Col/pleo: dirty brownish grains – none
Relief: low/-ve
Cleve: none/concoidal
Habit: nodules
Int. col: 1st – grey/white
Notes: not a true mineral rather a siliceous ooze – fine crystalline in xpl. Hardness same as quartz

Chalcedony and cavity filled with quartz

Chalcedony is a type of fibrous cryptocrystalline to fine grained silica that forms in pores, cavities and vugs in pre-existing rocks by precipitation from Si-rich fluids that pass through the rocks. Agate is a more brightly coloured variety of chalcedony that typically shows colour banding, with the colours being due to trace amounts of iron and manganese (or, increasingly, to dyes!). The concentrically banded geodes that you see in rock shops are vugs that have been partially to wholly filled with chalcedony/agate.

Chlorite

Chem: (Mg,Fe)₃(Si,Al)₄O₁₀(OH)2*(Mg,Fe)₃(OH)₆
Sheet / Phyllosillicate
T-O-T structure
Monoclinic
Hd: 2-2.5
Hs: green clear – green
Relief: high/positive
Cleve: perfect cleavage foliated masses – 2nd – Berlin Blue
Other: has inclusions

Coal

To be updated…

Dolomite

CaMg(CO3)₂
rhombohedral / trigonal
Hd: 3.5 – 4
Hs: Colourless, white, grey (due to impurities, it can be many colours)
Col/pleo: colourless/none
Relief: low-moderate
Cleve: perfect
Twin: black with pastel stripes, wiggly twins?!?
Habit: crystalline/granular
Ext: twinkling extinction
Int. Col: VERY HIGH – 4th order
Other: Sugary texture may only be observed in finely crystalline dolomite as opposed to curved crystals faces of coarse dolomite. HCL reaction is very poor to none. It is extremely difficult to separate dolomite from calcite using a thin section. Precipitate mineral!

Epidote

Chem: FeAl₂O-OH[Si₂O₇][SiO₄]
Sorosilicate
Monoclinic
6
silver/pistachio green yellow or green
colourless to greenish yellow
Relif: high/+ve
brittle – planar
lamellar (not common) fibrous, coarse to fine granular, massive.
3rd – bright green vitrous,
Notes: pearly (Regional and contact metamorphic rocks)

Garnet with pressure shadows

Please check the information for the mineral in the igneous table above.

Hematite (Ore) (B), Quartz (A)

On the XPL photo, you can see the radiating cement of quartz (A) and the think black-brown (to dark reddish) outline of hematite (B) Hs: red-brown streak, steel gray and metallic shiny

Kyanite

Chem: Al₂SiO₅
Type: Triclinic
Hd: 5.5-7
Hs: blue
Col/pleo: clear
Relief: high/+ve
Twin: simple twins (sometimes)
Habit: bladed / tabular
Ext: inclined
Int. Col: 1st – grey/yellow
Other: HIGH PRESSURE POLYMORPH, may show stepped appearance like “tree-bark” in thinsection

Mudstone

with mud interclasts

AND

with interclasts of fragments

Muscovite

Chem: KAl3Si3O10(OH)2
Sheet / Phyllosilicate
T-O-T+c structure
Comp. An: (Si4O10)4-
2 ; 5
Monoclinic
Hd: 2.5 / 4.0
Hs: silvery/white – colourless
Col/pleo: clear/none
Relief: moderate/+ve
Cleave: perfect cleavage
Twin: none
Habit:massive/platy parallel / pebbly 2nd – 3rd vitrous

Ooids

The second image (XPL) shows with stylolites…

Fossils in Packstone

to be updated…

Serpentine

to be updated…

Sillimanite

First two images are from an altered formation of the mineral in igneous rocks.

Chem: Al2SiO5
orthorhombic
Hd: 7.5
Hs: white/yellow
Col/pleo: clear/none
Relief: moderate to high/+ve
Habit: cylindrical/fibrous
Int. col: 2nd – pink

Staurolite

Chem: (Fe,Mg,Zn)₂Al₉(Si,Al)₄O₂0(OH)₄
Type: Orthosilicate / Nesosilicate
Monoclinic
Hd: 7-7.5
Hs: dark brown / black
Col/pleo: honey/potato yellow – none (ppl)
Relief: high/+ve
Cleve: subconcoidal
Ext: zoning?
Habit: intersecting prisms like a cross
Int. Col: 2nd – mid/high yellow
Other: poikiloblastic (air bubbles), “stauros” greek for cross – 2intersecting prisms HEXAGONAL SHAPED EUHEDRAL

Tremolite

Monoclinic
5.0-6.0
white, brown…
colourless – none
moderate/+ve
brittle
2@60/120
silky, fibrous
2nd – bright
turquoise similar to sillimanite in handsample

Zebra Dolostone

This type of rocks were originally limestone, now transformed into ‘Zebra’ dolomite.

At least partially fracture controlled, because of planar zones of coarse, light-coloured dolomite in darker, fine grained host. However, all of the rock now consists of dolomite, including the dark fine grained portions, suggesting an earlier period of pervasive dolomitization.

Halite and Gypsum

Halite is a Precipitate! Since it is white in colour, hand samples may be contaminated with other minerals causing it to appear in different coulours. The Halite is pretty much table salt, but do not taste it.

Gypsum has a sugary texture, curved crystal surfaces