Precious and Semi-precious Gemstone (gif-format)
Showing posts with label School. Show all posts
Showing posts with label School. Show all posts
Tuesday, July 26, 2016
Thursday, October 31, 2013
Conditions necessary for the formation of hydrothermal mineral deposits
Conditions necessary for the formation of hydrothermal mineral deposits include:
(1) Presence of hot water to dissolve and transport minerals,
(2) Presence of interconnected openings in the rock to allow the solutions to move,
(3) Availability of sites for the deposits, and
(4) Chemical reaction that will result in deposition.
(1) Presence of hot water to dissolve and transport minerals,
(2) Presence of interconnected openings in the rock to allow the solutions to move,
(3) Availability of sites for the deposits, and
(4) Chemical reaction that will result in deposition.
What are the five most common rock-forming mineral groups?
What are the five most common rock-forming mineral groups?
1. Silicates
2. Oxides
3. Sulphides
4. Carbonates
5. Sulphate
6. Halides
7. Phosphates e.t.c
1. Silicates
2. Oxides
3. Sulphides
4. Carbonates
5. Sulphate
6. Halides
7. Phosphates e.t.c
What factors controls the shape of a well formed crystal?
The following are major factors that influence the shape of a well formed crystal:
- Symmetry properties of the underlying lattice
- Rates of growth; - the overall shape of the crystal is determined by the relative rates of growth of the various faces. Thus the slower the growth rate, the larger the face.
- The conditions under which a crystal is grown can affect its habit. The Temperature, degree of super-saturation, nature of the solvent all have their effects, and these may affect the growth of different faces in different ways.
- The presence of impurities in the solution can radically alter the habit of a crystal, as seen in the following table for the growth of sodium chloride:
cubes dendrites octahedra large crystals needles
These effects presumably come about because these substances preferentially adsorb to certain faces, impeding their growth.
List down at least five examples of sulphide minerals.
List down at least five examples of sulphide minerals.
Sulfide ores/minerals
i. argentite (silver sulfide),
ii. cinnabar (mercury),
iii. galena (lead sulfide),
iv. molybdenite (molybdenum sulfide),
v. pentlandite (nickelsulfide]),
vi. realgar (arsenic sulfide),
vii. stibnite (antimony),
viii. sphalerite (zinc sulfide),
ix. pyrite (iron disulfide), and
x. chalcopyrite (iron-copper sulfide).
Sulfide ores/minerals
i. argentite (silver sulfide),
ii. cinnabar (mercury),
iii. galena (lead sulfide),
iv. molybdenite (molybdenum sulfide),
v. pentlandite (nickelsulfide]),
vi. realgar (arsenic sulfide),
vii. stibnite (antimony),
viii. sphalerite (zinc sulfide),
ix. pyrite (iron disulfide), and
x. chalcopyrite (iron-copper sulfide).
Five types of fossils
Five types of fossils are:
1. Mold (imprint) fossils
When a leaf, feather, bone or even a body of an organism leaves an imprint on sediment, which hardens and becomes rock
2. Cast fossils
When minerals fill in the hollows of an animal track, a mollusk shell, or another part of an organism
3. Fossil fuels
Fuels formed by the remains of dead plants and animals
4. Actual remains
It is the body of an organism, with all the parts intact, usually preserved in ice, amber, or tar.
5. Petrified wood
When minerals replace wood or stone to create either petrified wood or a mineralized fossil
1. Mold (imprint) fossils
When a leaf, feather, bone or even a body of an organism leaves an imprint on sediment, which hardens and becomes rock
2. Cast fossils
When minerals fill in the hollows of an animal track, a mollusk shell, or another part of an organism
3. Fossil fuels
Fuels formed by the remains of dead plants and animals
4. Actual remains
It is the body of an organism, with all the parts intact, usually preserved in ice, amber, or tar.
5. Petrified wood
When minerals replace wood or stone to create either petrified wood or a mineralized fossil
List the five stages in the formation of sedimentary rocks
List the five stages in the formation of sedimentary rocks
i. Weathering
ii. Erosion
iii. Transportation
iv. Deposition
v. Lithification (compaction & cementation)
i. Weathering
ii. Erosion
iii. Transportation
iv. Deposition
v. Lithification (compaction & cementation)
What is magmatic differentiation?
What is magmatic differentiation?
This is a complex process whereby a single melt can produce a wide variety of different rocks.
Various processes have been suggested to explain the variation of magma compositions observed within small regions. Among the processes are:
1. Distinct melting events from distinct sources.
2. Various degrees of partial melting from the same source.
3. Crystal fractionation.
4. Mixing of 2 or more magmas.
5. Assimilation/contamination of magmas by crustal rocks.
6. Liquid Immiscibility.
This is a complex process whereby a single melt can produce a wide variety of different rocks.
