School Science Lessons
Earth and space sciences
2009-08-29
Please send comments to: J.Elfick@uq.edu.au
Geology
See:
Chemistry index and minerals, alphabetical list of terms
35.22.4
Clay
35.22.4.01 Chemical weathering
35.22.4.1 Illite
35.22.4.2 Kaolinite
35.22.4.3 Montmorillonite
(smectite)
35.22.4.4 Bentonite
35.22.4.5 Fuller's earth,
35.22.4.6 Vermiculite
35.22.4.7 Halloysite
35.3.1
Minerals mined at the Broken Hill mines
35.3.1.1 Silver, Ag
35.3.1.2 Lead, Pb
35.3.1.3 Zinc, Zn
Ore minerals of the primary (sulfide) zone
35.3.2.1 Galena (lead sulfide, PbS
35.3.2.2 Sphalerite (zinc sulfide, ZnS)
Gangue (waste) minerals of the
primary (sulfide) zone
35.3.3.1 Bustamite, calcium manganese silicate,
MnCaSiO6
35.3.3.2
Rhodonite, manganese silicate ([Mn,
Ca]SiO3)
35.3.3.3
Garnet (spessartine) manganese aluminium
silicate (Mn3Al2Si3O12)
Other minerals of the primary
(sulfide) zone
35.3.35.1 Chalcopyrite, copper iron sulfide
35.3.35.2
Pyrite, iron sulfide
35.3.35.3
Pyrrhotite, iron sulfide
35.3.35.4
Rhodochrosite, manganese carbonate, MnCO3
Ore minerals of the oxidized zone
35.3.35.5 Anglesite, lead sulfate, PbSO4
35.3.35.6
Azurite (copper carbonate)
35.3.35.7
Cerussite, lead carbonate, PbCO3
35.3.35.8
Copper
35.3.35.9
Coronadite (lead manganese oxide)
35.3.35.10
Goethite, FeO(OH), hydrated iron oxide,
hydrous iron oxide
35.3.35.11
Gypsum, calcium sulfate
35.3.35.12
Malachite, copper carbonate
35.3.35.13
Pyromorphite, Pb5(PO4)3Cl
35.3.35.14
Silver, Ag
35.3.35.15
Smithsonite, zinc carbonate, ZnCO3
35.12.1 Touchstone
35.14.2 Opals
35.14.3 Amethyst
35.14.4
Chalcedony
35.21.8
Classify igneous rocks in hand
specimens
35.23
Metamorphic rocks
35.23.01
Classification of metamorphic rocks
35.23.1
Coal, coal dust explosions
35.23.2
Graphite
35.23.3
Marble, CaCO3
35.23.4
Petroleum, crude oil
35.23.5
Quartzite
35.23.6
Slate
35.23.7
Talc,
Mg3Si4O10(OH)2
35.24 Make
artificial igneous
rocks, alum crystals,
sulfur crystals
35.25 Making
artificial rocks, sedimentary rocks
35.26 Making
artificial rocks, metamorphic rocks
35.27 Folds
35.28
Joints
35.29 Faults
35.30
Examine sand with a magnifying glass
35.30.1 Quicksand
35.31 Tests
for
limestone
35.32 Sort
sediments
35.33
Piezoelectricity
35.33.1
Pyroelectricity, ferroelectricity
35.33.2 Dimorphism, aragonite
35.34 How
fossils form
35.34.1 Dendrites,
false fossils
35.35 Find
fossils
35.40.1
Mapping contours, geological structures, erosion
35.40.2
Isostasy models
35.3.1
Minerals mined at the Broken Hill mines, Australia
The minerals of Broken Hill are world famous because many of them are
rare and beautiful. Unfortunately, most of the rare minerals were found
in the top sections of the mine where the sulfide ore minerals had been
weathered and oxidized by groundwater to produce a dazzling array of
secondary minerals. These areas of the mine are long worked out and not
producing ore or mineral specimens anymore.
The main metals mined for at Broken Hill are as follows:
35.3.1.1 Silver, Ag, occurs in a variety of
minerals but most of the silver is found as trace amounts of silver
mineral locked up inside the lead mineral, galena. Sometimes silver
occurs as big lumps, nuggets, of the metal itself. Only silver ever
comes out of the ground as a metal. Lead and zinc are always
locked away minerals, as is most of the silver. Silver is largely used
in the photographic industry although it has uses in jewellery,
electronics and silverware.
35.3.1.2 Lead, Pb, occurs mainly as the lead
ore galena (lead sulfide, PbS) It is characterized by a metallic silver
lustre and cubic fracture. Cerussite (lead carbonate, PbC03) and
anglesite (lead sulfate, PbSO4) are found in areas where galena has
been weathered or exposed to oxidizing groundwater. Typically this
occurred at or near the surface. Lead was used in water pipes, roofing
and pigments but is now mostly used in batteries for vehicles and other
equipment.
35.3.1.3 Zinc, Zn, occurs mainly as sphalerite
(zinc sulfide, ZnS). At Broken Hill it has a black resinous appearance
but rarely shows as big crystals. Smithsonite (zinc carbonate, ZnCO3),
resulting from the weathering and oxidation of ore by groundwater, is
found in areas where the ore body was at or near the surface. Zinc is
used in galvanized coatings of iron and steel. It is also used in die
cast alloy products, pigments and other industrial and agricultural
applications.
Ore minerals of the primary (sulfide) zone
35.3.2.1 Galena (lead sulfide, PbS is the main
lead ore mineral at Broken Hill. The silvery metallic lustre and cubic
appearance characterize galena. It has a relative density of 7.35.
Galena is also the source of much of the silver at Broken Hill. Silver
atoms can substitute for lead atoms or be present within minerals such
as acanthite (Ag2S) that have formed within the galena.
