Задания по Flash / Lessons / Lesson08 / 08Start / Minerals

OBSIDIAN
Obsidian is a naturally occurring glass, produced by volcanoes. When felsic lava
cools rapidly and freezes without sufficient time for crystal growth, obsidian is
formed.
Obsidian is often found within the margins of felsic lava flows, where cooling is
more rapid. Obsidian has been used for centuries in making tools and weapons.
Most commonly made were arrowheads. Because of the lack of crystal structure,
obsidian blade edges can reach almost molecular thinness. Today, obsidian can
be found in both decorative arts and crafts as well as modern day surgical
scalpel blades.

GRANITE
Granite is a common igneous rock composed of quartz, orthoclase, and
hornblende, often accompanied by pyroxene or mica. It is named granite
because of its granular surface. Granite rocks are usually medium- to coarse-
grained, occasionally with some larger individual crystals that form a rock known
as porphyry. Granite naturally occurs in many shades. Typical rocks are pink to
gray and occasionally black, depending on their chemistry and mineralogy.

Granite is widely used, as it is one of the most durable stones for architectural
and artistic applications. Typical uses include sculpture, architectural trims,
curbstones, and veneer building stone.

BASALT
Basalt is the most common type of solidified lava. Because of the rapid cooling of
lava on the Earth's surface, basalt is usually fine-grained.
Basalt is formed in areas where tectonic plates diverge. Rapid cooling of basalt
lava forms a dark glassy rock. Lava bombs are also glassy because they cool
quickly while falling through the air after being blown out of a basaltic volcano.
Iceland is the only place in the world where a mid-oceanic ridge is exposed
above sea level. Because of this, Iceland is a good place to see many types of
basalt. More than ninety percent of all rocks in Iceland are some form of basalt.


SCORIA
Scoria is a volcanic rock that contains cavities that were created by gas bubbles
trapped in the lava. The rock itself is formed as gas-charged lava is thrown into
the air during an eruption and cools while falling through the air.
Volcanic cones of scoria can be left behind after eruptions, usually forming
mountains with a crater at the summit. Mount Wellington, in Auckland, New
Zealand, is an example of a mountain formed by scoria.

Scoria is well-known for its use in landscaping and decorative uses for gardens
and pathways.


RHYOLITE
Rhyolite is a colorful and lightweight volcanic rock. It is considered the extrusive
equivalent to the plutonic granite rock. Because of its high silica and low iron and
magnesium contents, rhyolites polymerize very quickly and form viscous lavas.
Rhyolites that cool too quickly do not grow crystals and instead form natural
glass, or “obsidian.”
Rhyolite weighs roughly one-half as much as limestone and granite, does not
crumble, and is virtually impervious to the elements, which makes it a popular
choice for decorative building. Additionally recognized for its beauty, this rock is
also commonly used today in landscape designs.

SHALE
Shale is the most common sedimentary rock. It is a fine-grained sedimentary
rock whose original constituents were clays or mud. It is characterized by thin
laminae that break with an irregular curving fracture, which is usually splintery,
and often parallel to the indistinguishable bedding plane. This property is called
fissility.
The process that forms shale is called “compaction”. The fine particles that
compose shale can remain suspended in air long after the larger and denser
particles of sand have deposited out. Shale is typically deposited in very slow-
moving water and is often found in lake deposits, floodplains, river deltas, and
beach sands. It can also be found deep on the continental shelf, in quiet waters.

BRECCIA
Breccia is typically a sedimentary rock that is composed of angular fragments
from a previous rock structure. Each small fragment is essentially cemented in a
matrix that may be of a similar or a different material. This composition also
makes breccia a clastic rock.
Breccia can be formed by submarine debris flows, avalanches, and mudflow.
Another way to find sedimentary breccia is as angular fragments of rocks in a
finer-grained groundmass. These are, in essence, lithified colluvium. Thick
sequences of sedimentary (colluvial) breccias are generally formed next to fault
scarps in grabens.
When highly compacted, breccia makes wonderful beads or brooches and is
often carved into various other decorative accessories.

