Using only the whole slide view under plane-polarised light (PPL), describe the appearance of the thin section U09 in terms of mineral shape and orientation, grain size, mineral colour and relief.

This question is based on the thin section U09 in the Virtual Microscope. You should make use of the overall field of view at all magnifications and also the rotation views in both plane-polarised light and between crossed polars.  

The question requires you to use your skills with the Virtual Microscope, mineral identification and PT analysis. 

You will need to consult both the table of mineral properties and the pressure-temperature grid (both reproduced below). You may assume that the composition of the region of U09 for which you are asked to make pressure-temperature estimates is similar to that of typical Dalradian metapelitic rocks and that all pressure–temperature conditions lie within the range of this grid. 

(a) (8 marks) Using only the whole slide view under plane-polarised light (PPL), describe the appearance of the thin section U09 in terms of mineral shape and orientation, grain size, mineral colour and relief. Note that no rotation points or viewing under crossed-polarised light (XPL) are required for this part of the question. Detailed mineral descriptions and identification are not required. (Advisory limit: 100 words for general description)  

(b) (24 marks) For rotation 1: 

(i) Based on the optical properties, describe in your own words and then identify the highest relief mineral present in rotation 1 which forms large crystals occupying a significant area of the field of view. (Advisory limit: 50 words) 

(ii) Describe in your own words and then identify the two most abundant colourless minerals from rotation 1. (Advisory limit: 50 words for each mineral) 

(iii) What is the relationship between the timing of deformation of the rock and growth of the mineral identified in (i)? You may use any part of the slide for your observations. Provide a brief explanation and a labelled figure (either a photograph of a relevant part of the virtual microscope image or a simple free-hand sketch) to illustrate your answer. You should ensure that your figure is clearly labelled with a scale and of an appropriate size. (Advisory limit: 50 words) 

(c) (8 marks) For rotation 3: 

Describe in your own words and then identify a greenish-grey mineral from rotation 3 which is found near to the high relief mineral, and also in the whole slide view at co-ordinates 10.00, 2.35. (Advisory limit: 50 words) 

(d) (4 marks) What type of metamorphic rock is sample U09? Very briefly justify your answer. (Advisory limit: 30 words) 

(e) (12 marks) 

(i) What is the maximum temperature and pressure that the rock could have experienced during metamorphism? In each case provide a brief explanation for how you determined the values. (Two numbers with appropriate units and advisory limit: 50 words for explanation of each value) 

(ii) Name the metamorphic facies to which this rock belongs. Give a brief explanation for your choice of facies and identify the source of the facies diagram you have used. (Advisory limit: 35 words for name of term and explanation) 

(f) (8 marks) A colleague has collected a sample from a nearby exposure in the field and reports that she has identified a high relief, aluminosilicate mineral with the formula Al₂SiO₅. Name two possible minerals that this might be, with a brief explanation. You may assume this rock is unusually rich in aluminium for a Dalradian sample. (Advisory limit: 100 words) 

Sample U09   

The Virtual Microscope 

Sample U09 in a new tab 

Table of mineral properties 

Name (abbreviation)	Formula	Hand specimen	Plane-polarised light	Cross-polarised light
Quartz (Q)	SiO2	Greasy grey, conchoidal fracture	Colourless, low relief	Grey and white, undulose extinction
Orthoclase (Or),  Albite (Ab),  Anorthite (An)	KAlSi3O8,  NaAlSi3O8,  CaAl2Si2O8	Pale grey, stumpy crystals with cleavage	Colourless, low relief	Grey and white with oblique extinction, twinning prominent
Chlorite (Chl)	Mg5Al2Si3O10(OH)8	Green, mica-like	Pleochroic, green, low relief	Very dark grey or blue, straight extinction
Biotite (Bt)	K(Mg,Fe)3AlSi3O10(OH)2	Black mica	Pleochroic, brown to yellow, prominent cleavage	Bright colours masked by pleochroism, straight extinction
Muscovite (Ms)	KAl3Si3O10(OH)2	White mica	Colourless, low relief prominent cleavage	Bright colours, straight extinction
Andalusite (And)	Al2SiO5	Pink or white stumpy crystals	Usually colourless, high relief, two cleavages	Grey to yellow, parallel extinction
Kyanite (Ky)	Al2SiO5	Blue or white blades	Colourless, high relief, prominent cleavage	Grey to blue, slightly oblique extinction, rare lamellar twins
Sillimanite (Sil)	Al2SiO5	Grey or white, fibrous, silky	Colourless fibres, high relief, good cleavage	Grey to blue, straight extinction
Garnet (Grt)	(Mg,Fe)3Al2Si3O12	Red or brown, equant, multifaceted	Colourless or pink, very high relief, fractures	Isotropic
Staurolite (St)	(Mg,Fe)4Al18Si7.5O44(OH)4	Brown stumpy crystals	Pale yellow, pleochroic, high relief	Grey to yellow, straight extinction
Cordierite (Crd)	(Mg,Fe)2Al4Si5O18	Dirty blue, stumpy crystals	Colourless, low relief	Grey to white, rare sector twinning
Chloritoid (Cld)