35.3.2.2 Sphalerite (zinc sulfide, ZnS) is the
main zinc ore mineral at Broken Hill. Good crystalline sphalerite is
unusual at Broken Hill. The colour of sphalerite varies with its
impurities. At Broken Hill it is black but some rare large crystals
have a deep red colour.
Gangue (waste) minerals of the
primary (sulfide) zone
35.3.3.1 Bustamite, calcium manganese silicate,
MnCaSiO6, also occurs in
the galena-rich lodes. It has a range of pink to orange to deep brown
colours.
35.3.3.2
Rhodonite, manganese silicate ([Mn,
Ca]SiO3) is the most abundant manganese mineral found in the
galena-rich ore bodies. It has a range of beautiful red-pink
colours.
35.3.3.3
Garnet (spessartine) manganese aluminium
silicate (Mn3Al2Si3O12, is
a port wine red mineral commonly
associated with galena ore.
Other minerals of the primary
(sulfide) zone
35.3.35.1 Chalcopyrite, copper iron sulfide,
occurs
in veins in garnet, quartzite and garnet sandstone in ore bodies. The
associated minerals are argentiferous galena and arsenopyrite.
35.3.35.2
Pyrite, iron sulfide, is found in
lining
cavities in faults and
fractures in ore bodies. The associated minerals are calcite and
rhodocrosite.
35.3.35.3
Pyrrhotite, iron sulfide, is found in
veins, zones and bands in ore bodies. It can be weakly magnetic but not
at Broken Hill. The associated minerals are calcite, galena, and
chalcopyrite.
35.3.35.4
Rhodochrosite, manganese carbonate, MnCO3
is a pink mineral found in fault zones along with other carbonate
minerals, e.g. calcite.
Ore minerals of the oxidized zone
35.3.35.5 Anglesite, lead sulfate, PbSO4,
is
another widespread
secondary mineral from the oxidized zones of the mine. It is found in
vughs (irregular voids) and fractures in all mines in the outcrop area.
The associated
minerals are marshite, iodargyrite, pyromorphite, stalactitic goethite,
and goethite matrix replaced by cerussite.
35.3.35.6
Azurite (copper carbonate) has a habit
consisting of short
tabular prisms, equidimensional plates, long spear-like crystals with
pyramidal terminations. The associated mineral is malachite.
35.3.35.7
Cerussite, lead carbonate, PbCO3,
occurs as ore grade concentrations. It is a secondary mineral from the
oxidized zones. Most Broken Hill cerussite is opaque white, but wine
yellow, yellow brown. smoky brown, transparent and translucent examples
are known. It occurs as reticulated masses, complex arrowheads “twinned
crystals”, and "jack straw" masses of tubular-shaped crystals. It is
found in ore bodies and is one of the most abundant minerals of the
oxidized zone. The associated minerals are malachite, azurite, and
bromian chlorargyrite.
35.3.35.8
Copper has arborescent forms in large cavities, four-sided wire prisms,
elongate octahedrons with repeated branches. Also, stalactitic or
dendritic masses in wire-like groups and “nail head” crystals.
The associated minerals are cuprite and malachite.
35.3.35.9
Coronadite (lead manganese oxide)
originally referred to as psilomelane, is massive,
stalactitic, shawls, cellular, botryoidal habit. It is
abundant in the upper levels of the oxidized zone and outcrop. The
associated minerals are goethite that forms the matrix for a variety of
secondary minerals.
35.3.35.10
Goethite, FeO(OH), hydrated iron oxide,
hydrous iron oxide, has a
habit
consisting of
botryoidal, mamillary, stalactitic masses and crusts. It is abundant in
the
gossanous capping of the ore bodies. The associated mineral is
coronadite.
35.3.35.11
Gypsum, calcium sulfate, has a habit
consisting of fibrous,
massive, colourless transparent crystals. It is located in ore bodies
in the
seams, cavities, water courses and crusts in abandoned workings. The
associated minerals are rosasite, linarite and dolomite.
35.3.35.12
Malachite, copper carbonate has
botryoidal and sometimes
velvety habit. It is found in ore bodies as powdery to compact fibrous
crusts and hemispherical aggregates. The associated minerals are
azurite and cerussite.
35.3.35.13
Pyromorphite is the most common lead
phosphate, Pb5(PO4)3Cl.
It is a secondary mineral from the oxidized zone. It has a large range
of habits and colours including coatings and sprays, simple hexagonal
prisms, stout hexagonal prisms, branching aggregates, mamillated,
botryoidal and colloform masses. It is found all along the lode
outcrop. The associated minerals are coronadite, cerussite, secondary
galena, and anglesite.
35.3.35.14
Silver has massive, wire habit The associated minerals are gold and
copper.
35.3.35.15
Smithsonite, zinc carbonate, ZnCO3,
is a widespread secondary mineral from the oxidized zones. It occurs as
rounded botryoidal aggregates resembling drops of wax and as
honeycombed masses in ore bodies. It is the most abundant secondary
carbonate after cerussite. The associated minerals are coronadite and
goethite.
35.12.1 Touchstone is a
form of schist used to assay gold by comparing the streak of the
sample to the streak of "touch needles" with known gold content.
35.14.2 Opals
See diagram 35.14.1
Opal is similar to chalcedony, but it is a hydrous silica. It has
non-metallic lustre, white streak, not good cleavage, conchoidal
fracture, white colour, vitreous lustre with colour patches, specific
gravity about 2, can scratch glass and be scratched by quartz. This
mineral has no definite atomic structure and never occurs as
crystals. Opal colour is not formed from impurities or chemicals
within the
gemstone. Opal has the same chemical structure as glass, SiO2.nH2O.