SANDSTONE
Sandstone is a sedimentary rock composed of sand-size minerals and rock
grains. Much like sand itself, sandstone can be any color. Because sandstone
beds often form highly visible cliffs and other topographic features, certain colors
of sandstone have been strongly identified with certain regions.
Some sandstone is resistant to weathering and is easy to work with. Additionally,
the individual grains that form sandstone make it very strong. These qualities
have made sandstone a common choice for use in building and paving.
Rock formations that are primarily sandstone are porous enough to allow
percolation of water, making them valuable aquifers. Fine-grained aquifers, such
as sandstones, are more apt to filter out pollutants from the surface than are
rocks with cracks and crevices, such as limestones or other rocks fractured by
seismic activity.

CONGLOMERATE
A conglomerate is a rock that consists of individual stones that have become
cemented together. Like breccias, conglomerates are sedimentary rocks
consisting of many fragments. The main difference between the two types of rock
is that breccias consist of several angular pieces, but the conglomerate is
primarily made up of rounded fragments. Both conglomerates and breccias are
characterized by having rock particles larger than sand.
Conglomerate is formed by river movement or ocean wave action. The
cementing agents that fill the spaces to form the solid rock conglomerate are
silica, calcite, or iron oxides.


LIMESTONE
Limestone is a sedimentary rock composed largely of the mineral calcite (calcium
carbonate). Limestone often contains small amounts of clay, silt, and sand.
Pure limestone is almost white in color. Because of impurities, such as clay,
sand, iron oxide and other organic materials, limestone often displays many
variations of color. Limestone may be crystalline, clastic, granular, or massive,
depending on the method of formation.
Limestone has many uses. It has been used as a building material for many
centuries. It is also commonly used to aid in the production of glass, paper, and
steel.

SLATE
Slate is a fine-grained, foliated metamorphic rock that tends to break into thin,
flat sheets. Slate is mainly composed of quartz and muscovite Often, it also
contains biotite, chlorite, hematite, pyrite, apatite, graphite, kaolin, magnetite, and
tourmaline.
Slate forms when shale is put under pressure with high temperatures. When this
happens, the clay begins to revert to mica minerals. This gives the surface of
slate a sheen typical of mica.
Slate is usually dark, but it can be colorful too. High-quality slate is an excellent
paving stone as well as a raw material for long-lasting roof tiles. Blackboards and
handheld writing tablets were once made of slate.

MARBLE
Marble is a metamorphic rock and is formed from limestone by heat and pressure
inside the earth's crust. The process in which marble is created is called re-
crystallization. Impurities that are present in the limestone during re-
crystallization affect the mineral composition of the marble that later forms.
The minerals that result from impurities are what give marble its wide variety of
colors. The purest marble is white. Marble containing serpentine is green. Marble
with limonite is yellow, and marble with hematite in it has a reddish color.
Marble has been highly valued for its beauty, strength, and resistance to fire.
Like the ancient Greeks of centuries before, we still use marble today in many
buildings and statues.

QUARTZITE
Quartzite is a hard, metamorphic rock that is made from sandstone. Through
heating and pressure, the original quartz sand grains and quartz silica cement
are fused into one. Pure quartzite is usually white or light grey in color but often
occurs in various shades of pink and red due to varying amounts of iron oxide.
Other colors are due to impurities of minor amounts of other minerals.
In true metamorphic quartzite, the individual quartz grains have re-crystallized to
form an interlocking mosaic of quartz crystals, often having a sugary appearance.
Minor amounts of former cementing materials, iron oxide, carbonate and clay,
are often re-crystallized and have migrated under the pressure to form streaks
and lenses within the quartzite.
Quartzite is often dyed and tumbled for decorative purposes in the glass and
ceramics industry.

GNEISS
Gneiss is a widely distributed and commonly used type of metamorphic rock.
Gneiss is rich in quartz and feldspar and has a banded and foliated texture.
Due to the physical and chemical similarity between many gneisses and plutonic
igneous rocks, some are used as building stones and for other structural
purposes.

SCHIST
Schist is a metamorphic rock, primarily notable for the prevalence of lamellar
minerals such as micas, chlorite, talc, hornblende and graphite. Quartz often
occurs in drawn-out grains to such an extent that a particular form called quartz
schist is produced.
Because it polishes so nicely, gneiss is widely used as an ornamental building
stone. Schist is also the source of the semi-precious gemstone called garnet.