Mei 2012 - Jejaring Kimia


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Mei 30, 2012

DOLOMITE [ Carbonates : Dolomite ]

Mei 30, 2012 2
CaMg(CO3)2, calcium magnesium Carbonate
In some cements, as a source of magnesium and as mineral specimens

Dolomite, which is named for the French mineralogist Deodat de Dolomieu, is a common sedimentary rock-forming mineral that can be found in massive beds several hundred feet thick. They are found all over the world and are quite common in sedimentary rock sequences. These rocks are called appropriately enough dolomite or dolomitic limestone. Disputes have arisen as to how these dolomite beds formed and the debate has been called the "Dolomite Problem". Dolomite at present time, does not form on the surface of the earth; yet massive layers of dolomite can be found in ancient rocks. That is quite a problem for sedimentologists who see sandstones, shales and limestones formed t

oday almost before their eyes. Why no dolomite? Well there are no good simple answers, but it appears that dolomite rock is one of the few sedimentary rocks that undergoes a significant mineralogical change after it is deposited. They are originally deposited as calcite/aragonite rich limestones, but during a process call diagenesis the calcite and/or aragonite is altered to dolomite. The process is not metamorphism, but something just short of that. Magnesium rich ground waters that have a significant amount of salinity are probably crucial and warm, tropical near ocean environments are probably the best source of dolomite formation.

Dolomite in addition to the sedimentary beds is also found in metamorphic marbles, hydrothermal veins and replacement deposits. Except in its pink, curved crystal habit dolomite is hard to distinguish from its second cousin, calcite. But calcite is far more common and effervesces easily when acid is applied to it. But this is not the case with dolomite which only weakly bubbles with acid and only when the acid is warm or the dolomite is powdered. Dolomite is also slightly harder, denser and never forms scalenohedrons (calcite’s most typical habit).

Dolomite differs from calcite, CaCO3, in the addition of magnesium ions to make the formula, CaMg(CO3)2. The magnesium ions are not the same size as calcium and the two ions seem incompatible in the same layer. In calcite the structure is composed of alternating layers of carbonate ions, CO3, and calcium ions. In dolomite, the magnesiums occupy one layer by themselves followed by a carbonate layer which is followed by an exclusively calcite layer and so forth. Why the alternating layers? It is probably the significant size difference between calcium and magnesium and it is more stable to group the differing sized ions into same sized layers. Other carbonate minerals that have this alternating layered structure belong to the Dolomite Group. Dolomite is the principle member of the Dolomite Group of minerals which includes ankerite, the only other somewhat common member.

Dolomite forms rhombohedrons as its typical crystal habit. But for some reason, possibly twinning, some crystals curve into saddle-shaped crystals. These crystals represent a unique crystal habit that is well known as classical dolomite. Not all crystals of dolomite are curved and some impressive specimens show well formed, sharp rhombohedrons. The luster of dolomite is unique as well and is probably the best illustration of a pearly luster. The pearl-like effect is best seen on the curved crystals as a sheen of light can sweep across the curved surface. Dolomite can be several different colours, but colourless and white are very common. However it is dolomite’s pink colour that sets another unique characteristic for dolomite. Crystals of dolomite are well known for their typical beautiful pink colour, pearly luster and unusual crystal habit and it is these clusters that make very attractive specimens.

Physical Characteristics

Colour : often pink or pinkish and can be colourless, white, yellow, gray or even brown or black when iron is present in the crystal
Luster : pearly to vitreous to dull
Transparency : crystals are transparent to translucent
Crystal System : trigonal; bar 3
Crystal Habits : include saddle shaped rhombohedral twins and simple rhombs some with slightly curved faces, also prismatic, massive, granular and rock forming. Never found in scalenohedrons
Cleavage : perfect in three directions forming rhombohedrons
Fracture : conchoidal
Hardness : 3.5 - 4
Specific Gravity : 2.86 (average)
Streak : white
Other : Unlike calcite, effervesces weakly with warm acid or when first powdered with cold HCl
Associated Minerals : include calcite, sulfide or minerals, fluorite, barite, quartz and occasionally with gold
Major Occurrences : include many localities throughout the world, but well known from sites in Midwestern quarries of the USA; Ontario, Canada; Switzerland; Pamplona, Spain and in Mexico
Best Indicators : typical pink colour, crystal habit, hardness, slow reaction to acid, density and luster

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Rino Safrizal
Jejaring Kimia Updated at: Mei 30, 2012

