1461_2014_lab2

ROYAL HOLLOWAY UNIVERSITY OF LONDON - DEPARTMENT OF EARTH SCIENCES Metamorphic Geology

METAMORPHIC PRACTICAL 2 (Term 1, 2014/15)

Aims and Objectives

The major aims of this practical are: 1. To introduce you to practical approaches to the identification of minerals in BOTH

hand specimen and especially thin section. 2. To allow you to practice identifying minerals. 3. To introduce you to some key metamorphic minerals.

INTRODUCTION

Identifying minerals is one of the fundamental skills needed by a geologist. As we discussed during our first lesson, there are no short-cuts in this procedure. Being able to distinguish different minerals and assess their character requires you to be able to understand and describe a range of mineral properties, and recall or be able to rapidly investigate what clues these provide as to mineralogy. Today’s lab is based around providing you with an opportunity to observe, investigate, and describe your impressions of a range of properties that can aid you in this process. You should use both hand specimens of key minerals as well as thin sections. A range of minerals - focusing on kyanite, garnet and micas - and respective thin sections are distributed around the benches in the class. You will need to move around from one to another during the course of the lab to inspect them all and complete the tasks set. At the end of this practical (at approx.. 11:45), we’ll wrap this up by me showing key features of all samples investigated.

EXERCISE – HAND SPECIMENS Kyanit e, Micas, Garnet

EXERCISE – THIN SECTIONS Use t he accompanying t hin sect ion descript ion form for describing t he t hin

sect ions available.

I’ ll ment ion during class t ime which minerals you should focus on in each sect ion!

C26.549: Garnet mica gneiss

C23.655: Muscovit e schist

There are only ~10 thin section available, so don’t hang on to them for too long.

C5.652: Kyanit e schist


You will have to hand-in a COMPREHENSIVE and DETAILED

descript ion of thin sect ion C5 652 next Friday, i.e. 21st Nov

2014. Use the enclosed descript ion template as a guideline

only, and try to include as many details as possible for each

mineral (by the way, the rock may contain more or fewer

minerals than listed there, so watch out…) .

Crucially, a DETAILED LABELLED drawing on a separate page

is paramount – and should show both PPL and XPL

observat ions. This will be assessed!

EXERCISE – THIN SECTIONS OF MINERALS There are also a few (old) addit ional t hin sect ions available t hat cont ain

predominant ly/ exclusively one of t he minerals we are focusing on t oday. They

have variable names & qualit y but should be a good addit ional resource. Don’t use

t hem t oo long and share t hem wit h your peers as much as possible.

EXERCISE - HARDNESS This tray contains 9 of the minerals in the Mohs hardness scale (sadly, no large diamonds in such sample sets…). Test the minerals against one another (or alternative hardness indicators) and list them in order of relative hardness. Could you identify each of the minerals again?

Minerals of the week:

1) Kyanite formula: Al2SiO5, (structurally Al2[O|SiO4])

Al-rich parageneses, very typical of metamorphic rocks

One form of three Al2SiO5 modifications in metamorphic rocks whose occurrence is very indicative of pressure-temperature conditions achieved. The other two modifications are sillimanite and andalusite, and you will see them introduced over the coming weeks.

Kyanite is the high-pressure –low(er) temperature modification.

Hand specimen: Typically prismatic, elongate, tabular or bladed crystals. Often very

distinctive blue colour.

Color: blue, sometimes white or gray


Luster: vitreous

Hardness: Anisotropic hardness: 5 parallel to the length of the crystals and 7 perpendicular to the long direction !!! (thus name in German = Disthen from dis ("twice") und sthenos

("force") – due to anisotropy of hardness)

Thin section:

High relief (relative to qtz, fsp), i.e. high refractive index

Low to medium birefringence (max yellow-orange of 1st order)

Always oblique extinction

Normally 2 very well-developed cleavages that intersect at a high angle (not right angle) (all dependent on thin section orientation, though)

Often porphyroblastic appearance (idiomorph - hypidiomorph)

2) Mica sheet silicate: basic formula:

Mica are all water bearing with (OH)- groups being structurally bound!

e.g. http://virtual-museum.soils.wisc.edu/muscovite/index.html

White mica: W1Y2[Si3AlO10](OH)2 - dioctahedral mica: Y-position: two trivalent (3+) cations (2 x 3+ = 6+) Y in white mica is (primarily) occupied by Al

3+ (in phengite there also is some Mg,

Fe in Y position; see below)

Biotite: W1Y3[Si3AlO10](OH)2 - trioctahedral mica: - Y-position: three divalent (2+) cations (3 x 2+ = 6+) Y in biotite is occupied by Mg

2+, Fe

2+

W posit ion: K, but also Na [ bot h as 1+ ions] ; rarely Ca (2+

ion)

White mica:

In met amorphic rocks, t here is more composit ional variabilit y for muscovit e t han

in magmat ic rocks. Thus they should be referred to as ’white mica’ rat her t han

muscovit e. It should be not ed t hough t hat in t hin sect ion t he chemically dif ferent

whit e micas (see below) cannot be dist inguished.

http://virtual-museum.soils.wisc.edu/muscovite/index.html


We chemically dist inguish – besides muscovit e – phengit e & paragonit e.

