1A Collections

Microcrystalline

Fri, 21 Jun 2019 16:38:12 +0100

Title

Microcrystalline

Subject

Rocks with a crystalline structure in which the individual crystals cannot be identified by the naked eye, but are large enough to be identified using standard optical microscopy are described as microcrystalline.

Description

Compare with cryptocrystalline.

Microperthite texture

Fri, 21 Jun 2019 16:40:44 +0100

Title

Microperthite texture

Subject

Parallel lamellae or trains of blebs of one mineral, all in the same optical orientation, are enclosed in a single “host” crystal of another mineral. This texture is generally attributed to solid-state exsolution of the daughter phase from the host.

Description

Have a look at the alkali granite (L100) and the hornblende biotite granite (L102) in the 1A Reference Series.

Mohs scale

Fri, 21 Jun 2019 16:41:24 +0100

Title

Mohs scale

Subject

Mineral hardness is described using the Mohs scale. The scale runs from very soft at 0 up to very hard at 10. The positions of minerals are determined by their relative hardness. For example, quartz can scratch calcite, but calcite cannot scratch quartz. Thus quartz is harder than calcite.

Nautiloid

Thu, 20 Jun 2019 13:07:42 +0100

Title

Nautiloid

Description

Taxonomy

Phylum: Mollusca

Class: Cephalopoda

Diagnostic features

Simple suture

Chambers

Central siphuncle

Straight, curved or coiled shell

Stratigraphic range

Cambrian to recent

Straight forms: Cambrian to Permian

Coiled forms: Devonian to Present

Way of life

Nektonic (jet propulsion)

Predatory

Buoyancy controlled by changing gas/liquid contents of chambers

Marine

Preservation

Commonly preserved as internal moulds

Modern nautiloids preserved with original shell material

Learn about types of preservation

Advanced notes

Nautiloids originated in the Cambrian period, and radiated during the Ordovician. These early nautiloids had conical shaped shells. In the Silurian some nautiloids used curved shells, and by the Devonian some were coiled. The evolution of the coiled shell from the straight is thought to be driven, at least in part, by increased mobility of a coiled shell compared to a long straight one.

Nautiloids could be confused with ammonoids. You can distinguish them by the location of the siphuncle and the complexity of the suture pattern. Nautiloids, unlike ammonoids, are not extinct, although only six species remain today (compared to thousands in the Palaeozoic). This means that if presented with modern shell material it is likely a nautiloid and not an ammonoid.

Specimens

Browse nautiloids

External links

Telling planispiral fossils apart

Olivine

Mon, 17 Jun 2019 12:48:39 +0100

Title

Olivine

Description

Formula

(Mg,Fe)₂[SiO₄]

Structure

Orthosilicate

Crystal System

Crystal system: orthorhombic (+) (-)

Physical Properties

Colour in hand specimen: pale green
Cleavage: none, instead it has characteristic curving cracks
Hardness: 7

Optical Properties

Colour in plane polarised light: colourless
Relief: moderate
Birefringence: 0.035; 2nd order pink

Mineral Specimens

M100 Olivine
North Carolina
A mass of rather irregular crystals of olivine
Yellow-green colour
Vitreous lustre
No cleavage

Rock Specimens

L141 Gabbro
L162 Basalt, porphyritic
L172 Ultramafic cumulate

Advanced Notes

Olivine has two end-member compositions: forsterite, Mg₂[SiO₄], and fayalite, Fe₂[SiO₄]. The physical and optical properties detailed above refer to forsterite. Forsteritic olivine is a common constituent of mafic and ultra-mafic rocks.
Cracks are often filled with serpentinite alteration.

Mineral Name

olivine
forsterite
fayallite

Ooid

Fri, 21 Jun 2019 16:48:52 +0100

Title

Ooid

Subject

Sub-spherical sedimentary grains comprising a nucleus surrounded by concentric layers of carbonate precipitate. Ooids form abiogenically. They are sometimes referred to as ooliths.

Description

Have a look at the oolitic ironstone (L361) and the oolitic limestone (L353) in the 1A Reference Series.

Orthopyroxene

Mon, 17 Jun 2019 13:14:02 +0100

Title

Orthopyroxene

Description

Formula

(Mg,Fe)₂Si₂O₆

Structure

Chain silicate

Crystal System

Crystal system: orthorhombic (+) (-)

Physical Properties

Colour in hand specimen: black
Cleavage: two at 90°
Hardness: 5-6

Optical Properties

Colour in plane polarised light: colourless, reddish or greenish
Pleochroism: variable weak to strong
Relief: moderate
Birefringence: 0.007-0.020; 1^st order grey to 2^nd order purple
Extinction: straight

Advanced Notes

Orthopyroxene has two end-member compositions: enstatite, Mg₂Si₂O₆, and ferrosilite, Fe²⁺₂Si₂O₆.

Mineral Name

orthopyroxene

Orthosilicate

Fri, 21 Jun 2019 16:49:13 +0100

Title

Orthosilicate

Subject

Silicate minerals in which the SiO4 tetrahedra do not share oxygen atoms with one another are called orthosilicates, or island silicates. These minerals do not contain bridging oxygen atoms.

Overgrowth

Fri, 21 Jun 2019 16:49:40 +0100

Title

Overgrowth

Subject

The crystallisation of one mineral on the surface of another, producing a rim or part of a rim.

Pearly [lustre]

Fri, 21 Jun 2019 16:50:43 +0100

Title

Pearly [lustre]

Subject

Unsurprisingly the quality and intensity of light reflected from minerals displaying “pearly” lustre is reminiscent of pearls. This effect is produced by the reflection of light from many thin, coplanar, transparent sheets within the mineral. Muscovite, for example, has pearly lustre.

Description

Have a look at the muscovite mica (M301, M302) in the 1A Reference Series.