<![CDATA[1A Collections]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/browse?sort_field=added&sort_dir=a&page=18&output=rss2 Tue, 09 Apr 2024 21:24:34 +0100 p1acollections@esc.cam.ac.uk (1A Collections) Zend_Feed http://blogs.law.harvard.edu/tech/rss <![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[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[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[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[Cleavage]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/180

Title

Cleavage

Subject

Many minerals display very fine, parallel, straight lines called cleavage. Cleavages are the manifestation of planes of weakness within the crystal structure of the mineral. A single crystal may display cleavage in more than one orientation. For example, amphiboles have cleavages which intersect at ~56°, whereas pyroxenes have cleavages which intersect at 90°. 

]]> Fri, 21 Jun 2019 15:16:13 +0100 <![CDATA[Colour index]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/181

Title

Colour index

Subject

Rocks can be classified according to their colour index (CI). This describes the proportion of ferromagnesian minerals, such as olivine, pyroxenes, amphiboles and biotite mica, they contain. Colour index is measured in volume percent. The terms used to describe the varying proportions of ferromagnesian minerals are:

]]> Fri, 21 Jun 2019 15:20:43 +0100 <![CDATA[Conchoidal fracture]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/182

Title

Conchoidal fracture

Subject

When a brittle material lacking cleavage (plane of weakness) fractures, it breaks into curved fragments. This is conchoidal fracture. Flint, quartz and glass display conchoidal fracture.

Description

Have a look at the rhyolite glass (L113), the flint nodule (L370), and quartz (M411) in the 1A Reference Series.
]]> Fri, 21 Jun 2019 15:24:05 +0100 <![CDATA[Cross-hatched twinning]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/183

Title

Cross-hatched twinning

Subject

Where one set of parallel twin planes intersects another at 90°, twinning is produced in a cross-hatched or tartan pattern. This is a diagnostic characteristic of microcline (an alkali feldspar), and is occasionally referred to as microcline twinning.
]]> Fri, 21 Jun 2019 15:24:33 +0100 <![CDATA[Cryptocrystalline]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/184

Title

Cryptocrystalline

Subject

Rocks with a crystalline structure in which the individual crystals are so small that they cannot be identified using standard optical microscopy are described as cryptocrystalline.

Description

Compare with microcrystalline.
]]> Fri, 21 Jun 2019 15:31:20 +0100 <![CDATA[Crystal lattice]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/185

Title

Crystal lattice

Subject

An ordered arrangement of atoms within a solid. This typically exhibits long range order in the form of a repeating motif (unit cell) on each lattice point, to build up the whole structure.
]]> Fri, 21 Jun 2019 15:35:48 +0100