Various processes have been suggested to explain the variation of magma compositions observed within small regions. Among the processes are:
1. Distinct melting events from distinct sources.
2. Various degrees of partial melting from the same source.
3. Crystal fractionation.
4. Mixing of 2 or more magmas.
5. Assimilation/contamination of magmas by crustal rocks.
6. Liquid Immiscibility.
Why sandstone and limestone do not show foliation when metamorphosed?
Why sandstone and limestone do not show foliation when metamorphosed?
Sandstone and limestone rocks are usually uniform in composition. Also rock made up of all one mineral (e.g. quartz = Quartzite, calcite = Marble; exception = Hornfels) so the minerals do not segregate into layers.
Foliated textures result when the new metamorphic minerals (many of which are platy micas such as Biotite and Muscovite) line up producing a distinct layering in the rock. The layering produces three distinctly different looking rocks; those with slaty cleavage (e.g. Slate), schistosity (e.g. Schist), and mineral banding (or Gneiss Texture). Minerals (e.g., quartz sandstone or limestone), produces rocks that are characterized by fine or coarse interlocking crystals that do not display foliation.
Sandstone and limestone rocks are usually uniform in composition. Also rock made up of all one mineral (e.g. quartz = Quartzite, calcite = Marble; exception = Hornfels) so the minerals do not segregate into layers.
Foliated textures result when the new metamorphic minerals (many of which are platy micas such as Biotite and Muscovite) line up producing a distinct layering in the rock. The layering produces three distinctly different looking rocks; those with slaty cleavage (e.g. Slate), schistosity (e.g. Schist), and mineral banding (or Gneiss Texture). Minerals (e.g., quartz sandstone or limestone), produces rocks that are characterized by fine or coarse interlocking crystals that do not display foliation.
What is sedimentary Facies?
What is sedimentary Facies?
This is the characteristics of a rock or series of rocks reflecting their appearance, composition, and conditions of formation or characteristics of stratified sedimentary body/rock distinguished from others by its appearance, composition and conditions of formation.
This is the characteristics of a rock or series of rocks reflecting their appearance, composition, and conditions of formation or characteristics of stratified sedimentary body/rock distinguished from others by its appearance, composition and conditions of formation.
What is a porphyritic rock and how is formed?
What is a porphyritic rock and how is formed?
Rocks having large crystals in a fine groundmass of minerals/Containing relatively large isolated crystals in a mass of fine texture. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.
Rocks having large crystals in a fine groundmass of minerals/Containing relatively large isolated crystals in a mass of fine texture. Porphyritic textures develop when conditions during cooling of a magma change relatively quickly. The earlier formed minerals will have formed slowly and remain as large crystals, whereas, sudden cooling causes the rapid crystallization of the remainder of the melt into a fine grained (aphanitic) matrix. The result is an aphanitic rock with some larger crystals (phenocrysts) imbedded within its matrix. Porphyritic texture also occurs when magma crystallizes below a volcano but is erupted before completing crystallization thus forcing the remaining lava to crystallize more rapidly with much smaller crystals.
SUPERGENE ENRICHMENT AND REACTION EQUATIONS
GOSSAN FORMATION AND SUPERGENE ENRICHMENT
Reaction Equations
1. 2FeS2 + 15/2O2 + 4H2O Fe2O3 + 4SO42- +8H+
Pyrite
Sulphuric acid enhances the breakdown of accompanying sulphides:
2. 2 CuFeS2 +17/2O2 +2H2O ---> Fe2O3 +2Cu2+ + 4 SO42- + 4H+
Chalcopyrite
Copper is leached downwards and may reprecipitate as Cu sulphates (or carbonates)
above the water table or copper sulphides (esp. chalcocite Cu2S) below the water table
(by replacement of pyrite). This can result in substantial increase in metal content.
Reaction Equations
1. 2FeS2 + 15/2O2 + 4H2O Fe2O3 + 4SO42- +8H+
Pyrite
Sulphuric acid enhances the breakdown of accompanying sulphides:
2. 2 CuFeS2 +17/2O2 +2H2O ---> Fe2O3 +2Cu2+ + 4 SO42- + 4H+
Chalcopyrite
Copper is leached downwards and may reprecipitate as Cu sulphates (or carbonates)
above the water table or copper sulphides (esp. chalcocite Cu2S) below the water table
(by replacement of pyrite). This can result in substantial increase in metal content.
