https://wserv3.esc.cam.ac.uk/p1acollections/items/browse?page=24&sort_field=added&sort_dir=a&output=atom <![CDATA[1A Collections]]> 2024-04-09T21:25:38+01:00 Omeka https://wserv3.esc.cam.ac.uk/p1acollections/items/show/236 <![CDATA[<h2>Phenocryst</h2>]]> macrocryst and porphyroblast.]]> 2019-07-04T09:53:19+01:00

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.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/237 <![CDATA[<h2>Platy</h2>]]> 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.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/238 <![CDATA[<h2>Pleochroic halo</h2>]]> Have a look at the alkali basalt (L100) in the 1A Reference Series.]]> 2019-07-04T09:53:20+01:00

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.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/239 <![CDATA[Pleochroism]]> Have a look at hornblende amphibole (M201, M211) and biotite mica (M303, M304) in the 1A Reference Series.]]> 2019-07-04T09:53:21+01:00

Title

Pleochroism

Subject

Some minerals are pleochroic. When rotated under plane polarised light they change colour. Biotite and the amphibole hornblende are pleochroic.

Description

Have a look at hornblende amphibole (M201, M211) and biotite mica (M303, M304) in the 1A Reference Series.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/240 <![CDATA[<h2>Plutonic</h2>]]> 2019-07-04T09:53:21+01:00

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.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/241 <![CDATA[<h2>Poikilitic</h2>]]> Have a look at the gabbro (L141) in the 1A Reference Series.]]> 2019-07-04T09:53:21+01:00

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.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/242 <![CDATA[<h2>Polycrystalline</h2>]]> Have a look at the quartzites (L520, L521) in the 1A Reference Series.]]> 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.
]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/243 <![CDATA[<h2>Porosity</h2>]]>

Compare with permeability.

]]> 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.

]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/244 <![CDATA[<h2>Porphyritic</h2>]]> 2019-07-04T09:53:22+01:00

Title

Porphyritic

Subject

An igneous rock containing phenocrysts in a fine-grained groundmass is said to have a porphyritic texture.

]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/245 <![CDATA[<h2>Porphyroblast</h2>]]>

Compare with phenocryst.

]]> 2019-07-04T09:53:23+01:00

Title

Porphyroblast

Subject

A porphyroblast is the metamorphic equivalent of a phenocryst. Do not confuse them! It is a large crystal set into a finer-grained fabric, in a metamorphic rock.

Description

Compare with phenocryst.

]]>