<![CDATA[1A Collections]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/browse?page=16&sort_field=Dublin+Core%2CTitle&output=rss2 Tue, 09 Apr 2024 21:24:06 +0100 p1acollections@esc.cam.ac.uk (1A Collections) Zend_Feed http://blogs.law.harvard.edu/tech/rss <![CDATA[Crinoid]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/117

Title

Crinoid

Description

Taxonomy

Phylum: Echinodermata

Class: Crinoidea

Diagnostic Features

Crown

Stalk

Pentaradial symmetry

Ossicles

Calyx

Pinnule

Holdfast

Way of life

Benthic (some pseudoplanktonic)

Filter feeders

Stratigraphic range

Ordovician to present

Advanced Notes

The heyday of the crinoids was the Palaeozoic, where they dominated shallow marine environments. Modern crinoids are exclusively deep marine, and the diversity of forms is far less than it was during the Paleozoic.

Were they victims of the increased grazing and predation pressure of the Mesozoic Marine Revolution?

Specimens

Browse crinoids

Links

youtube: modern crinoid behaving pseudoplanktonically. You can see the pinnules, crown and multiple stalks.

flickr: beautiful photographs of both living and fossil crinoids. Compare the levels of detail you can see, admire the colours, and spot the distinctive features that make these crinoids identifiable.

Learn about types of preservation

crinoid_sketch.png
crinoid_ossicle.png
]]> Fri, 21 Jun 2019 09:06:13 +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 <![CDATA[Dolomite]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/65

Title

Dolomite

Description

Formula

CaMg(CO3)2

Structure

Non-silicate

Crystal System

Crystal system: trigonal (-)

Physical Properties

Colour in hand specimen: white, often tinged with yellow or brown 
Cleavage: rhombohedral 
Hardness: 3.5-4 

Optical Properties

Colour in plane polarised light: colourless 
Pleochroism: none 
Relief: low 
Birefringence: 0.179-(0.185); >4th order 
Extinction: inclined 
Twinning: lamellar, common

Mineral Specimens

M543 Dolomite, associated with siderite and copper pyrites 
Cerro Blanco, Chile 
Rhombohedral crystals of dolomite with curved faces, associated with siderite (iron carbonate) and copper pyrite 
White, slightly grey 
Hardness 3.5 – 4

Rock Specimens

L352 Limestone, dolomitic

Mineral Name

dolomite
dolomite_1.jpg
dolomite_CS.jpg
]]> Mon, 17 Jun 2019 16:21:55 +0100 <![CDATA[Dull [lustre]]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/186

Title

Dull [lustre]

Subject

Minerals displaying little or no lustre are described as dull. The surfaces of these minerals comprise coarse granulations, which scatter light in all directions. Clay minerals, for example, have dull lustre.

Description

See: adamantine
]]> Fri, 21 Jun 2019 15:37:02 +0100 <![CDATA[Echinoid]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/118

Title

Echinoid

Description

Taxonomy

Phylum: Echinodermata

Class: Echinoidea

Diagnostic features

Ambulacra (tube feet)

Periproct (opening for anus)

Peristome (opening for mouth)

Pentaradial symmetry (regular echinoids)

Bilateral symmetry imposed upon pentaradial symmetry (irregular echinoids)

Stratigraphic range

Ordovician to Present (Irregular Echinoids from Jurassic to Present)

Way of life

Regular echinoids:

Grazers with functional jaw apparatus

Long spines for protection and movement

Epifaunal at a wide range of depths

Use tube feet for respiration and movement

Irregular echinoids:

Feed on organic matter in sediment, front shaped to funnel sediment into mouth

Use fine spines to move

Infaunal burrowers

Use tube feet for respiration

Advanced notes

Grazing echinoids leave a distinctive pentagonal feeding trace Gnathicnus pentax.

Learn about types of preservation

Specimens

Browse echinoids

echinoid_sketch.png
irregular_echinoid_sketch.png
]]> Fri, 21 Jun 2019 09:12:57 +0100 <![CDATA[Eolian]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/187

Title

Eolian

Description

See aeolian.
]]> Fri, 21 Jun 2019 15:37:56 +0100 <![CDATA[Epidote]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/46

Title

Epidote

Description

Formula

Ca2Al2O∙(Al,Fe3+)OH[Si2O7][SiO4]

Structure

Orthosilicate

Crystal System

Crystal system: monoclinic (-)

Physical Properties

Colour in hand specimen: green
Cleavage: one, perfect
Hardness: 6

Optical Properties

Colour in plane polarised light: yellow-green
Pleochroism: yes
Relief: high
Birefringence: 0.015-0.051; 1st order orange to 3rd order pink
Extinction: inclined
Twinning: lamellar, not common

Rock Specimens

L354 Limestone, shelly

Mineral Name

epidote
Epidote_CS.jpg
]]> Mon, 17 Jun 2019 13:10:21 +0100 <![CDATA[Equant]]> https://wserv3.esc.cam.ac.uk/p1acollections/items/show/188

Title

Equant

Subject

A crystal which is of roughly equal size in all directions is described as equant. Although this description implies that equant crystals tend towards a spherical form, the term describes both rounded and angular crystals.

Description

See: ultramafic cumulate (L171)
]]> Fri, 21 Jun 2019 15:39:14 +0100