Talc is one of the less common semi precious stones and number
one on Mohs scale.
The Mohs scale of mineral hardness characterizes the scratch
resistance of various minerals through the ability of a harder
material to scratch a softer material. It was created in 1812 by
the German mineralogist Friedrich Mohs and is one of several
definitions of hardness in materials science.
On the Mohs scale, a pencil "lead" (graphite) has a hardness of 1;
a fingernail, 2.2-2.5; a copper penny, 3.2-3.5; a pocketknife 5.1;
a knife blade, 5.5; window glass plate, 5.5; and a steel file, 6.5.
A streak plate (unglazed porcelain) has a hardness of 7.0. Using
these ordinary materials of known hardness can be a simple way to
approximate the position of a mineral on the scale.
1.Talc (Mg3Si4O10(OH)2)
2.Gypsum (CaSO4·2H2O)
3.Calcite (CaCO3)
4.Fluorite (CaF2)
5.Apatite (Ca5(PO4)3(OH-,Cl-,F-))
6.Orthoclase Feldspar (KAlSi3O8)
7.Quartz (SiO2)
8.Topaz (Al2SiO4(OH-,F-)2)
9.Corundum (Al2O3)
10.Diamond (C)
Talc is a mineral composed of hydrated magnesium silicate with the
chemical formula H2Mg3(SiO3)4 or Mg3Si4O10(OH)2. In loose form, it
is the widely used substance known as talcum powder. It occurs as
foliated to fibrous masses, its monoclinic crystals being so rare
as to be almost unknown. It has a perfect basal cleavage, and the
folia are non-elastic, although slightly flexible.
It is very soft and sectile (can be cut with a knife); with a Mohs
hardness of 1, it can be easily scratched by a fingernail. It has a
specific gravity of 2.5–2.8, a clear or dusty luster, and is
translucent to opaque. Talc is not soluble in water, but it is
slightly soluble in dilute mineral acids. Its colour ranges from
white to grey or green and it has a distinctly greasy feel. Its
streak is white.
Soapstone is a metamorphic rock composed predominantly of
talc.
Formation
Talc is a metamorphic mineral resulting from the metamorphism of
magnesian minerals such as serpentine, pyroxene, amphibole,
olivine, in the presence of carbon dioxide and water. This is known
as talc carbonation or steatization and produces a suite of rocks
known as talc carbonates.
Talc is primarily formed via hydration and carbonation of
serpentine, via the following reaction;
1.serpentine + carbon dioxide ? talc + magnesite + water
Mg3Si2O5(OH)4 + 3CO2 ? Mg3Si4O10(OH)2 + 3 MgCO3 + 3 H2O
Talc can also be formed via a reaction between dolomite and silica,
which is typical of skarnification of dolomites via silica-flooding
in contact metamorphic aureoles;
2.dolomite + silica + water ? talc + calcite + carbon dioxide
CaMg(CO3)2 + 4 SiO2 + H2O ? Mg3Si4O10(OH)2 + 3 CaCO3 + 3 CO2
Talc can also be formed from magnesian chlorite and quartz in
blueschist and eclogite metamorphism via the following metamorphic
reaction:
3.chlorite + quartz ? kyanite + talc + water
In this reaction, the ratio of talc and kyanite is dependent on
aluminium content with more aluminous rocks favoring production of
kyanite. This is typically associated with high-pressure,
low-temperature minerals such as phengite, garnet, glaucophane
within the lower blueschist facies. Such rocks are typically white,
friable, and fibrous, and are known as whiteschist.
Talc is a tri-octahedral layered mineral; its structure is similar
to that of pyrophyllite, but with magnesium in the octahedral sites
of the composite layers.
Crystal structure of Talc.
Talc is used in many industries such as paper making, plastic,
paint and coatings, rubber, food, electric cable, pharmaceuticals,
cosmetics, ceramics, etc. A coarse grayish-green high-talc rock is
soapstone or steatite and has been used for stoves, sinks,
electrical switchboards, etc. It is often used for surfaces of lab
counter tops and electrical switchboards because of its resistance
to heat, electricity and acids. Talc finds use as a cosmetic
(talcum powder), as a lubricant, and as a filler in paper
manufacture. Talc is used in baby powder, an astringent powder used
for preventing rashes on the area covered by a diaper (see diaper
rash). It is also often used in basketball to keep a player's hands
dry. Most tailor's chalk is talc, as is the chalk often used for
welding or metalworking.
Must involve visitors undertaking some educational task that
relates to the Earth science at the site. This could involve
measuring or estimating the size of some feature or aspect of
the
site, collecting and recording data (such as time of a tidal
bore),
or sending an e-mail to the cache owner with the answer to
Earth
science related questions they obtained by reading an
information
display. While photographs may be requested, they do not take
the
place of other logging requirements. Taking a photograph alone
or
asking people to do internet research does NOT meet these
logging
guidelines. Requests for specific content in the photograph
(must
include the visitor's face, for example) will be considered
an
additional logging requirement and must be optional. Cache
owners
may not delete the cache seeker's log based solely on
optional
tasks.
To Log your earthcache find, please do the
following (answers by email please, not in your log): You can go
ahead and log your find then email the answers to me within 7 days,
if not your log will be deleted.
Click on hennieventer top of the page
and
Send Message
Questions:
1. How does the rock feel when you slip your fingers over
it?
2.How does the Talc look comparing the other rocks around
it?
3. Examine the rocks on the left and on the right side of the
Talc. Talc is primarily formed via hydration and carbonation of
serpentine. There are 3 different ways mentioned in the listing.
How would you say to your understanding doing this EarthCache was
the talc formed at this location?
4.Looking at the Talc formation at coordinates, How high and
wide is the Talc visible?
5. Take a photo with your GPS at Coordinants. Place photo
with
your log.
I have used sources available to me by visiting the site, the
Polokwane library, internet, research, and asking questions to
get
information for this earth cache.
Acknowledgements:
Geological Journeys (A Travelers Guide to South Africa’s
rocks and land forms) Nick Norman and Gavin Whitfield.
Wikipedia
I am not a geologist. Since Earth caching is educational I
also
leant a lot through the research. Never known my finger nail was
twise as hard than a rock. Hope you will enjoy this
Earthcache.