However, the molecular structure is different and this difference
causes the colour. The opal molecules form in a regular symmetrical
pattern. White light enters the opal, and the molecules act as myriads
of prisms, and the light is consequently refracted out as various
colours. The most famous sources of opal are the Lightning Ridge Opal Mines.
1. Solid light (white) opal occurs as two types, milky and crystal.
Milky opal is opaque, with the colours visible on the surface only.
Crystal opal is transparent with the colours being visible from within
the depths of the stone.
2. Opal triplets consist of thin slices of opal affixed to a background
of black glass. A dome of clear quartz crystal is then glued to the
upper surface. The opal slice is so thin that it becomes totally
transparent. Thus, the black background causes the colours to
darken dramatically. The crystal dome is to protect and magnify
the opal.
3. Opal doublets are similar in construction to triplets, but without
the crystal dome. Thus, a slice of opal is glued to a piece of black
glass (or similar substance) and the actual opal is then
polished. The opal is generally thicker than a triplet, with
better quality opal so doublets are more valuable.
4 Dark (black) opals have dark colours, similar to doublets and
triplets. Here, however, the dark background is a natural phenomenon.
Thus, a black opal is, in fact, a natural doublet with a band of colour
sitting on a dark background. Black opals are very rare, so very
valuable.
5. Boulder opals are mined in Queensland, Australia, where
ironstone boulders occur with thin veinlets of opal running through
them. The stones are cut as natural doublets, with part of the seam of
opal as the face, and the ironstone as the natural backing. Boulder
opals have a similar appearance to black opals, but have less value.
6. Boulder opal matrix is used when the ironstone / opal amalgam
is such that full boulder opals cannot be cut. So the fine veinlets and
dark ironstone are polished together.
7. Andamooka matrix consists of a mixture of opal and porous rock, and
is white in colour. A regular stone is cut from this amalgam and placed
in a sugar solution that soaks into the rock. sulfuric acid is then
applied to carbonize the sugar and turn it black. Thus, the fine
slivers of opal now have a black background to give the finished
article the appearance of a black opal.
8. Synthetic opals were developed in France several years ago and are
virtually never sold in Australia by virtue of the fact that they are
synthetic and have little acceptance.
Australia provides the world with 95% of all precious opal. There are
eight varieties of opal available in Australia. Before contemplating
the purchase of an opal, it is important to understand each type and
the principles of their valuation.
Opal valuation
The three basic criteria of evaluation for all opals, light or dark in
colour
1. Colour
The more red visible in an opal, the more valuable. The colour
hierarchy is red, orange, green, blue.
2. Brightness
Brightness is most important aspect of opal valuation. The brighter and
stronger the colour, the better the quality. Thus a bright green stone
can be more valuable than a dull red one.
3. Patterns
The larger the splashes of colour, the better the quality. "Pinfire" or
"sheen" patterns are the least valuable. The ultimate pattern is the
extremely rare and valuable "harlequin" with a symmetrical square
checkerboard appearance.
Misconceptions
1. Opal is not soft. It has the same hardness as glass.
2. Opal is not unlucky. This was a rumour started about the year
1900 by London diamond merchants to try to protect their then monopoly.
For centuries before that, opal had been considered a stone of good
fortune.
3. Opal does not shrink in settings.
4. Opal does not lose its colour in the sun or snow (or anywhere).
5. Opal is not affected by water. However, in the past, triplets have
been glued with a resin that does not agree with moisture. This resin
has come apart, causing the opal to appear cloudy. Most triplets now
have a water resistant glue so check this before purchasing, and always
obtain a guarantee.
35.14.3 Amethyst
A violet-blue variety of quartz. (Greek: not intoxicate, amethyst was
thought to be a charm against inebriety)
35.14.4
Chalcedony
It is fibrous with very small crystals of quartz and the silca
mineral moganite. Chalcedony may be in the form of the following
gemstones: Agate, Aventurine, Bloodstone, Carnelian, Chrysoprase,
Heliotrope, Jasper, Onyx, Sard
Agate is concentrically banded in crazy patterns it is called
agate. Onyx is in flat layers. Sardonyx is in the form of white and
brown red bands. Cornelian is red due to iron impurities. Thunder
eggs are in the form of a rock shell filled with agate. Bloodstone
occurs as an opaque or
translucent mineral, bright or dark green in colour and interspersed
with small red spots.
35.21.8
Classify igneous rocks in hand
specimens
After Al
Grenfell The Australian Science Teachers Journal Vol. 32 No. 3
See diagram 5.21.1a | See
diagram 5.21.1b
Use a magnifying glass to classify magmatic rocks by texture and
mineralogy. Volcanic types and plutonic types of igneous rocks have
cooled and
solidified at different rates typically in different physical
environments
giving different textures. Plutonic
rocks are the granites, some porphyries and other igneous unstratified
crystalline rocks thought to have formed at great depth and pressure in
the earth.
Plutonic rocks have individual grains coarse
enough to be individually identifiable usually >1 mm diameter.
Volcanic
rocks have no visible crystals
Classify plutonic rocks using the
modification of the IUGS (International Union of Geological Sciences)
classification of plutonic rocks. Use the triangular coordinate system
in
diagram 35.21.1a. Use diagram 35.21.1b to estimate the volumetric
abundance of the
major rock forming minerals.
Divide the surface and sub volcanic magma
systems
of volcanic rocks into two broad categories with differing flows of
energy and
modes of eruption.
The magmas are:
1. blown out as pieces of ejecta or
2. erupted or intruded as coherent units.
So you can distinguish
corresponding hand
specimens on the presence or absence of volcaniclastic texture.
Table
1.
shows
the chief types of pyroclastic rocks.