Mei 28, 2012

Oxygen [O]

Mei 28, 2012 1

A. Characteristic

An : 8
N : 8
Am : 15.9994 (3) g/mol
Group No : 16
Group Name : Chalcogen
Block : p-block
Period : 2
State : gas at 298 K
Colour : colourless as a gas, liquid is pale blue
Classification : Non-metallic
Boiling Point : 90.20K (-182.95oC)
Melting Point : 54.36K (-218.79oC)
Density : 1.429g/l

B. Discovery Information

Who : Joseph Priestley, Karl Wilhelm Scheele
When : 1774
Where : England/Sweden

C. Name Origin

Greek : oxus (acid) and gennan (generate). "Oxygen" in different languages.

D. Sources

Obtained primarily from by liquification and then fractional distillation of the air. World wide production is around 100 million tons.

E. Abundance

Universe : 10000 ppm
Sun : 9000 ppm
Atmosphere : 2.095 x 105 ppm
Earth’s Crust : 4.74 x 105 ppm
Human : 6.1 x 108 ppb by weight; 2.4 x 108 ppb by atoms

F. Uses

Used in steel making, production of methanol (CH3OH), welding, water purification, cement and rocket propulsion. It is also required for supporting life and combustion. Oxygen is a major component of air, produced by plants during photosynthesis, and is necessary for aerobic respiration in animals.

G. Notes

Liquid and solid O2 are both a light blue colour. Ozone (O3) is a deeper blue colour. Oxygen is the second most common element on Earth, composing around 46% of the mass of Earth’s crust and 28% of the mass of Earth as a whole, and is the third most common element in the universe. Forms almost 21% of atmosphere.

H. Hazards

Certain derivatives of oxygen, such as ozone (O3), singlet oxygen, hydrogen peroxide (H2O2), hydroxyl radicals and superoxide (O2-), are highly toxic. Highly concentrated sources of oxygen promote rapid combustion and therefore are fire and explosion hazards in the presence of fuels.

I. Allotropes of Oxygen

1. Dioxygen [ O2 ]
A gas at room temperature. Dioxygen exists as a diradical (contains two unpaired electrons) and is the only allotrope of any element with unpaired electrons.
2. Ozone [ O3 ]
Ozone was discovered by Christian Friedrich Schonbein in 1840, who named it after the Greek word for smell (ozein), from the peculiar odour in lightning storms. The actual odour from a lightning strike is from electron that have been freed during the rapid chemical changes that take place, and not ozone.
Ozone is a pale blue gas at standard temperature and pressure. It forms a dark blue liquid below -112oC and a dark blue solid below -193oC. Ozone is a powerful oxidizing agent. It is also unstable, decaying to ordinary oxygen (O2). It is present in low concentrations throughout the Earth’s atmosphere: ground level ozone is an air pollutant with harmful effects on lung function and in the upper atmosphere it prevents damaging ultraviolet light from reaching the Earth’s surface.
It is also formed from O2 by electrical discharges such as lightning, and by action of high energy electromagnetic radiation.
Some kinds of electrical equipment generate significant levels of ozone. This is especially true of devices using high voltages, such as television sets, laser printers, and photocopiers. Ozone is widely used in disenfecting water, killing bacteria and cleaning and bleaching fabrics.
3. Tetraoxygen [ O4 ]
The most recently discovered allotrope of oxygen. It is a deep red solid and is created by pressurizing O2 to the order of 20 GPa. Its properties are being studied for use in rocket fuels and similar applications, as it is a much more powerful oxidizer than either O2 or O3.

J. Reactions of Oxygen

Oxygen does not react with acids or bases under normal conditions.
1. Reactions with water
Oxygen will not react with water.
2. Reactions with air
Oxygen gas does not react with itself or nitrogen under normal conditions. However the effect of ultraviolet light upon oxygen gas is to form the blue gas ozone, O3.
3. Reactions with halogens
Irradiation of a low pressure mixture of oxygen and fluorine gases will produce dioxygen difluoride.
O2(g) + F2(g) --> F2O2(g)

K. Isotopes of Oxygen

16O [8 neutrons]
Abundance: 99.762
Stable with 8 neutrons
17O [9 neutrons]
Abundance: 0.038%
Stable with 9 neutrons
18O [10 neutrons]
Abundance: 0.205%
Stable with 10 neutrons
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Rino Safrizal
Jejaring Kimia Updated at: Mei 28, 2012

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