https://wserv3.esc.cam.ac.uk/p1acollections/items/browse?page=21&sort_field=Dublin+Core%2CTitle&output=atom <![CDATA[1A Collections]]> 2024-04-09T21:25:04+01:00 Omeka https://wserv3.esc.cam.ac.uk/p1acollections/items/show/220 <![CDATA[<h2>Lustre</h2>]]> 2019-07-04T09:53:13+01:00

Title

Lustre

Subject

A combination of the intensity and quality of the light reflected from the surface of a mineral. A number of terms are used to describe the range of lustres displayed by minerals, including adamantine, dull, metallic, pearly, resinous and vitreous.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/221 <![CDATA[<h2>Macrocryst</h2>]]> Compare with phenocryst.]]> 2019-07-04T09:53:14+01:00

Title

Macrocryst

Subject

A relatively large crystal, usually occurring within an igneous rock. Unlike the term phenocryst, it does not imply a petrogenetic link between the crystal and the surrounding magma.

Description

Compare with phenocryst.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/59 <![CDATA[<h2>Magnetite</h2>]]>

Formula

Fe3O4

Structure

Non-silicate

Crystal System

Crystal system: cubic

Physical Properties

Colour in hand specimen: black
Streak: black
Cleavage: none
Hardness: 7.5-8

Optical Properties

Colour in plane polarised light: almost opaque
Relief: high
Isotropic

Mineral Specimens

M501 Magnetite 
Brazil 
Crystals showing the characteristic octahedral form 
Silver-black colour 
Metallic lustre

M502 Magnetite 
Hay Tor, Devon 
A mass of small octahedral crystals embedded in a metamorphic rock 
Silver-black 
Metallic lustre 
No cleavage

Rock Specimens

L102 Hornblende biotite granite
L120 Granodiorite, accessory 
L141 Gabbro, olivine decomposition, accessory 
L162 Basalt, porphyritic, feldspar inclusion 
L172 Ultramafic cumulate, olivine alteration

Mineral Name

magnetite
iron oxide
opaques
]]> 2019-07-04T09:52:20+01:00

Title

Magnetite

Subject

Magnetite is the most abundant oxide in igneous rock.

Description

Formula

Fe3O4

Structure

Non-silicate

Crystal System

Crystal system: cubic

Physical Properties

Colour in hand specimen: black
Streak: black
Cleavage: none
Hardness: 7.5-8

Optical Properties

Colour in plane polarised light: almost opaque
Relief: high
Isotropic

Mineral Specimens

M501 Magnetite 
Brazil 
Crystals showing the characteristic octahedral form 
Silver-black colour 
Metallic lustre

M502 Magnetite 
Hay Tor, Devon 
A mass of small octahedral crystals embedded in a metamorphic rock 
Silver-black 
Metallic lustre 
No cleavage

Rock Specimens

L102 Hornblende biotite granite
L120 Granodiorite, accessory 
L141 Gabbro, olivine decomposition, accessory 
L162 Basalt, porphyritic, feldspar inclusion 
L172 Ultramafic cumulate, olivine alteration

Mineral Name

magnetite
iron oxide
opaques
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/222 <![CDATA[<h2>Melanocratic</h2>]]> 2019-07-04T09:53:14+01:00

Title

Melanocratic

Subject

Melanocratic rocks have a high colour index. That is to say that 60-90 vol% of the rock is made up of ferromagnesian minerals.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/223 <![CDATA[<h2>Mesocratic</h2>]]> 2019-07-04T09:53:14+01:00

Title

Mesocratic

Subject

Mesocratic rocks have a moderate colour index. That is to say that 30-60 vol% of the rock is made up of ferromagnesian minerals.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/224 <![CDATA[<h2>Metallic [lustre]</h2>]]> Have a look at pyrite (M520) in the 1A Reference Series.]]> 2019-07-04T09:53:15+01:00

Title

Metallic [lustre]

Subject

Minerals displaying a metal-like lustre are described as metallic. These minerals have surfaces which are perfectly reflective. Pyrite, for example, has metallic lustre.

