Na[AlSi3O8] – Ca[Al2Si2O8]
Framework silicate
Crystal system: triclinic (+) (-)
Colour in hand specimen: white
Cleavage: two at 90°
Hardness: 6-6.5
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.010, 1st order grey
Extinction: inclined
Twinning: lamellar, very common
M403 Plagioclase feldspar, oligoclase
Ytterby, Sweden
A cleavage fragment of white plagioclase feldspar
Pearly lustre on cleavage planes
Two sets of cleavage at 90°
Fine parallel striations due to lamellar twinning
M404 Plagioclase feldspar, labradorite
Labrador
Dark grey, light where fragments are small due to cracking along cleavage
Schiller (blue iridescence), characteristic of labradorite
Very fine parallel striations due to lamellar twinning
Plagioclase feldspar forms a continuous solid solution between the sodic endmember (albite, or Ab) and the calcic endmember (anorthite, or An). The series is sub-divided into albite (An0-10), oligoclase (An10-30) andesine (An30-50), labradorite (An50-70), bytownite (An70-90) and anorthite (An90-100).
Na[AlSi3O8] – Ca[Al2Si2O8]
Framework silicate
Crystal system: triclinic (+) (-)
Colour in hand specimen: white
Cleavage: two at 90°
Hardness: 6-6.5
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.010, 1st order grey
Extinction: inclined
Twinning: lamellar, very common
M403 Plagioclase feldspar, oligoclase
Ytterby, Sweden
A cleavage fragment of white plagioclase feldspar
Pearly lustre on cleavage planes
Two sets of cleavage at 90°
Fine parallel striations due to lamellar twinning
M404 Plagioclase feldspar, labradorite
Labrador
Dark grey, light where fragments are small due to cracking along cleavage
Schiller (blue iridescence), characteristic of labradorite
Very fine parallel striations due to lamellar twinning
Plagioclase feldspar forms a continuous solid solution between the sodic endmember (albite, or Ab) and the calcic endmember (anorthite, or An). The series is sub-divided into albite (An0-10), oligoclase (An10-30) andesine (An30-50), labradorite (An50-70), bytownite (An70-90) and anorthite (An90-100).
Chain silicate
Crystal system: monoclinic (+)
Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5.5-6
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
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.
Chain silicate
Crystal system: monoclinic (+)
Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5.5-6
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
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.
Chain silicate
Crystal system: orthorhombic (+) (-)
Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5-6
Colour in plane polarised light: colourless, reddish or greenish
Pleochroism: variable weak to strong
Relief: moderate
Birefringence: 0.007-0.020; 1st order grey to 2nd order purple
Extinction: straight
Orthopyroxene has two end-member compositions: enstatite, Mg2Si2O6, and ferrosilite, Fe2+2Si2O6.
Chain silicate
Crystal system: orthorhombic (+) (-)
Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5-6
Colour in plane polarised light: colourless, reddish or greenish
Pleochroism: variable weak to strong
Relief: moderate
Birefringence: 0.007-0.020; 1st order grey to 2nd order purple
Extinction: straight
Orthopyroxene has two end-member compositions: enstatite, Mg2Si2O6, and ferrosilite, Fe2+2Si2O6.
Orthosilicate
Crystal system: orthorhombic (+) (-)
Colour in hand specimen: pale green
Cleavage: none, instead it has characteristic curving cracks
Hardness: 7
Colour in plane polarised light: colourless
Relief: moderate
Birefringence: 0.035; 2nd order pink
M100 Olivine
North Carolina
A mass of rather irregular crystals of olivine
Yellow-green colour
Vitreous lustre
No cleavage
Orthosilicate
Crystal system: orthorhombic (+) (-)
Colour in hand specimen: pale green
Cleavage: none, instead it has characteristic curving cracks
Hardness: 7
Colour in plane polarised light: colourless
Relief: moderate
Birefringence: 0.035; 2nd order pink
M100 Olivine
North Carolina
A mass of rather irregular crystals of olivine
Yellow-green colour
Vitreous lustre
No cleavage
Fine-grained, greenish black rock comprising dark and white minerals.
Contains amygdales, mostly white, though some have black cores.
Fine-grained plagioclase laths and irregular clinopyroxene (augite) grains, replaced with calcite. Many of the plagioclase laths have been almost entirely altered.
The amygdales are filled with zeolite, growing in spherulitic and radiate patterns. Many have rims of calcite and fine-grained chlorite.
Some vein material - granular quartz with sutured contacts.
Zeolites are produced at low temperatures. They are commonly precipitated in basaltic volcanic piles.
Cenozoic
Lava, Skye
Fine-grained, greenish black rock comprising dark and white minerals.
Contains amygdales, mostly white, though some have black cores.
Fine-grained plagioclase laths and irregular clinopyroxene (augite) grains, replaced with calcite. Many of the plagioclase laths have been almost entirely altered.
The amygdales are filled with zeolite, growing in spherulitic and radiate patterns. Many have rims of calcite and fine-grained chlorite.
Some vein material - granular quartz with sutured contacts.
Zeolites are produced at low temperatures. They are commonly precipitated in basaltic volcanic piles.
Plagioclase feldspar
- Tabular phenocrysts making-up ~50% of the rock. Characteristic lamellar twinning. Some are zoned.
Olivine
- Phenocrysts have been extensively altered, rendering them almost unrecognisable. The olivine has decomposed to magnetite and serpentine. Relict olivine grains display characteristic curving cracks and euhedral form.
Clinopyroxene (augite)
- Interstitial grains making-up ~35% of the rock. Poikilitic texture. Inclined extinction. Interference colours from first order pink to second order orange. Pale grey under plane polarised light. Some simple twinning.
Accessory minerals
- Opaque, interstitial, probably magnetite.
Coarse grained → slowly cooled, intrusive.
Hydrothermal alteration of olivine after crystallisation of the rock.
Plagioclase feldspar
- Tabular phenocrysts making-up ~50% of the rock. Characteristic lamellar twinning. Some are zoned.
Olivine
- Phenocrysts have been extensively altered, rendering them almost unrecognisable. The olivine has decomposed to magnetite and serpentine. Relict olivine grains display characteristic curving cracks and euhedral form.
Clinopyroxene (augite)
- Interstitial grains making-up ~35% of the rock. Poikilitic texture. Inclined extinction. Interference colours from first order pink to second order orange. Pale grey under plane polarised light. Some simple twinning.
Accessory minerals
- Opaque, interstitial, probably magnetite.
Coarse grained → slowly cooled, intrusive.
Hydrothermal alteration of olivine after crystallisation of the rock.