Table 2. shows classification of
non-fragmented volcanic rocks, e.g. aphyric lava by colour.
Table 3.
shows
classification of porphyritic volcanic rocks by phenocryst assemblages.
Each
table can be expanded to accommodate additional volcanic rocks that may
be
relatively uncommon generally but locally abundant.
Classification of igneous rock hand specimens
1a Fine-grained < l. mm
aphanitic, (Volcanic rocks)
1b Average grain diameters > 1 mm
phaneritic, (Plutonic rocks), Go to 5a 5b
2a Pyroclasts present, (Pyroclastic rocks), See
Table 1
2b Pyroclasts absent, grains interlocked, (Non-fragmental volcanic
rocks)
3a Ground mass glassy, (Obsidian)
3b
Ground mass crystalline
4a Aphyric phenocrysts absent, (Aphyric
lava), See
Table 2
4b Porphyritic phenocrysts present, (Porphyritic volcanics), See Table 3
5a
Medium to coarse grained, (Plutonic rocks)
5b Pegmatite (>30 mm), (Pegmatite)
Table 1. Pyroclastic rock types
Rock type: Features
Agglomerate (Pyroclasts >32 mm blocks and bombs rounded pyroclasts
predominant)
Volcanic breccia (Pyroclasts >32 mm blocks and bombs;
angular pyroclasts predominant)
Lapilli tuff (Pyroclasts 4 to 32 mm and of any
shape)
Tuff (Pyroclasts < 4 mm and of any shape)
Ignimbrite (Welded
tuff with unsorted nature > 50% fragments < 4 mm pumice clasts
common
often flattened and with frayed terminations)
Table 2.
Aphyric lava
Rock type (colour)
Mafic
lava (dark coloured)
Felsic lava (light coloured)
Table 3.
Porphyritic volcanic rocks
Rock type:
phenocryst mineralogy (Plutonic equivalent)
Basalt: +- olivine +- augite +-
plagioclase (Gabbro)
Andesite: plagioclase +- mafic phases (Diorite)
Dacite: plagioclase +- quartz + mafic phases (Tonalite)
Rhyodacite:
plagioclase + alkaline feldspar + quartz +- mafic phases (Granodiorite)
Rhyolite: alkali feldspar +- quartz +- mafic phases (Granite)
Trachyte:
alkali feldspar + mafic phases (Syenite)
35.23
Metamorphic rocks are the result of heat and
pressure
applied to igneous and sedimentary rocks. The pressure causes mineral
grains to
align in a single plane so the rock tends to split in this direction.
This
alignment is called foliation. However, marble and quartzite are
metamorphic
rocks but are not foliated. Metamorphic rocks are similar to igneous
rocks in
that they are hard and have interlocked mineral grains. Thermal
metamorphism,
contact metamorphism, is caused by heat when molten lava heats rocks to
form
fine grain rocks with no bands or layers, e.g. hornfels. Regional
metamorphism
refers to the changes caused by extensive heat and pressure to produce
coarse grain,
banded rocks, e.g. gneiss.
The three varieties of foliation are as
follows:
1. Gneissic or banded foliation shows distinct bands of different
minerals. The
thicker bands are usually feldspar.
2. Schistosis foliation is caused
by the
parallel arrangement of platy minerals, e.g. mica.
3. Slaty cleavage
refers to
the tendency of a rock to split into thin, even slabs, e.g. slate.
The
cleavage
is the result of the parallel planar arrangement of microscopic mineral
grains.
35.23.01
Classification of metamorphic rocks
1. Foliated, banded or platy:
1.1 Coarsely banded, bands irregular in thickness (gneiss)
1.2 Schistose,
regular banding, medium in thickness, and platy (schist)
1.3 Slaty regular
fine banding and platy (slate)
2. Non-foliated, massive or granular:
2.1 Mainly calcite or dolomite
(marble)
2.2 Mainly quartz - quartzite
2.3 Mainly serpentine and / or talc (serpentine or talc)
2.4 Mainly organic, grey or black (graphite and
anthracite coal)
35.23.1
Coal
is mainly carbon from woody material,
algae and any plant
debris that collected millions of years ago in swamps. The heat and
pressure
caused by overlying deposits of sand and clay caused the formation of
coal. The
older the coal the greater the percentage of carbon. Peat is a lowest
quality
coal. It has a high percentage of water. Lignite or brown coal
is older
than peat and has received much more compression. Bituminous coals are
hard,
black and brittle. Anthracite is black and shiny.
A coal dust explosion occurs when a concentration of coal dust is
lifted and ignited. Ignition of a small quantity of methane gas in a
coal mine generates a pressure wave that can lift coal dust and ignites
it. Unless the reaction is limited, the coal dust will continue to lift
and burn and create an even larger pressure wave so that a coal dust
explosion occurs throughout the coal mine. Such explosions can be
prevented in two ways
1. Eliminate both ignition sources for methane from electrical
equipment or frictional sparking and eliminate accumulations of methane
by ventilation.
2. Preventing accumulation of fine coal dust and make inert coal dust
accumulations by stone dusting, i.e. application of finely crushed
limestone, called stone dust, to make the fine coal dust
incombustible. Also, by storing stone dust as passive explosion
barriers, any explosion causes dispersal of the stone dust to
make the airborne dust cloud incombustible.
Collect different types of
coal and break them with a hammer. You may find fossils in the peat and
softer
coal. Anthracite breaks with a conchoidal (shell-like) fracture similar
to
when you smash the corner of a piece of glass. Find the weight and
volume of the
coal samples and calculate the density Burn the coal samples to heat
waters and
estimate that sample produces the most heat per gram of coal.
35.23.2
Graphite, like diamond, consists of the
crystallized carbon, C.
