<![CDATA[1A Collections]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/browse?tags=metamorphism&output=rss2 Tue, 09 Apr 2024 21:16:20 +0100 p1acollections@esc.cam.ac.uk (1A Collections) Zend_Feed http://blogs.law.harvard.edu/tech/rss <![CDATA[Chlorite]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/53

Title

Chlorite

Subject

Chlorite is a common high-temperature alteration product of ferromagnesian minerals (such as biotite) in igneous rocks. It is stable in the upper part of the crust and is a major constituent of the metamorphic rocks known as greenschists.

Description

Formula

(Mg,Fe2+,Fe3+,Mn,Al)12[(Si,Al)8O20](OH)16

Structure

Sheet silicate

Crystal System

Crystal system: monoclinic (+) (-)

Physical Properties

Colour in hand specimen: green 
Cleavage: one, good 
Hardness: 2-3

Optical Properties

Colour in plane polarised light: colourless or green 
Pleochroism: weak to moderate 
Relief: low 
Birefringence: anomalous (blue or brown colours that do not appear in the Michel-Levy chart) 
Extinction: straight

Rock Specimens

L100 Alkali granite 
L101 Granite 
L120 Granodiorite 
L130 Andesite, porphyritic (porphyrite) 
L161 Basalt, amygdaloidal 
L201 Tuff 
L504 Garnet schist

Mineral Name

chlorite
chlorite_CS.jpg
]]> Mon, 17 Jun 2019 15:31:03 +0100 <![CDATA[Garnet]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/45

Title

Garnet

Description

Formula

(Mg,Fe,Ca)3Al2Si3O12

Structure

Orthosilicate

Crystal System

Crystal system: cubic

Physical Properties

Colour in hand specimen: typically red
Cleavage: none
Hardness: 6-7.5

Optical Properties

Colour in plane polarised light: colourless, pale pink 
Pleochroism: none
Relief: high
Isotropic

Mineral Specimens

M130 Garnet schist
Portsoy, Banffshire
Euhedral porphyroblasts of dark-red garnet, with quartz, mica and other minerals in a medium-grade metamorphic rock.

Rock Specimens

L504 Garnet schist

Advanced Notes

The garnet group encompasses a wide range of compositions, and consequently a range of optical and physical properties. The properties detailed above are typical of pyrope (Mg3Al2Si3O12), almandine (Fe2+3Al2Si3O12) and grossular (Ca3Al2Si3O12).

Mineral Name

garnet
Garnet_HS.jpg
]]> Mon, 17 Jun 2019 13:07:10 +0100 <![CDATA[L201 Tuff]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/17

Title

L201

Tuff

Description

Hand Specimen

Fine-grained, blue-green, sedimentary texture. 
Slaty cleavage. 
Some patches of lighter and finer-grained material.

Thin-section

Slaty cleavage due to the alignment of chlorite and muscovite. 
Cross-cut by the blobs of the fine-grained material. These are orange-brown in thin-section. Could be water-lain ash, ripped-up to form clasts. 
Very fine-grained, brown matrix, probably volcanic ash. 
Carbonate, quartz and opaque clasts slightly larger than the matrix.

Rock History

Fine-grained volcanic ash deposited by an explosive volcanic eruption has undergone low grade metamorphism to produce a slaty cleavage.

Rock Name

tuff
L201_HS.jpg
L201_TS_ppl.jpg
L201_TS_xpl.jpg
]]> Fri, 14 Jun 2019 17:27:47 +0100