Factors for hydrothermal alteration
Factors for hydro-thermal alteration
a) Temperature;
b) Pressure; - low pressure favors alteration
c) Rock type; - rock composition, and texture control alteration and hence permeability.
d) Permeability;
e) Fluid composition;
f) Duration of activity.
a) Temperature;
b) Pressure; - low pressure favors alteration
c) Rock type; - rock composition, and texture control alteration and hence permeability.
d) Permeability;
e) Fluid composition;
f) Duration of activity.
Conditions for secondary enrichment (prerequisite conditions for the secondary sulfide enrichment)
Conditions for secondary enrichment (prerequisite conditions for the secondary sulphide enrichment)
A warm climate, in so far as it favors chemical action, is favorable to sulphide enrichment. Deposits in high latitudes are not so likely to show extensive migration .of the metals, because low temperature decreases chemical activity, and freezing prevents solution.
Since water is the agent of ore enrichment, abundant rainfall is favorable to the formation of secondary ores.
As a rule, the relief is great in areas of high altitudes, and erosion is consequently more rapid. Moreover, in such areas temperatures are lower and conditions are less favorable to solution.
Deposits located at very high altitudes, where rocks are disintegrated by frost and carried away
un-weathered as talus and boulders, are not so likely to be extensively enriched as are deposits
that lie at lower altitudes.
In so far as strong relief supplies head, it is favorable to deep and rapid circulation of
underground water, and it is likewise favorable to relatively deep enrichment. In base-leveled
(flat land) regions underground circulation is sluggish and the nearly stagnant waters cannot
descend far into the zone of primary sulphides without losing the valuable metals which they
dissolve higher up.
to the lower horizons the metals dissolved near the surface may be scattered.
Permeability is essential for sulphide enrichment. If the primary deposits are not permeable the
solutions that pass downward through the oxidized zones will move laterally along the contact
between oxidized and sulphide ores and ultimately will escape into fractures in the wall rock or
reissue as springs at some level below the points of entry. If they do not encounter a reducing
environment the metals may be scattered.
- The climate, (temperature, rainfalls)
A warm climate, in so far as it favors chemical action, is favorable to sulphide enrichment. Deposits in high latitudes are not so likely to show extensive migration .of the metals, because low temperature decreases chemical activity, and freezing prevents solution.
Since water is the agent of ore enrichment, abundant rainfall is favorable to the formation of secondary ores.
- Altitude
As a rule, the relief is great in areas of high altitudes, and erosion is consequently more rapid. Moreover, in such areas temperatures are lower and conditions are less favorable to solution.
Deposits located at very high altitudes, where rocks are disintegrated by frost and carried away
un-weathered as talus and boulders, are not so likely to be extensively enriched as are deposits
that lie at lower altitudes.
- Relief;
In so far as strong relief supplies head, it is favorable to deep and rapid circulation of
underground water, and it is likewise favorable to relatively deep enrichment. In base-leveled
(flat land) regions underground circulation is sluggish and the nearly stagnant waters cannot
descend far into the zone of primary sulphides without losing the valuable metals which they
dissolve higher up.
- Permeability of the deposits is an essential condition, for if solutions cannot find access
to the lower horizons the metals dissolved near the surface may be scattered.
Permeability is essential for sulphide enrichment. If the primary deposits are not permeable the
solutions that pass downward through the oxidized zones will move laterally along the contact
between oxidized and sulphide ores and ultimately will escape into fractures in the wall rock or
reissue as springs at some level below the points of entry. If they do not encounter a reducing
environment the metals may be scattered.
- The duration of the period of weathering as well as time taken to concentrate the deposit
Mineral resource and mineral reserve
Mineral resource vs mineral reserve Mineral resource of a country (or an area) means the total available economically viable mineral stored in that country (or in the area). Mineral Reserve is the availability of a particular mineral in an occurrence that can be economically exploited. Please remember, reserve can be of proved, estimated or probable category, depending on the degree of the intensity of geological investigation carried out to assess the potentiality of that occurrence. But mineral resources are usually tentative. In other words, Reserve pertains to a particular mineral while the Resource is the sum total of all the economic minerals.
Mineral resources are defined as natural concentrations of minerals or, bodies of rock that are, or may become, of potential economic interest due to their inherent properties.
The classification includes the more important groups of primary ores:
1. Sedimentary beds; mechanical, chemical, organic, etc.
2. Magmatic segregations; consolidated from molten magmas.
1. Contact-metamorphic deposits; deposited in intruded rocks by fluids passing from consolidating intruding rocks.
2. Hydrothermal fluids;
Pegmatite veins; deposited by "aqueo-igneous" magmatic solutions.