Graphite has metallic lustre, can mark paper, grey to black colour,
black streak, cleavage in one direction, and relative density 2.1. It
is
soft,
black and opaque. It is greasy to touch and leaves a
grey dust
on the fingers so it is a good lubricant for machinery and "lead"
pencils. The softer
lead
pencils, the "B" grade, contain more graphite in the "lead". By
contrast diamond is crystallized carbon but it is one of the hardest
minerals and is
colourless and
transparent. Graphite is a good conductor of electricity but diamond
does not
conduct electricity. Graphite
is also
used as stove polish and dry lubricants in the electrical industry.
It occurs in crystalline igneous and metamorphic rocks and is
also made
artificially
by heating coke in a furnace. The central electrode of a dry cell
battery is
made of carbon.
35.23.3
Marble, CaCO3, is a crystalline
limestone formed
by the heating of limestone rock under pressure, thermal metamorphism.
If
limestone is heated strongly, it gives off carbon dioxide leaving quick
lime,
calcium oxide, CaO. If limestone is heated under great pressure it
melts and
does not lose carbon dioxide. If it then cools slowly, it
recrystallizes as
marble. Marble is a beautiful building stone, valued for
its
smoothness and hardness. Pure white marble is recrystallized calcite
and looks
like a sugar cube. Different colours in marble are caused by
impurities,
e.g.
dolomite, silica, iron, clay minerals. A common method of preparing
carbon
dioxide is to add hydrochloric acid to marble chips. Not all polished
stones are
marbles.
35.23.4
Petroleum, crude oil, is a mixture of
hydrocarbons often
with sulfur and nitrogen. Most scientists think it was formed from the
remains
of marine organisms buried at great depths, although some people
suggest an
inorganic origin. Oil deposits usually occur in sedimentary rocks with
a thick
layer of rock above and below. The oil may float on a layer of water
and be
under a layer of natural gas, a mixture of gaseous hydrocarbons, mainly
methane.
Crude oil is usually a dark green, brown or black oily liquid with a
characteristic smell. It always occurs with gas and water. A waxy
form of
petroleum is made from coal.
35.23.5
Quartzite is an altered and exceedingly
hard sandstone.
The grains have been bound together by a cement formed by the
dissolving action
of heated water. The constituent grains have been recrystallized to
form dense interlocked cementation. When sandstone is broken, only the
cement
holding the
sand grains together is damaged. If a piece of quartzite is shattered,
both the
sand grains and cement are broken because the particles are so strongly
bound
together. Quartzite has a glistening appearance caused by its
sugar-like crystal
structure. Quartzite is white when pure, but most of it contains mica,
iron,
feldspar or other mineral particles that alter the colour to grey,
brown, red, yellow, green or black. Owing to its extreme hardness,
quartzite has few commercial uses except road-making.
35.23.6
Slate
is a dense rock with a texture so
fine that the
individual grains in it cannot be seen by the eye or even through a
magnifying
glass. Slate comes from clay, mudstone and shales altered by
heat and pressure. It was once laid down as alluvial material at the
bottom of
lakes and oceans. Fossils of long dead marine plants and animals can be
seen
perfectly preserved in it. Most slate is grey, dark grey or black,
depending on
how much plant material it contains. It may also be green because
of the
chloride in sea water, or red, purple, yellow or brown because of
iron
stains. Slate has a well-marked cleavage so the surface of the flat
broken
piece of slate feels soft and silky. Although easily cut, slate resists
the weather so it is for roofs.
Formerly, school children learnt their lessons on small
slate
boards that they could write on with chalk and later erase the writing.
35.23.7
Talc,
Mg3Si4O10(OH)2,
has green to yellow to white colour, hardness 1, white streak, pearly
to
greasy lustre with a silvery
sheen, good but not visible cleavage, occurring in
plates or granular form, greasy
to
touch, relative density 2.7 to 2.8. Talc forms as a secondary mineral
after
metamorphosis.
Pieces of talc
break into
distinctly thin, easily bent layers that remain in
that shape. So talc is flexible but not elastic. Talc occurs as compact
masses, not crystals. Talc is used to make talcum powder, dry
lubricants, fireproof materials, linoleum, paper filler, a filler in
glazing and in cosmetics and floor
coverings. Soapstone is a compact form of
talc
used for carving. Tailors use small pieces of talc, French chalk, to
mark cloth.
Handle a talc specimen and note that it
has a pearly surface, is greasy to
touch, and can be easily scratched by the fingernail. Use it to mark
paper.
35.24 Make
artificial igneous
rocks, alum crystals,
sulfur crystals
1. Crystallization of alum solutions is similar to the formation of
coarse
grained and fine grained igneous rocks. Fill a test-tube one quarter
full of
powdered potash alum,
[Al2(SO4)3.K2(SO4).24H2O]
[also shown as KAl(SO4)2.12H2O].
Slowly add
just enough boiling water to dissolve the alum. Hold the test-tube in a
flame so
that the mixture boils gently. 1. Pour half the solution into a
shallow
metal container. Place a piece of string partly in the liquid and add a
seed
crystal. Stir the alum solution in the container so it cools quickly.
2. Hang
another piece of string in the test-tube so that it reaches the bottom
and add a
seed crystal. Place the test-tube where it will cool slowly. Examine
the two
solutions the following day and note the sizes of the crystals formed.
2. Melt some sulfur in a test-tube. Fit a filter paper into a funnel
and pour the
molten sulfur into it. As the sulfur cools it begins to solidify, first
forming
a crust on the surface. As soon as the crust has formed, remove the
filter paper
from the funnel and unfold it, so that the still liquid sulfur in the
lower part
of the filter can flow away from the crust. Note a mass of small
crystals on the
underside of the crust. Use a magnifying glass to observe the shape of
these
crystals.