Phengit e: K(Mg,Fe2+

)xAl

2-x[ Si

3+xAl

1-xO

10] (OH)

2

Paragonit e: NaAl2[ Si

3AlO

10] (OH)

2

Muscovit e: KAl2[ Si

3AlO

10] (OH)

2

Margarit e CaAl2[ Si

2Al

2O

10] (OH)

2 –rare

Sericit e: very f ine-grained whit e mica in low-grade met amorphic rocks, of t en

alt erat ion product of feldspar alt erat ion.

Biot ite:

Biot it e: K(Mg,Fe2+

)3[ Si

3AlO

10] (OH)

2

Phlogopit e: KMg3[ Si

3AlO

10] (OH)

2

MOHS mineral hardness scale

Relative hardness (MOHS)

Mineral name

Mineral formula Mnemonic

Other associations

1 Talc Mg3[Si4O10](OH)2 The Very soft

2 Gypsum CaSO4 × 2 H2O Girls

3 Calcite CaCO3 Can

4 Fluorite CaF2 Flirt

5 Apatite Ca5(PO4)3(OH,F) And You have got in your teeth & bones!

6 Orthoclase KAlSi3O8 Other

7 Quartz SiO2 Queer

8 Topaz Al2[SiO4](OH,F)2 Things

9 Corundum Al2O3 Can As gems: ruby (red), sapphire (blue)

10 Diamond C Do


Metamorphic Rocks: Thin Sect ion Descript ion * = delete as necessary; Fill in descriptors as much as possible! Not all descriptors will be applicable to all minerals!!!! Thin section number:……………………….. Name: …………………………

Mineral 1 Name: ………………………………… Relief: *Low/moderate/high *Opaque/Transparent Shape: Cleavage: *none visible/weak/strong *If 2 cleavages, approx intersection angle: Colour and pleochroism (ppl): Abundance (approximate % of mineral in rock)

Alteration noticeable:

Birefringence: *low (grey/white) *medium (strong oranges, blues, pinks) *high (washed out pink/green) Specify typically encountered colours – colour range: Extinction: *straight *undulose Twinning *none/simple/multiple/tartan Zoning *yes/no Mineral 2 Name: ………………………………… Relief: *Low/moderate/high *Opaque/Transparent Shape: Cleavage: *none visible/weak/strong *If 2 cleavages, approx intersection angle: Colour and pleochroism (ppl): Abundance (approximate % of mineral in rock)

Alteration noticeable: Birefringence: *low (grey/white) *medium (strong oranges, blues, pinks) *high (washed out pink/green)

Specify typically encountered colours – colour range: Extinction: *straight *undulose Twinning *none/simple/multiple/tartan Zoning *yes/no Mineral 3 Name: ………………………………… Relief: *Low/moderate/high *Opaque/Transparent Shape: Cleavage: *none visible/weak/strong *If 2 cleavages, approx intersection angle: Colour and pleochroism (ppl): Abundance (approximate % of mineral in rock)

Alteration noticeable: Birefringence: *low (grey/white) *medium (strong oranges, blues, pinks) *high (washed out pink/green) Specify typically encountered colours – colour range: Extinction: *straight *undulose Twinning *none/simple/multiple/tartan Zoning *yes/no Mineral 4 Name: ………………………………… Relief: *Low/moderate/high *Opaque/Transparent Shape: Cleavage: *none visible/weak/strong *If 2 cleavages, approx intersection angle: Colour and pleochroism (ppl): Abundance (approximate % of mineral in rock)


Birefringence: *low (grey/white) *medium (strong oranges, blues, pinks) *high (washed out pink/green) Specify typically encountered colours – colour range: Extinction: *straight *undulose Twinning *none/simple/multiple/tartan Zoning *yes/no Sketch of general texture – and any additional mineral descriptions


Metamorphic Rocks: Thin Sect ion Descript ion * = delete as necessary; Fill in descriptors as much as possible! Not all descriptors will be applicable to all minerals!!!! Thin section number:……………………….. Rock Name: …………………………










Metamorphic Rocks: Thin Sect ion Descript ion * = delete as necessary; Fill in descriptors as much as possible! Not all descriptors will be applicable to all minerals!!!! Thin section number:……………………….. Name: …………………………









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1461_2014_lab2