1A Collections

Pyrite

Mon, 17 Jun 2019 16:01:43 +0100

Title

Pyrite

Description

Formula

FeS₂

Structure

Non-silicate

Crystal System

Crystal system: cubic

Physical Properties

Colour in hand specimen: pale brassy yellow; black when fine-grained
Streak: greenish or brownish
Lustre: metallic
Cleavage: not obvious
Hardness: 6-6.5

Optical Properties

Colour in plane polarised light: opaque

Mineral Specimens

M520 Slate containing pyrite
Coniston, Lancashire
Cubic crystals, embedded in a well-cleaved slate
Golden colour
Metallic lustre
Indistinct cleavage in places

M412 Quartz (with galena and pyrite)
North Wales
A mass of small hexagonal quartz prisms, with galena, pyrite and zinc blende.
Transparent, white to light grey
Galena: purple-black, shiny, platy habit, metallic
Pyrite: metallic lustre, gold colour, cubic habit, “Fools’ gold”

Rock Specimens

L112 Rhyolite
L371 Chert, bedded

Mineral Name

pyrite
opaques

Plagioclase feldspar

Mon, 17 Jun 2019 15:45:29 +0100

Title

Plagioclase feldspar

Description

Formula

Na[AlSi₃O₈] – Ca[Al₂Si₂O₈]

Structure

Framework silicate

Crystal System

Crystal system: triclinic (+) (-)

Physical Properties

Colour in hand specimen: white
Cleavage: two at 90°
Hardness: 6-6.5

Optical Properties

Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.010, 1^st order grey
Extinction: inclined
Twinning: lamellar, very common

Mineral Specimens

M403 Plagioclase feldspar, oligoclase
Ytterby, Sweden
A cleavage fragment of white plagioclase feldspar
Pearly lustre on cleavage planes
Two sets of cleavage at 90°
Fine parallel striations due to lamellar twinning

M404 Plagioclase feldspar, labradorite
Labrador
Dark grey, light where fragments are small due to cracking along cleavage
Schiller (blue iridescence), characteristic of labradorite
Very fine parallel striations due to lamellar twinning

Rock Specimens

L100 Alkali granite
L101 Granite
L102 Hornblende biotite granite
L112 Rhyolite
L120 Granodiorite
L130 Andesite, porphyritic (porphyrite)
L141 Gabbro
L162 Basalt, porphyritic
L172 Ultramafic cumulate
L504 Garnet schist

Advanced Notes

Plagioclase feldspar forms a continuous solid solution between the sodic endmember (albite, or Ab) and the calcic endmember (anorthite, or An). The series is sub-divided into albite (An_0-10), oligoclase (An_10-30) andesine (An_30-50), labradorite (An_50-70), bytownite (An_70-90) and anorthite (An_90-100).

Mineral Name

plagioclase
feldspar

Alkali feldspar

Mon, 17 Jun 2019 15:40:50 +0100

Title

Alkali feldspar

Description

Formula

(K,Na)[AlSi₃O₈]

Structure

Framework silicate

Crystal System

Crystal system: monoclinic (-) or triclinic (+) (-)

Physical Properties

Colour in hand specimen: white or pink
Cleavage: two at 90°
Hardness: 6-6.5

Optical Properties

Colour in plane polarised light: colourless
Pleochroism: none
Relief: low
Birefringence: ~0.008; 1^st order greys
Extinction: inclined
Twinning: usually simple or lamellar;
Microcline (KAlSi₃O₈) displays characteristic cross-hatched twinning

Mineral Specimens

M400 Alkali feldspar, microcline
A single crystal of pale orange-pink alkali feldspar.
Pearly lustre.

M401 Alkali feldspar, orthoclase
Norway
A fragment of opaque, pale grey alkali feldspar.
Two sets of good cleavage at right angles, with a pearly lustre on the cleavage faces.

M402 Alkali feldspar, orthoclase
Shap, Westmorland
Porphyritic granite.
Large phenocrysts of pink feldspar, two sets of cleavage at 90°, simple twinning.
Also white plagioclase feldspar, quartz and biotite.