3. Melt sulfur in a test-tube then pour it into a large beaker of
water so that it solidifies rapidly to form plastic sulfur. Take it out
of the
water and examine it after two hours. The solid sulfur formed is very
hard and
you cannot see crystals with a magnifying glass. However, very tiny
crystals may
be seen with a microscope.
35.25 Making
artificial rocks, sedimentary rocks
1. Use a hammer to grind different coloured sedimentary rocks, keeping
the colours
separated. Put coloured powdered particles in a glass
jar as different layers. Let water trickle down the
inside of the jar so
as not to disturb the layering until the water is 1 cm above the
sediments.
Put the jar in the
sun and let the water evaporate. Wrap the jar in a thick cloth and
break it with a hammer.
2.
Mix Portland cement with water and put it in a mould until it hardens.
Break the set cement with a hammer and examine the outside and inside
surfaces.
3. Mix dry cement
with twice as much sand or gravel to form concrete. Add
water, mix thoroughly, and place it in a mould. Leave the concrete to
harden for
several days.
Break the set concrete with a hammer and examine the outside and inside
surfaces. Note whether the concrete is easier or harder to break than
the Portland cement.
4. Mix
plaster
of Paris with a small amount of water and put it in a mould until it
hardens. Stir rapidly or it
will harden while being mixed. Break the set plaster with a hammer and
examine the outside and inside
surfaces. Note whether the plaster is easier or harder to break than
the Portland cement or the concrete.
35.26 Making
artificial rocks, metamorphic rocks
Fire a shaped piece of clay that has first been dried and put on a
piece of
broken pottery and heated it in a large crucible over a Bunsen
burner.
35.27 Folds
See diagram 35.27: Folds
Folds occur where parallel layer form an arch, anticline, or form a
trough, syncline. The line along which the direction of dip changes is
the hinge line. Arrange carpets or blankets in layers on the floor.
Push the layers horizontally to create anticlines and synclines.
35.28 Joints
Joins are fractures in rocks where no relative movement occurs each
side of the fracture. Joints can be caused by cooling shrinkage and
increased tension within the rock.
35.29 Faults
See diagram 35.29: Faults
A fault is a fracture in rock where some displacement has occurred. The
fault plane of the fracture can be vertical but usually there is a dip
of the fault. Dip-slip movement is where the direction of movement on
the fault plane, is parallel to the dip of the fault, i.e. up or down.
Strike-slip movement is where the direction of movement on the fault
plane is parallel to the strike of the fault, i.e. sideways. This
movement results in tear faults. The throw of a fault is the vertical
displacement between blocks of rock. The heave of a fault is the
horizontal displacement of blocks of rock. The hanging wall is the
surface with rock above it. The footwall is the surface with rock below
it. A normal fault is a dip-slip fault with the hanging wall on the
downthrow side, i.e. it appears that a block has slipped down the
fault. A reverse fault is a dip-slip fault with the hanging wall on the
upthrow side, i.e. it appears that a block has been pushed up the
fault. A trough fault is caused by downthrow movement between two
parallel faults to form a graben or rift valley. Upthrow between two
parallel faults results in a horst. A series of parallel faults is
called step faulting.
Make layers of modelling clay, Plasticine. Cut the layers vertically
then cut the layers at an angle to create a model of a fault.
35.30
Examine sand with a magnifying glass
The nearly colourless crystals are probably the mineral
quartz. Look for other
minerals in the sand.
35.30.1 Quicksand
Areas of quicksand have a source of upwards pressure from a spring
below. The sand becomes suspended and frictionless so will not support
weight on it. A person caught in quicksand should try to float on the
back and keep the arms below the surface. The nose and mouth should
remain above the surface and allow a slow and laborious paddling to the
edge of the quicksand.
35.31 Tests
for
limestone
Drop lemon juice, or
vinegar, or dilute hydrochloric acid on rock specimens. Limestone will
effervesce or bubble caused by carbon dioxide gas given off.
Marble, a metamorphic rock made
from
limestone, will also respond to this test.
35.32 Sort
sediments
Thoroughly mix equal
portions
of gravel, coarse sand particles, and clay particles. Place this
mixture in a
glass jar, not more than half full. Fill the jar with water. Place a
cap on the
jar and shake vigorously. Allow the material to settle. The components
will
arrange themselves in order, with the heavier particles at the bottom
and the
clay particles on the top.
35.33
Piezoelectricity
See 32.1.2:
Pressure,
piezoelectricity (Electronics)
Piezoelectricity is the phenomenon in certain crystals anc ceramics
when application of mechanical stress causes electric charge with the
voltage proportional to the stress. It occurs in crystals of cane
sugar, quartz, Rochelle salt (potassium sodium
tartrate-4-water), topaz, tourmaline,
Demonstrate piezoelectricity by dusting the cooling
or
warming crystal with a dust of red lead and sulfur that has passed
through a
silk or nylon screen. A simple bellows can be made from a plastic nasal
spray or
deodorant bottle in that the aperture has been enlarged to allow a
sizeable
spray to be emitted. Place in the bottle a mixture of about 2 parts red
lead to
1 part sulfur. Put a small piece of silk or nylon stocking over the
mouth of the
bottle. Tighten this with a rubber band. The dust particles receive
electric
charges as they pass through the screen formed by the stocking. They
settle on
the end of the crystal that attracts them. The red lead gets a
positive charge
and goes to the negative end of the crystal. The sulfur gets a negative
charge
and settles on the positive end of the crystal.
