(K,Na)[AlSi3O8]
Framework silicate
Crystal system: monoclinic (-) or triclinic (+) (-)
Colour in hand specimen: white or pink
Cleavage: two at 90°
Hardness: 6-6.5
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.008; 1st order greys
Extinction: inclined
Twinning: usually simple or lamellar;
Microcline (KAlSi3O8) displays characteristic cross-hatched twinning
M400 Alkali feldspar, microcline
A single crystal of pale orange-pink alkali feldspar.
Pearly lustre.
M401 Alkali feldspar, orthoclase
Norway
A fragment of opaque, pale grey alkali feldspar.
Two sets of good cleavage at right angles, with a pearly lustre on the cleavage faces.
M402 Alkali feldspar, orthoclase
Shap, Westmorland
Porphyritic granite.
Large phenocrysts of pink feldspar, two sets of cleavage at 90°, simple twinning.
Also white plagioclase feldspar, quartz and biotite.
(K,Na)[AlSi3O8]
Framework silicate
Crystal system: monoclinic (-) or triclinic (+) (-)
Colour in hand specimen: white or pink
Cleavage: two at 90°
Hardness: 6-6.5
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.008; 1st order greys
Extinction: inclined
Twinning: usually simple or lamellar;
Microcline (KAlSi3O8) displays characteristic cross-hatched twinning
M400 Alkali feldspar, microcline
A single crystal of pale orange-pink alkali feldspar.
Pearly lustre.
M401 Alkali feldspar, orthoclase
Norway
A fragment of opaque, pale grey alkali feldspar.
Two sets of good cleavage at right angles, with a pearly lustre on the cleavage faces.
M402 Alkali feldspar, orthoclase
Shap, Westmorland
Porphyritic granite.
Large phenocrysts of pink feldspar, two sets of cleavage at 90°, simple twinning.
Also white plagioclase feldspar, quartz and biotite.
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
[See labelled diagram above]
Aragonite shell
Septa
Suture (different for goniatites, ceratites and ammonites, see second diagram above)
Chambers
Siphuncle
Planispiral shape, involute or evolute (see third diagram on right hand side)
Devonian to Cretaceous
Nektonic (jet propelled)
Predatory
Marine
Internal mould
Learn about types of preservation
Ammonoids could passively remove or add water to chambers via the siphuncle. This allowed them to change their density, and maintain neutral buoyancy with the surrounding seawater. This meant that they were able to swim as efficiently as possible.
Ammonoids are extremely good index fossils on account of their diverse morphologies, good preservation potentials and widespread fossil locations.
Phylum: Mollusca
Class: Cephalopoda
Subclass: Ammonoidea
[See labelled diagram above]
Aragonite shell
Septa
Suture (different for goniatites, ceratites and ammonites, see second diagram above)
Chambers
Siphuncle
Planispiral shape, involute or evolute (see third diagram on right hand side)
Devonian to Cretaceous
Nektonic (jet propelled)
Predatory
Marine
Internal mould
Learn about types of preservation
Ammonoids could passively remove or add water to chambers via the siphuncle. This allowed them to change their density, and maintain neutral buoyancy with the surrounding seawater. This meant that they were able to swim as efficiently as possible.
Ammonoids are extremely good index fossils on account of their diverse morphologies, good preservation potentials and widespread fossil locations.
(Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al)5Si6-7.5Al2-0.5O22(OH)2
Chain silicate
Crystal system: monoclinic (+) (-)
Colour in hand specimen: black
Cleavage: two at ~56°
Hardness: 5-6
Colour in plane polarised light: pale green, green, light yellow-brown to brown
Pleochroism: yes
Relief: moderate
Birefringence: ~0.02; 2nd order purple
Extinction: inclined
Twinning: simple, lamellar, common
M210 Amphibole, hornblende
Bilen, Bohemia
Single, black prismatic crystal.
At the ends of the crystal two sets of cleavage planes are visible, intersecting at ~56°
M211 Amphibolite
Isle of Coll, Inner Hebrides
A metamorphic rock consisting almost entirely of crystals of the amphibole hornblende
Black
Brilliant lustre
Two sets of cleavage planes intersect at ~56°
The amphibole group encompasses a wide range of compositions. Even the hornblendes, which are a sub-group of the amphiboles, encompass a range of compositions (see mineral formula, above). The properties above refer to hornblende.
(Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al)5Si6-7.5Al2-0.5O22(OH)2
Chain silicate
Crystal system: monoclinic (+) (-)
Colour in hand specimen: black
Cleavage: two at ~56°
Hardness: 5-6
Colour in plane polarised light: pale green, green, light yellow-brown to brown
Pleochroism: yes
Relief: moderate
Birefringence: ~0.02; 2nd order purple
Extinction: inclined
Twinning: simple, lamellar, common
M210 Amphibole, hornblende
Bilen, Bohemia
Single, black prismatic crystal.
At the ends of the crystal two sets of cleavage planes are visible, intersecting at ~56°
M211 Amphibolite
Isle of Coll, Inner Hebrides
A metamorphic rock consisting almost entirely of crystals of the amphibole hornblende
Black
Brilliant lustre
Two sets of cleavage planes intersect at ~56°
The amphibole group encompasses a wide range of compositions. Even the hornblendes, which are a sub-group of the amphiboles, encompass a range of compositions (see mineral formula, above). The properties above refer to hornblende.