1A Collections

Foliation

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Title

Foliation

Subject

This refers to any pervasive planar feature in a rock. It is produced by the arrangement of the constituent grains into sheet-like structures, usually in compositionally distinct layers. Foliations can be primary (e.g. bedding, fissility, flow banding) or secondary (e.g. cleavage).

Form

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Title

Form

Description

See habit.

Galena

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Title

Galena

Description

Formula

PbS

Structure

Non-silicate

Crystal System

Crystal system: cubic

Physical Properties

Colour in hand specimen: lead grey
Streak: lead grey
Lustre: metallic
Cleavage: one, perfect
Hardness: 2.5

Optical Properties

Colour in plane polarised light: opaque
Relief: high

Mineral Specimens

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”

Mineral Name

galena

Garnet

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Title

Garnet

Description

Formula

(Mg,Fe,Ca)₃Al₂Si₃O₁₂

Structure

Orthosilicate

Crystal System

Crystal system: cubic

Physical Properties

Colour in hand specimen: typically red
Cleavage: none
Hardness: 6-7.5

Optical Properties

Colour in plane polarised light: colourless, pale pink
Pleochroism: none
Relief: high
Isotropic

Mineral Specimens

M130 Garnet schist
Portsoy, Banffshire
Euhedral porphyroblasts of dark-red garnet, with quartz, mica and other minerals in a medium-grade metamorphic rock.

Rock Specimens

L504 Garnet schist

Advanced Notes

The garnet group encompasses a wide range of compositions, and consequently a range of optical and physical properties. The properties detailed above are typical of pyrope (Mg₃Al₂Si₃O₁₂), almandine (Fe²⁺₃Al₂Si₃O₁₂) and grossular (Ca₃Al₂Si₃O₁₂).

Mineral Name

garnet

Gastropod

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Title

Gastropod

Description

Taxonomy

Phylum: Mollusca

Class: Gastropoda

Diagnostic Features

Coiled shell - planispiral or helical - consists of body whorl and spire

Mostly aragonitic shell

Shell consists of one large chamber (no septa)

Aperture

Siphonal notch

Way of life

Marine, non-marine, land-dwelling

Carnivorous, scavenging, deposit feeding and more

Advanced notes

Because of their conservative shell shape it is difficult to use gastropods as environmental or stratigraphic indicators.

Specimens

Browse gastropods

External links

Telling planispiral fossils apart

Glass

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Title

Glass

Subject

When magma is quenched (cooled exceedingly rapidly) even very small crystals are unable to grow. Instead, a volcanic glass is formed. It is amorphous (having no crystalline structure), and is consequently isotropic.

Description

Have a look at the rhyolite glass (L113) in the 1A Reference Series.

Glomerocryst

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Title

Glomerocryst

Subject

A glomerocryst is a clump of phenocrysts. A rock in which clumping of phenocrysts is abundant and pervasive is said to have a glomeroporphyritic texture.

Glomeroporphyritic

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Title

Glomeroporphyritic

Subject

This is an igneous texture in which phenocrysts clump together, within a fine-grained groundmass.

Granular (or phaneritic)

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Title

Granular (or phaneritic)

Subject

This texture refers to an igneous or metamorphic rock which consists entirely of interlocking, roughly equant grains, large enough to be seen with naked eye.

Description

Have a look at the hornblende biotite granite (L102) in the 1A Reference Series.

Graphic texture

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Title

Graphic texture

Subject

The regular intergrowth of two minerals (usually quartz and feldspar), which crystallised simultaneously. It is called “graphic” because the intergrowths resemble writing.