35.33.1
Pyroelectricity, ferroelectricity
Some piezoelectric crystals, e.g. tourmaline, are also
pyroelectric, i.e. electricity is generated when the crystal is heated
and opposite charges gather on the opposite faces of asymmetric
crystals. So the heated crystals generate electricity.
Other substances are pyroelectric but are termed ferroelectric because
the the direction of charge changes if external electric fields are
applied, e.g. gallium nitrate (GaN) and lithium tantalate (LiTaO3).
35.33.2 Dimorphism,
aragonite
Aragonite and calcite are the two polymorphs of calcium carbonate, CaCO3.
Aragonite has orthorhombic crystals with repeated twinning in
"pseudo-hexagonal form", i.e. the angle is 63o48'
instead of 65o, to form flower-like aggregates,
"flos-ferri", flowers of iron. The carbonates of cadmium, iron,
magnesium, manganese, zinc, and the double carbonates of calcium
and magnesium are isomorphous with calcite. However, the carbonates of
barium, lead, and strontium are isomorphous with aragonite. Sodium
nitrate crystals are isomorphous with calcite but potassium nitrate
crystals are isomorphous with aragonite at room temperature but changes
to the calcite form at high temperature. Aragonite forms in the shells
of molluscs and freshwater corals. Fossil coral shells, ammolite,
is an iridescent gemstone.
35.34 How
fossils form
A fossil is any evidence of a
form of life that lived some time in the past. Most fossils are found
in layers
of sedimentary rock. Fossils formed by burial are usually
found when
the sedimentary rock containing them is split open. Cover a leaf with
petroleum
jelly and place it on a pane of glass or other smooth surface. Make a
circular
mould about 2 cm deep and place it around the leaf. Hold the mould in
place by
pressing modelling clay around the outside. Now mix up some plaster of
Paris and
pour it over the leaf. When the plaster has hardened, you can remove
the leaf,
and you will have an excellent leaf print. Some fossils were made this
way by
having silt deposited over them, which later hardened into sedimentary
rock.
Repeat this experiment using a greased clam or oyster shell to make the
imprint.
35.34.1 Dendrites,
false fossils
Manganese and iron oxides and hydroxides may form deposits branching
through limestone, dendrites. These branches may be mistaken for
fossils, but they are only chemical in origin.
35.35 Find
fossils
In some localities, fossils may be found
in stone quarries or where there are rock outcrops. Try to find someone
in the
community who knows about fossils and then plan a field trip with the
class to
collect some of them. If there are no fossils in your locality, you may
have to
depend on state or national museums to send you a few. A letter to the
state or
national museum may prove helpful.
35.40.1
Mapping contours, geological structures, erosion
After N.E. Austin The
Australian Science Teachers Journal Vol. 33 No. 1
See diagram
335.9.1
1. Show relief on the map with contour lines. To develop skills in
contour line interpretation by experimental means use landform models
of the
three fundamental surface forms: planar concave and convex. They
exist in five
spatial forms as in figure 1.
A. Make the five forms A to E from flexible
white cardboard. Contour lines are lines joining places of equal
altitude i.e.
for small regions of the Earth's surface the intersection of equally
spaced
horizontal planes with the Earth's surface. For landform modelling you
can
produce such planes with plastic sheets held vertically or 35 mm
slides e.g. S1
and S2 as in figure 2 with the back light or projector projecting
horizontally.
2. Project S1 horizontally on the five
spatial surface forms A to E
then look
vertically down on to the models. You can vary the inclination of each
of the
models A to E from 10o to 90o. Also you can vary
the
concavity or convexity of models B to E if the white cardboard used to
make them
is flexible. The figure 3 shows what you see when looking vertically
down using
S1. In A B and C contour lines are straight lines on all surfaces
that can be
generated by the translation in space of any horizontal straight line
moving
parallel to itself. In D and E contour lines are curved on all
surfaces that can be generated by translation in space of any straight
line inclined
to the
horizontal moving parallel to itself along a curved path.
3. Figure 4 shows what you see when
looking vertically down using S2. In A B
and C contour line spacing decreases with increasing steepness of the
landform.
Figure 3D and figure 4D show that if contour lines are concave when
viewed from
the direction of low altitude to high latitude the landform is
concave. Figure
3E and figure 4E show if contour fines are convex when viewed from the
direction
of low altitude to high altitude the landform is convex. The same
spatial forms
described above can be used in developing concepts of outcrop in
relation to
geological structures as in figure 35. In these experiments the
projector can be
positioned to take into account varying orientations of geological
structure and
landform.
35.40.2
Isostasy models
After N.E. Austin The Australian
Science Teachers Journal Vol. 32 No. 3
See diagram
335.9.2
1. Make a tank from acrylic or glass sheet. Keep a clearance of 2
mm at the sides and ends of the tank. Make a water inlet at the base of
the tank
to helps filling and draining with a garden hose. Adjust the mass of
wooden
blocks of various lengths by inserting rolled lead sheet and float the
blocks in
a water tank e.g. use 50 mm x 25 mm redwood timber density 0.6 g
cm-3. Cut a basic length 38 cm long. Calculate its true
volume, V.
Multiply this volume by 0.7 to find its adjusted mass M. Drill a 9 mm
hole
centrally upward through the base. Cut a short length of 9 mm dowel to
act as a
sealing plug in this hole. Put rolled sheet lead in the hole to adjust
the
relative density to 0.7. Put on a balance the 38 cm block the prepared
plug
and the rolled lead sheet necessary to bring the total mass up to the
calculated
adjusted mass. Use lead shot in the final mass adjustment. Insert the
prepared
lead in the prepared hole after rolling the sheet to fit. Glue in the
prepared
plug.
2. To make an Airy's model cut 15 lengths
of the redwood timber
between 38 and
19 cm long. Drill and prepare plugs as above. Calculate the adjusted
mass M for
each length as above or use formula: Ma LM / 38 where L length in
centimetres M mass of density adjusted 38 cm basic length in grams.
