<![CDATA[1A Collections]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/browse?sort_field=added&page=10&sort_dir=d&output=rss2 Tue, 09 Apr 2024 21:23:16 +0100 p1acollections@esc.cam.ac.uk (1A Collections) Zend_Feed http://blogs.law.harvard.edu/tech/rss <![CDATA[Carlsbad twinning]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/179

Title

Carlsbad twinning

Subject

Description

See simple twins.
]]> Fri, 21 Jun 2019 14:59:01 +0100 <![CDATA[Bladed]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/178

Title

Bladed

Subject

Crystals with a flattened, elongate habit, resembling blades of grass are described as bladed. Bladed crystals are more elongate than platy crystals, and thinner than tabular crystals.
]]> Fri, 21 Jun 2019 14:58:18 +0100 <![CDATA[Birefringence]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/177

Title

Birefringence

Subject

Light passing through a mineral is resolved into two permitted vibration directions. In an anisotropic mineral light travels at different velocities along the two vibration directions. The ray travelling in the slow vibration direction will be retarded compared with the ray travelling in the fast vibration direction. On leaving the mineral grain the two rays will interfere, producing interference colours.

When the slow ray exits the mineral grain the fast ray will already have exited the grain and travelled an additional distance know as the optical path difference, or retardation (Δ). This distance is proportional to the thickness of the mineral grain (t) and the difference in the refractive indices of the two vibration directions (n1-n2):

Δ = t(n1-n2)

The birefringence is the difference in the refractive indices of the two vibration directions (n1-n2).

When rotated under cross-polarised light anisotropic minerals display changing interference colours called birefringencecolours. The birefringence (n1-n2) of the mineral can be determined from these colours using a Michel-Levy chart.

]]> Fri, 21 Jun 2019 14:45:25 +0100 <![CDATA[Argillaceous]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/176

Title

Argillaceous

Subject

Rocks comprising, or containing a significant proportion of, clay are described as argillaceous.

Description

Have a look at the argillaceous limestone (L350) in the 1A Reference Series.

]]> Fri, 21 Jun 2019 14:40:52 +0100 <![CDATA[Aphanitic (or aphyric)]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/175

Title

Aphanitic (or aphyric)

Subject

Igneous rocks made up of grains too small to be seen with the naked eye have an aphanitic texture. These rocks do not contain phenocrysts. They are non-porphyritic.
]]> Fri, 21 Jun 2019 14:39:13 +0100 <![CDATA[Anisotropic]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/174

Title

Anisotropic

Subject

The speed of light through any medium is inversely proportional to the refractive index of the medium. The refractive index of a mineral is determined by the arrangement and type of atoms within its structure.

If the structure of the mineral allows light to travel at different velocities depending on the direction in which the light is passing through it, the mineral is anisotropic. That is to say, it has different refractive indices in different orientations.

Description

Compare with isotropic.
]]> Fri, 21 Jun 2019 14:38:39 +0100 <![CDATA[Anhedral]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/173

Title

Anhedral

Subject

A crystal displaying none of its characteristic crystal faces is called anhedral.

Description

Have a look at the granodiorite (L120) for anhedral quartz, biotite and plagioclase.
]]> Fri, 21 Jun 2019 14:35:20 +0100 <![CDATA[Amygdale]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/172

Title

Amygdale

Subject

A vesicle (necessarily in an igneous rock), which has been filled in by a mineral crystallising from late-stage fluids. Amygdales are commonly produced by the crystallisation of quartz, calcite or zeolite.

Description

Have a look at the amygdaloidal basalt (L161) in the 1A Reference Series.
]]> Fri, 21 Jun 2019 14:31:07 +0100 <![CDATA[Amorphous]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/171

Title

Amorphous

Subject

Amorphous materials are non-crystalline solids. That is to say that they do not display long-range order in their structures. Glass is amorphous.

Description

Have a look at the rhyolite glass (L113) in the 1A Reference Series.
]]> Fri, 21 Jun 2019 14:27:54 +0100 <![CDATA[Alteration]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/170

Title

Alteration

Subject

Rocks undergo mineralogical change at low pressures, produced by reactions with circulating fluids. The replacement of the original minerals in the rock tends to take place primarily along cracks and around the edges of grains, though whole mineral grains can be replaced. The decomposition of olivine to serpentine is a common example of alteration.

Description

Common alteration minerals include clays for plagioclase, serpentine for olivine, and chlorite for ferromagnesian minerals such as biotite.
]]> Fri, 21 Jun 2019 14:25:45 +0100