Description

Have a look at pyrite (M520) in the 1A Reference Series.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/52 <![CDATA[<h2>Mica, biotite</h2>]]>

Formula

K2(Mg,Fe2+)6-4(Fe3+,Al,Ti)0-2[Si6-5Al2-3O20](OH,F)4

Structure

Sheet silicate

Crystal System

Crystal system: monoclinic (-)

Physical Properties

Colour in hand specimen: brown or black 
Cleavage: one, good 
Hardness: 2-3

Optical Properties

Colour in plane polarised light: pale yellow, pale green, green or brown 
Pleochroism: strong 
Relief: low 
Birefringence: 0.028-0.08; 2nd order green-yellow to >4th order 
Extinction: straight

Mineral Specimens

M303 Mica, biotite 
Cornwall 
Large irregular biotite crystals in a feathery arrangement 
Dark brown / black / golden 
Metallic lustre 
Platy habit due to the single perfect cleavage

M304 Mica, biotite 
Orlando, Canada 
Irregular plates of biotite split along cleavage planes 
One good cleavage 
Brilliant metallic lustre

Rock Specimens

L100 Alkali granite 
L101 Granite 
L102 Hornblende biotite granite 
L120 Granodiorite 
L130 Andesite, porphyritic (porphyrite) 
L504 Garnet schist

Mineral Name

mica
biotite
]]> 2019-07-04T09:52:18+01:00

Title

Mica, biotite

Description

Formula

K2(Mg,Fe2+)6-4(Fe3+,Al,Ti)0-2[Si6-5Al2-3O20](OH,F)4

Structure

Sheet silicate

Crystal System

Crystal system: monoclinic (-)

Physical Properties

Colour in hand specimen: brown or black 
Cleavage: one, good 
Hardness: 2-3

Optical Properties

Colour in plane polarised light: pale yellow, pale green, green or brown 
Pleochroism: strong 
Relief: low 
Birefringence: 0.028-0.08; 2nd order green-yellow to >4th order 
Extinction: straight

Mineral Specimens

M303 Mica, biotite 
Cornwall 
Large irregular biotite crystals in a feathery arrangement 
Dark brown / black / golden 
Metallic lustre 
Platy habit due to the single perfect cleavage

M304 Mica, biotite 
Orlando, Canada 
Irregular plates of biotite split along cleavage planes 
One good cleavage 
Brilliant metallic lustre

Rock Specimens

L100 Alkali granite 
L101 Granite 
L102 Hornblende biotite granite 
L120 Granodiorite 
L130 Andesite, porphyritic (porphyrite) 
L504 Garnet schist

Mineral Name

mica
biotite
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/51 <![CDATA[<h2>Mica, muscovite</h2>]]>

Formula

K2Al4[Si6Al2O20](OH,F)4

Structure

Sheet silicate

Crystal System

Crystal system: monoclinic (-)

Physical Properties

Colour in hand specimen: colourless 
Cleavage: one, good 
Hardness: 2.5-3

Optical Properties

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: 0.036-0.049; 2nd order pink to 3rd order yellow 
Extinction: straight

Mineral Specimens

M301 Mica, muscovite 
Lincoln County, North Carolina 
Well-formed tabular crystals showing the pseudohexangonal symmetry 
Colourless, mirror- like, metallic lustre 
Platy, flaky habit 
Soft

M302 Mica, muscovite    
Mysore, India 
Plates of mica artificially cut to shape 
Colourless 
One perfect cleavage

Rock Specimens

L110 Rhyolite, porphyritic (quartz porphyry) 
L201 Tuff 
L504 Garnet schist

Mineral Name

mica
muscovite
]]> 2019-07-04T09:52:18+01:00

Title

Mica, muscovite

Description

Formula

K2Al4[Si6Al2O20](OH,F)4

Structure

Sheet silicate

Crystal System

Crystal system: monoclinic (-)

Physical Properties

Colour in hand specimen: colourless 
Cleavage: one, good 
Hardness: 2.5-3

Optical Properties

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: 0.036-0.049; 2nd order pink to 3rd order yellow 
Extinction: straight

Mineral Specimens

M301 Mica, muscovite 
Lincoln County, North Carolina 
Well-formed tabular crystals showing the pseudohexangonal symmetry 
Colourless, mirror- like, metallic lustre 
Platy, flaky habit 
Soft

M302 Mica, muscovite    
Mysore, India 
Plates of mica artificially cut to shape 
Colourless 
One perfect cleavage

Rock Specimens

L110 Rhyolite, porphyritic (quartz porphyry) 
L201 Tuff 
L504 Garnet schist

Mineral Name

mica
muscovite
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/225 <![CDATA[<h2>Micaceous</h2>]]> Have a look at muscovite mica (M301, M302) and biotite mica (M303, M304) in the 1A Reference Series.]]> 2019-07-04T09:53:16+01:00

Title

Micaceous

Subject

A rock containing a significant proportion of micas is described as micaceous. The micas are a group of sheet silicate minerals, which include biotite and muscovite.

Description

Have a look at muscovite mica (M301, M302) and biotite mica (M303, M304) in the 1A Reference Series.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/226 <![CDATA[<h2>Microcline twinning</h2>]]> See cross-hatched twins.]]> 2019-07-04T09:53:16+01:00

Title

Microcline twinning

Description

See cross-hatched twins.
]]>