Rock Specimens

L100 Alkali granite
L101 Granite
L102 Hornblende biotite granite
L110 Rhyolite, porphyritic (quartz porphyry)

Mineral Name

alkali feldspar

L120 Granodiorite

Fri, 14 Jun 2019 09:43:53 +0100

Title

L120

Granodiorite

Subject

Age

Caledonian, Devonian

Location

Boulder, Moor of Rannoch

Description

Hand Specimen

Mostly moderately coarse-grained (typically 1-2 mm), some regions finer-grained (typically ~0.5 mm).
“Salt and pepper” appearance being speckled black and white. Two black phases, biotite mica and hornblende amphibole. White regions comprise quartz and white feldspar.

Thin-section

Hornblende amphibole
- Pleochroic in greens and cream. Some of the grains display simple twinning. Two cleavages intersect at 56°.

Biotite
- Subhedral to anhedral grains, pleochroic in brown and straw. Straight, speckled extinction. Chlorite partially replaces biotite – late stage alteration.

Quartz
- Anhedral, interstitial grains. They display undulose extinction and contain abundant tiny inclusions, but are characteristically fresh and unaltered.

Feldspar, mainly plagioclase
- Mostly anhedral grains, though some are lath-shaped. Alteration. Zonation.

Accessory minerals:
- Sphene
- Magnetite
- Apatite

Rock History

Rock history Hornblende is euhedral. It is, therefore a primary igneous phase, and must have crystallised from a wet melt.

Magma cooled relatively slowly, allowing large crystals to grow.

Rock Name

granodiorite

L113 Rhyolite, glassy (obsidian)

Thu, 13 Jun 2019 14:38:27 +0100

Title

L113

Rhyolite, glassy (obsidian)

Subject

Age

Cenozoic

Location

Yellowstone, U.S.A.

Description

NB, the term ‘obsidian’ refers to a jet-black glassy rock. It usually has a rhyolitic composition, but not always.

Hand Specimen

Black rock with the characteristic vitreous lustre and conchoidal fracture of glass.
The surfaces are scattered with tiny spherical holes. These are vesicles.

Thin-section

Holding the thin-section up to the light, the spherical vesicles within the brown glass are clearly visible.

Under the microscope the pale brown glass is scattered with tiny opaque speckles in a weak flow texture.

Spherical vesicles are mostly filled in with mounting medium; notice its distinctive appearance under cross-polarised light. There are a small number of opaque infilled circles. These could be artefacts or black clumps of iron oxide.

Tiny crystals are scattered throughout the glass, indicating that some devitrification has occurred. Circles of devitrification, probably nucleated on oxide crystals.

Rock History

Glass → Very rapidly cooled magma. Probably extrusive.
Vesicles: perfectly spherical, indicating that they were not being sheared by magma flow, i.e. the magma was not flowing.

Rock Name

glassy rhyolite
glass
obsidian

L100 Alkali granite

Wed, 12 Jun 2019 12:05:16 +0100

Title

L100

Alkali granite

Subject

Specimen Age

Variscan , Permo-Carboniferous

Description

Hand Specimen

This rock has a porphyritic texture. Phenocrysts of white feldspar up to 30mm long are surrounded by a coarse groundmass of quartz, biotite and white feldspar.

Thin-section

Alkali feldspar (orthoclase)
- Grains typically 2-4mm long, subhedral to anhedral, dirty looking due to alteration. Many display a microperthite texture.

Plagioclase feldspar
- Grains typically 1-4mm long, subhedral, characteristic lamellar twinning. Many of the grains look “dirty” under plain polarised light, and have high birefringence speckles under cross-polarised light. They have been partially altered to white mica. Some grains are zoned.

Quartz
- Grains typically 1-4mm across, anhedral. They are characteristically clean and unaltered when compared with feldspar grains, but do contain many tiny inclusions.

Biotite
- Grains similar size as the rest of the groundmass. Strongly pleochroic in pale straw and dark reddish brown. Speckled extinction. Some sections show the one good cleavage of biotite. Small black blobs and circles are dotted all over the crystals. These are pleochroic haloes, produced by zircons. Partially decomposed to chlorite.

Accessory minerals
- Apatite
- Small irregular grains of muscovite mica

Rock History

This is a coarse-grained rock, which must have cooled slowly as part of an intrusion. Its granitic composition (essential quartz and feldspar) indicates that it was produced in a continental setting.

Rock Name

alkali granite