Composition:
25.37 %
Mg (42.06 % MgO), 14.57 % Fe (18.75 % FeO), 18.32 % Si
(39.19 % SiO2
), 41.74 % O
Physical properties:
Cleavage: [001]
Good, [010] Distinct
Color: Yellowish green,
Olive green, Greenish black, Reddish
brown.
Density: 3.27 - 3.37,
Average = 3.32
Diaphaniety:
Transparent to translucent
Fracture: Brittle -
Conchoidal - Very brittle fracture producing small, conchoidal
fragments.
Habit: Massive -
Granular - Common texture observed in granite and other igneous
rock.
Hardness: 6.5-7 -
Pyrite-Quartz
Luminescence:
Non-fluorescent.
Luster: Vitreous
(Glassy)
Streak:
white
The olivine group of
minerals is made up of a magnesium
(Forsterite -
Mg2SiO4) and a iron
(Fayalite -
Fe2SiO4) end member. It is one of the
minerals that first year geology students come into contact
with as it is one of the minerals that crystallise first in
Bowen’s
Reaction
series.
Minerals in the olivine group crystallize in
the
orthorhombic system
(
space group Pbnm) with
isolated silicate tetrahedra, meaning that olivine is a
nesosilicate.
Transparent olivine is sometimes used as a
gemstone called
peridot (a type of Forsterite),
the French word for olivine. It is also called chrysolite, from the
Greek words for gold and stone. Some of the finest gem-quality
olivine has been obtained from a body of mantle rocks on
Zabargad island in the Red
Sea.
Olivine/peridot occurs in both
mafic
and
ultramafic igneous rocks and as a
primary mineral in certain metamorphic rocks. Mg-rich olivine
crystallizes from magma that is rich in magnesium and low in
silica. That magma crystallizes to mafic rocks such as
gabbro
and
basalt
. Ultramafic rocks such as
peridotite and
dunite
can be residues left after extraction of magmas, and
typically they are more enriched in olivine after extraction of
partial melts. Olivine and high pressure structural variants
constitute over 50% of the Earth's upper mantle, and olivine is one
of the Earth's most common minerals by volume. The
metamorphism of impure
dolomite or other sedimentary
rocks with high magnesium and low silica content also produces
Mg-rich olivine, or forsterite.
Uses:
A worldwide search is on for cheap processes to sequester
CO2 by mineral reactions. Removal by reactions with
olivine is an attractive option, because it is widely available and
reacts easily with the (acid) CO2 from the atmosphere.
When olivine is crushed, it weathers completely within a few years,
depending on the grain size. All the CO2 that is
produced by burning 1 litre of oil can be sequestered by less than
1 litre of olivine. The reaction is exothermic but slow. In order
to recover the heat produced by the reaction to produce
electricity, a large volume of olivine must be thermally well
isolated. Then it can produce power, while at the same time
removing CO2. The end-products of the reaction are
silicon dioxide, magnesium carbonate and small amounts of iron
oxide.
The aluminium foundry industry uses olivine sand
to cast objects in aluminium. Olivine sand requires less water than
silicon based sand while providing the necessary strength to hold
the mould together during handling and pouring of the metal. Less
water means less gas (steam) to vent from the mould as metal is
poured into the mould.
Green
sand
The term “green
sand” is used to describe a beach made up of olivine crystals.
One such beach, Papakolea Beach (also known as Green Sand
Beach, Mahana Beach and, erroneously, Pu'u Mahana) is a green
sand beach located at
South Point, in the
Ka'u district of the island of
Hawai'i . It is one of only two green sand beaches in the world,
the other being in
Guam and as the name implies, the
beach gets distinctive colouring from olivine crystals found in a
nearby cinder cone. Note that the green sand is more
abundant at the bottom of the cinder cone.
Papakolea Beach is located
in a bay circled by
Pu'
u Mahana, a
cinder cone formed over 49,000
years ago and associated with the southwest rift of
Mauna Loa. Since its last
eruption, the cinder cone has partially collapsed and been
partially eroded by the ocean.
The cinder cone is olivine-rich.
Olivine is locally known as "Hawaiian Diamond"
and is notably found in
O
'ahu's
famous
Diamond Headlandmark. The source
of the green coloration of the beach sands is due to the olivine
crystals which are winnowed from the eroding headland by the action
of the sea. Olivine, being denser and tougher than the ash
fragments, glass and black
pyroxene of the rest of the rocks
and lava flows, tends to accumulate on the beach whereas the
usual
volcanic sand is swept out to sea.
Although these crystals are eventually washed away as well, the
constant erosion of the cinder cone ensures a steady supply of sand
for the foreseeable future—eventually, however, the supply will run
out and the beach will look like any other.
The formation of the cinder cone is currently a
point of contention, with some arguing that lava flowing into the
sea was suddenly cooled, forming an edifice on the coastline and
others noting that the cone was most likely too far away from the
ocean at the time of formation to make such an event possible.
Regardless of how it was formed according to the United States
Geological Survey, the last lava flow in the area ended over 10,000
years ago, making the area one of the more stable features in the
geologically turbulent
Ka
?u region. As such, the geologic history of
the site can be seen in the rock surrounding the beach and bay,
which are not subject to erosion and thus display geologic layers
formed by previous eruptions, lava flows, and other volcanic
events. In addition, the current patterns of erosion can be seen
first-hand, as only the portions at the bottom of the cinder cone
collapse have been subject to the waves and turned into green sand;
the remaining portions appear gray.
Validating the
EarthCache find
You already know that you will find the beach
full of olivine grains so I’ll refrain from asking you about that.
However, a closer inspection of the olivine grains will reveal
something about their size. Therefore, in order to claim your
found of this EarthCache
please tell me:
1- What is the size of the grains of olivine you
see?
Are they:
A - Less than 1 mm in
diameter?,
B - Between 1 and 2 mm
in diameter?
C - > 2 mm in
diameter?
D - > 5 mm in
diameter?
(I know that weathering is an active process here
and the grain size may vary but I have several sand samples and
your answer will be judged fairly) AND
2-
What is the colour of the other grains that you find mixed in with
the olivine grains?
If you have a nice pic
of you on the beach or with the beach as a backdrop, please include
it with your log.
Getting
there
Once on the Big Island (
Hawaii or Kona), take Highway 11 (the main highway) south to Ka
Lae. There will be a sign pointing you further south to South
Point, which incidentally is the southernmost point of the U.S.A.
Eventually you will come to a parking lot and a little further off
you have a boat ramp. Facing the boat ramp, look left and you will
see a “Government Property” sign with a trail. Follow that well
marked trail for some 9.7 km (~6 miles) to the EarthCache and
Papakolea Beach . Take sun screen and plenty of water with you and
don’t go with your beach sandals – wear good, comfortable hiking
shoes.
Refs:
http://webmineral.com/data/Olivine.shtml
http://en.wikipedia.org/wiki/Olivine
http://en.wikipedia.org/wiki/Papakolea_Beach
http://en.wikipedia.org/wiki/Bowen%27s_reaction_series
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