1A Collections

Permeability

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Title

Permeability

Subject

The ability of a material to allow fluids to pass through it. A permeable material contains an interconnected system of channels though which fluids can diffuse. An impermeable material does not allow fluids to diffuse through it, either because it has no porosity, or because its porosity is unconnected.

Description

Compare with porosity.

Phaneritic

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Title

Phaneritic

Description

See granular.

Phenocryst

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Title

Phenocryst

Subject

Phenocrysts are igneous crystals which have grown in the magma that they are found within. They are large crystals, visible to the naked eye. Importantly, they are significantly larger than the surrounding crystals of the groundmass.

Description

Compare with macrocryst and porphyroblast.

Plagioclase feldspar

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Title

Plagioclase feldspar

Description

Formula

Na[AlSi₃O₈] – Ca[Al₂Si₂O₈]

Structure

Framework silicate

Crystal System

Crystal system: triclinic (+) (-)

Physical Properties

Colour in hand specimen: white
Cleavage: two at 90°
Hardness: 6-6.5

Optical Properties

Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.010, 1^st order grey
Extinction: inclined
Twinning: lamellar, very common

Mineral Specimens

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

Rock Specimens

L100 Alkali granite
L101 Granite
L102 Hornblende biotite granite
L112 Rhyolite
L120 Granodiorite
L130 Andesite, porphyritic (porphyrite)
L141 Gabbro
L162 Basalt, porphyritic
L172 Ultramafic cumulate
L504 Garnet schist

Advanced Notes

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 (An_0-10), oligoclase (An_10-30) andesine (An_30-50), labradorite (An_50-70), bytownite (An_70-90) and anorthite (An_90-100).

Mineral Name

plagioclase
feldspar

Platy

2019-07-04T09:53:20+01:00

Title

Platy

Subject

Crystals which are very thin and sheet-like are described as having a platy form. They are thinner than tabular crystals and less elongate than bladed crystals.

Pleochroic halo

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Title

Pleochroic halo

Subject

Pleochroic haloes are tiny spheres of dark discolouration within crystal grains. They are attributed to the presence of zircons trapped within crystals of other minerals, such as biotite mica. Zircon (Zr[SiO4]) is a common accessory mineral of many igneous rocks. It often crystallises early, and becomes trapped within grains of later-crystallising phases. Zircons contain a high proportion of radioactive elements. When these decay, they damage the crystal structure of the surrounding grain, producing a pleochroic halo.

Description

Have a look at the alkali basalt (L100) in the 1A Reference Series.

Plutonic

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Title

Plutonic

Subject

Plutonic rocks and textures are those associated with deep magmatic intrusions, where cooling is slow. Crystals are thus able to grow very large, producing coarse-grained textures. This term is restricted to large igneous bodies more than hundreds of metres in diameter.

Poikilitic

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Title

Poikilitic

Subject

An igneous texture in which a small number of large grains enclose many smaller grains of another phase.

Description

Have a look at the gabbro (L141) in the 1A Reference Series.

Polycrystalline

2019-07-04T09:53:21+01:00

Title

Polycrystalline

Subject

A crystalline sample of a single mineral, containing many crystals. Quartzite, for example, is polycrystalline quartz.

Description

Have a look at the quartzites (L520, L521) in the 1A Reference Series.

Porosity

2019-07-04T09:53:22+01:00

Title

Porosity

Subject

The proportion of the material occupied by space, measured in volume percent. A porous material contains fluid-filled spaces, or pores, while a non-porous material does not. NB, make sure you understand the difference between porosity and permeability.

Description

Compare with permeability.