<![CDATA[1A Collections]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/browse?tags=ultramafic&output=rss2 Tue, 09 Apr 2024 21:16:31 +0100 p1acollections@esc.cam.ac.uk (1A Collections) Zend_Feed http://blogs.law.harvard.edu/tech/rss <![CDATA[Clinopyroxene (augite)]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/49

Title

Clinopyroxene (augite)

Description

Formula

(Ca,Mg,Fe2+,Fe3+,Ti,Al)2[(Si,Al)2O6]

Structure

Chain silicate

Crystal System

Crystal system: monoclinic (+)

Physical Properties

Colour in hand specimen: black 
Cleavage: two at 90° 
Hardness: 5.5-6

Optical Properties

Colour in plane polarised light: colourless, pale brown or pale grey 
Pleochroism: variable weak to moderate 
Relief: moderate 
Birefringence: 0.018-0.033; 1st order pink to 2nd order orange 
Extinction: inclined 
Twinning: simple, common

Rock Specimens

L141 Gabbro 
L161 Basalt, amygdaloidal 
L162 Basalt, porphyritic 
L172 Ultramafic cumulate

Advanced Notes

Clinopyroxene contains calcium. It encompasses a range of solid solutions from (Mg,Fe)2[Si2O6] to Ca(Mg,Fe)[Si2O6]. The physical and optical properties detailed above refer to augite, a clinopyroxene common in mafic igneous rocks.

Mineral Name

clinopyroxene
augite
opx_phase_diagram.jpg
]]> Mon, 17 Jun 2019 13:16:27 +0100 <![CDATA[L172 Ultramafic cumulate]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/14

Title

L172

Ultramafic cumulate

Subject

Age

Precambrian

Location

Lewisian Complex, Dyke
Assynt, Sutherland

Description

Hand Specimen

Greenish black, coarse-grained rock, comprising olivine, clinopyroxene and plagioclase.
Grains typically 1-2 mm across. 

Thin-section

Olivine 
- Subhedral to euhedral grains making up ~45% of the rock. Alteration to magnetite and serpentine, particularly along the curving cracks. Cracks across the thin-section, could be due to expansion of the rock as the olivine was altered.

Clinopyroxene 
- Interstitial phase making up ~35% of the rock. Moderate relief, inclined extinction and first order pink to second order orange interference colours.

Plagioclase feldspar 
- Interstitial phase making up ~20% of the rock. Low first order interference colours and lamellar twinning.

Accessory minerals - Biotite, small, interstitial grains, altered.

This is a cumulate rock. It has undergone late-stage shear banding.

Rock History

Olivine phenocrysts crystallised first. 
Clinopyroxene grew around the olivine phenocrysts. 
Interstitial feldspar filled in the remaining gaps. 
Shearing of the rock produced bands of deformation. 
Hydrothermal alteration.

Rock Name

ultramafic cumulate
L172_HS.jpg
L172_TS_ppl.jpg
L172_TS_xpl.jpg
]]> Fri, 14 Jun 2019 17:08:51 +0100