Slate
Colour: Black and shades of blue, green, brown and buff.
Texture: fine-grained.
Structure: By definition, slates are characterized by a single, perfect
cleavage (slaty cleavage), enabling it to be split into parallel-sided slabs.
On the cleavage surfaces sedimentary structures such as bedding and graded
bedding can often be seen. Fossils may be preserved but are invariably distorted.
Folds are often apparent in the field.
Mineralogy: Too fine-grained to distinguish with the naked eye. Pyrite porphyroblasts
often occur, usually as cubes.
Field relations: Slates are produced by low-grade regional metamorphism
of pelithic sediments (shales, mudstones) or fine-grained tuffs. They may
be associated with other metamorphic sedimentary or volcanic rocks."
(Hamilton et al 1976 , 150)
slate point
Quartzite
Colour: White, grey, reddish.
Texture: Medium-grained; usually of a granoblastic texture.
Structure: Usually massive but primary sedimentary features may be preserved,
such as bedding, graded bedding or current bedding.
Mineralogy: Essentially composed of tightly interlocking grains of quartz.
A little feldspar or mica may also be evident. White varieties are distinguished
from marble by their greater hardness.
Field relations: Quartzites are metamorphosed quartz sandstones and are
found in association with other metamorphosed sedimentary rocks such as
phyllite, schist and marble." (Hamilton
et al 1976 188)
quartzite flakes
Quartz
Quartz is composed mainly by SiO2, and its structure is crystalline.
Quartz occurs in many varieties. Rock crystal is
colourless quartz and sub-varieties include ghost quartz in which growth
stages are marked by inclusions, and rutilated quartz (sagenite), which
contains hair-like rods of rutile. Amethyst is purple; milk quartz is white;
rose quartz is rose-red or pink, and is usually found massive rather than
as crystals. Citrine is yellow and transparent and resembles topaz. Smoky
quartz (sometimes called cairngorm) is smoky brown to nearly black. Some
quartzes contain impurities that not only impart a colour but render them
opaque. Ferruginous quartz is an example of this and is commonly brick-red
or yellow.
Distinguishing features: Crystal form, conchoidal fracture, vitreous lustre,
hardness.
Occurrence: Quartz is one of the most widely distributed minerals. (Hamilton
et al 1976 128)
Rock crystal
Rock crystal is quartz in crystalline form and is translucent.
It has conchoidal fracture and therefore ideal for making tools
Flint
Colour: Blue-grey, grey, to nearly black when fresh, but weathers to a whitish,
powdery crust (patina).
Texture: Very fine-grained and smooth; conchoidal fracture. Rough on weathered
surfaces.
Structure: Flint and chert form rounded nodules of widely differing forms,
but chert also forms massive beds. Flint nodules are often hollow and may
contain a fossil, such as a sponge or echinoid.
Mineralogy : Composed of silica, mainly the variety chalcedony. Some authors
distinguish flint and chert compositionally but the differences, if any,
are slight.
Field relations: Flint and chert nodules occur typically in limestone and
chalk. They are usually patchily distributed but often concentrated along
one bedding plane. Their origins are not fully understood; but some appear
to be secondary replacements of the host rock, whilst others may represent
primary deposition on the sea bed of colloidal silica. (Hamilton
et al 1976 204)
