Framework silicate
Crystal system: trigonal (+)
Colour in hand specimen: colourless
Lustre: vitreous
Pleochroism: none
Cleavage: none
Hardness: 7
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: 0.009; 1st order grey
M410 Quartz
Characteristic prismatic crystal form: hexagonal cross-section, needle-like; hexagonal prism surmounted by a hexagonal pyramid
Colourless, transparent, milky in places
M411 Quartz
Madagascar
Colourless, transparent quartz fragment, milky due to cracks and crystal boundaries
Vitreous lustre, conchoidal fracture, no cleavage
M412 Quartz (with galena and pyrite)
North Wales
A mass of small hexagonal quartz prisms, with galena, pyrite and zinc blende.
Transparent, white to light grey
Galena: purple-black, shiny, platy habit, metallic
Pyrite: metallic lustre, gold colour, cubic habit, “Fools’ gold”
Framework silicate
Crystal system: trigonal (+)
Colour in hand specimen: colourless
Lustre: vitreous
Pleochroism: none
Cleavage: none
Hardness: 7
Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: 0.009; 1st order grey
M410 Quartz
Characteristic prismatic crystal form: hexagonal cross-section, needle-like; hexagonal prism surmounted by a hexagonal pyramid
Colourless, transparent, milky in places
M411 Quartz
Madagascar
Colourless, transparent quartz fragment, milky due to cracks and crystal boundaries
Vitreous lustre, conchoidal fracture, no cleavage
M412 Quartz (with galena and pyrite)
North Wales
A mass of small hexagonal quartz prisms, with galena, pyrite and zinc blende.
Transparent, white to light grey
Galena: purple-black, shiny, platy habit, metallic
Pyrite: metallic lustre, gold colour, cubic habit, “Fools’ gold”
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).