Deposits of the deep vein zone; formed at high temperature and under great pressure, generally in and along fissures.
Deposits formed at moderate and shallow depths by ascending hot solutions.
Deposits formed at and near the surface by ascending hot solutions.
Deposits formed at moderate and shallow depths by cold meteoric solutions.
Mineral resources are defined as natural concentrations of minerals or, bodies of rock that are, or may become, of potential economic interest due to their inherent properties.
The classification includes the more important groups of primary ores:
- Syngenetic deposits; contemporaneous with the inclosing rocks:
1. Sedimentary beds; mechanical, chemical, organic, etc.
2. Magmatic segregations; consolidated from molten magmas.
- Epigenetic deposits;, deposited later than the inclosing rocks:
1. Contact-metamorphic deposits; deposited in intruded rocks by fluids passing from consolidating intruding rocks.
2. Hydrothermal fluids;
Pegmatite veins; deposited by "aqueo-igneous" magmatic solutions.
Deposits of the deep vein zone; formed at high temperature and under great pressure, generally in and along fissures.
Deposits formed at moderate and shallow depths by ascending hot solutions.
Deposits formed at and near the surface by ascending hot solutions.
Deposits formed at moderate and shallow depths by cold meteoric solutions.
Tuesday, October 22, 2013
Why Placer Deposits are mined despite of their low Grade?
Answer:
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The name is from the Spanish word placer, meaning "alluvial sand". Placer mining is an important source of gold.
Minerals commercially mined from placer deposits include:
Types of placer deposits include:
Placer materials must be both dense and resistant to weathering processes. To accumulate in placers, mineral particles must be significantly denser than quartz (whose specific gravity is 2.65), as quartz is usually the largest component of sand or gravel. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.
Majority of placer deposit are small and often ephemeral as they formon the earth's surface usually at or above the local base level , so that many are removed by erosion before they can be burried.
Placer deposits are mostly of low grade but can be exploited because:
In geology, a placer deposit or placer is an accumulation of valuable minerals formed by gravity separation during sedimentary processes. The name is from the Spanish word placer, meaning "alluvial sand". Placer mining is an important source of gold.
Minerals commercially mined from placer deposits include:
- Gold
- Platinum group metals
- Tin, in the mineral cassiterite
- Diamonds
- Rare earth elements, from the mineral monazite
- Thorium, from the mineral monazite
- Titanium, from the mineral ilmenite
- Uranium, from Precambrian paleoplacers
Types of placer deposits include:
- alluvium placer deposit,
- eluvium placer deposit,
- aeolian placer deposit,
- beach placers deposit, and
- paleoplacers deposit.
Placer materials must be both dense and resistant to weathering processes. To accumulate in placers, mineral particles must be significantly denser than quartz (whose specific gravity is 2.65), as quartz is usually the largest component of sand or gravel. Placer environments typically contain black sand, a conspicuous shiny black mixture of iron oxides, mostly magnetite with variable amounts of ilmenite and hematite. Valuable mineral components often occurring with black sands are monazite, rutile, zircon, chromite, wolframite, and cassiterite.
Majority of placer deposit are small and often ephemeral as they formon the earth's surface usually at or above the local base level , so that many are removed by erosion before they can be burried.
Placer deposits are mostly of low grade but can be exploited because:
- They are loose, easy worked on materials which require no crushing. Relatively semi-mobile separating or hydraulic mining plants can be used. Mining takes the form of dredging, about the cheapest of all mining methods
- Placer deposit tend to concentrate minerals at a shallow depth and may contain valuable minerals such as Gold. Good example is the Witwatersrand in S. Africa.
Why continental crust is older than Oceanic Crust?
Why continental crust is older than Oceanic Crust?
Answer:
Because of this process; no oceanic crust older than 200 million years exist on the earth.
Answer:
- An important difference between continental and oceanic crust is their difference in density. Continental crust has a lower average density (2.6g/cm3) than does oceanic crust (3.0g/cm3). This density difference allows the continents to float permanently on the upper mantle, persisting more or less intact billions of years. Oceanic crust in contrast, is barely able to float on the mantle (which has a density of 3.3g/cm3).
- As oceanic crust ages and cools, it accumulates a heavy under-layer of cooled mantle rocks; the resulting two-layer structure eventually sink of its own weight (because of its own weight) into the mantle, where it is melted down and recycled.
Because of this process; no oceanic crust older than 200 million years exist on the earth.
- About 16% of mantle consists of recycled oceanic crust rocks; while only about 0.3% of mantle consists of recycled continental crust rocks.
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