Adjust
all lengths as before. Check all lengths in a one litre measuring
cylinder of
water by flotation.
3. To make a Pratt's model cut the same
number of lengths are cut as
before.
The shortest possible length now is 28 cm so cut the lengths cut are
between 38
and 28 cm. Prepare all drilled lengths and plugs. Use a longer drill
hole for
short lengths because you must adjust all lengths to the same mass M
of the
basic adjusted 38 cm. The shorter the length the greater the relative
density.
4. To make an erosion block cut a 38 cm
length of 100 mm x 50 mm
timber.
Cut away the top corners of this block using a fine saw. Join the off
cuts back
together using an aluminium plate. Replace this block in its original
position
and drill the plate and whole block for a suitable locating pin. Adjust
this
whole block to a relative density of 0.7 as above. Introduce the larger
erosion
block into the Airy model. Remove the four shortest blocks from this
set and
introduce this block near the centre of the array. Remove the cut away
upper
corner block to simulate erosion and observe the isostatic
readjustment. Replace
this cutaway to simulating snowfalls and observe the isostatic
readjustment.
35.22.4
Clay
consists of decomposed weathered rock,
usually
granite, or others that contain feldspar. Pure clay is a dazzling
white, has a
soft, oily feel and is easily broken. Damp clay is sticky and has a
special
smell. It absorbs water and becomes plastic when wet. Clays do not
split along
bedding planes, i.e. a surface parallel to the original deposition
surface. It
has particle size less than 0.004 mm diameter, 1/256 mm. Clay minerals
can take
up or lose water according to temperature and amount of available
water. Modelling clay, Plasticine, plastilina, is manufactured for use
mainly by children. Bole is a non-plastic clay that contains iron oxide
giving it a yellow to brown colour.
35.22.4.01 Chemical
weathering reactions
Some weathering reactions need water and some need acid to produce
aluminium silicate clays.. Both types of reactions reduces the size of
mineral particles with the exclusion of sliicic acid.
1. Al2(OH)4(Si2O5) + 5H2O
--> 2Al(OH)3 + 2H4SiO4
kaolinite + water --> gibbsite + silicic acid
2. Al2(OH)4(Si2O5) + 6H+
--> 2Al3+ + 2H4SiO4 + H2O
kaolinite + acid --> aluminium ion + silicic acid + water
3. 2KAl2(OH)2(Si3AlO10) + 3H2O
+ 2H+ --> 3Al2(OH)4(Si2O5)
+2K+
muscovite +water + acid --> kaolinite + potassium ion
4. 2KAlSi3O8 + 9H2O + 2H+-->
Al2(OH)4(Si2O5) + 2K+
+ 4H4SiO4
feldspar + water + acid --> kaolinite + potassium ion + silicic acid
5. Al2(OH)2(Si2O5)2
+ 5H2O --> Al2(OH)4(Si2O5)
+ 2H4SiO4
montmorillonite + water --> kaolinite + silicic acid
6. 2KAlSi3O8 + 12H2O + 2H+
--> KAl2(OH)2(Si3AlO10)
+2K+ + 6H4SiO4
feldspar = water + acid --> muscovite + potassium ion + silicic acid
7. 2KAl2(OH)2(Si3AlO10) + 9H2O
+2H+ --> 3Al(OH)3 + K+ + 3H4SiO4
muscovite + water + acid --> gibbsite +silicic acid
8. 2Al(OH)3 + 3H+ --> Al3+ + H2O
gibbsite + acid --> aluminium ion + water
Clay
minerals include the following groups:
35.22.4.1 Illite,
KAl4(Si, Al)8O20(OH)4, KAl4(Si,Al)8O18.2H2O
is
the
most
common clay mineral.
35.22.4.2 Kaolinite,
Al2(OH)4 (Si2O5),
Al4Si4O10(OH)8, weathering
product of feldspars, known as
white
clay, pipe clay, ball clay, Cornish clay and China clay, (also dickite
and nacrite minerals) is used in the
manufacture of fine porcelain that is almost
pure kaolin. The cheaper grades are made with the addition of feldspar.
It is a soft white powder, insoluble in water, dilute acids or
alkalies. Kaolin clay contains mainly kaolinite and some illite
35.22.4.3
Montmorillonite, smectite (Na,
Ca)(Al, Mg)6(Si4O10)3(OH)6.nH2O,
forms
from volcanic ash and occurs in Fuller's
earth and bentonite. These minerals easily
exchange cations and take
up and lose water, so are called "swelling clays".
35.22.4.4 Bentonite is often
included in products to improve the water holding capacity of soil
because of its water absorbing and water retaining properties.
Bentonite is a clay-like mineral consisting of hydrous aluminum
silicate. It is of very fine grain size, can absorb large amounts of
water and has a very high plasticity. It is used in the manufacture of
colloidal solutions.
35.22.4.5 Fuller's
earth, a hydrated compound of silica and alumina, has a grey
brown colour
and a smooth greasy feel. It is used as a filtering material to remove
the basic
colours of vegetable, animal and mineral oils and as a pigment filler.
35.22.4.6 Vermiculite (Mg, Fe, Al)3(Al,
Si)4O10(OH)2.4H2O, is used
as a potting medium in horticulture and as heat insulating material.
The name
comes from the
property
of forming long worm-like structures when heated.
Collect clay samples in your
region. Make clay pots and leave them in the sun to dry.
Note which clay makes the best pot. Examine samples of potting mix for
the
presence
of vermiculite.
35.22.4.7 Halloysite,
Al4Si4(OH)8O10.4H2O,
clay mineral