The Rock - GRANITE
- Igneous Rock Type: Intrusive
- Related to: Rhyolite, pegmatite, syenite
- Chemistry: Acidic
- Color: White, pink, orange, gray, black
- Texture: Phaneritic (easy to see crystals)
- Origins: Orogenic Plutons
- Common Minerals: Quartz, feldspars, hornblende
- Accessory Minerals: Tourmaline,
earth oxides, beryl,
- Uses: Building material, decorative counter tops, tiles,
tombstones, roads, jewelry, curling stones, marbles
Granite is possibly
the most common igneous rock type known to the general public. Granite,
which is named for its "granular" or phaneritic texture, has crystals that
tend to be easily seen, although they are generally small. It is
a rock that has been used for centuries for many different purposes such
as building material. Granite was used with limestone
as a building material for the pyramids of Egypt. Its durability, beauty
and abundance make it a preferred choice of stone over most others. Granite
is also a source of many mineral specimens. Unfortunately, most of the
crystals in a granite form anhedral crystals or crystals that lack their outward
shape. This is due to the way that the crystals grow into each
other to form interlocking crystal frameworks. Although this gives granite
its great durability, it limits its desirability as a source of mineral specimens.
Occasionally there are pockets within a granite where crystals can form very
The crystals of granite form while the molten material inside the Earth's
crust cools relatively slowly . Molten rock or magma that would have
formed granite had it stayed in the Earth's crust, but instead managed
to erupt onto the surface of the Earth, forms a rock called rhyolite. The two rock types have the same chemistry.
Rhyolite however does not generally have the same texture and crystals
are generally too small to see. If granite type rock has crystals that
grow larger than a large pebble (roughly 3 cm or about 1 inch across) then
it is called a pegmatite.
minerals that are found in granite are primarily
potassium or K-feldspars, hornblende and
micas. Quartz is usually the last
mineral to crystallize and fills in the extra space between the other minerals. Quartz's hardness, lack of chemical reactivity
and near lack of cleavage give granite a significant amount of its desirable
durable properties. The quartz will appear gray, but is actually colorless
and is reflecting and fusing the colors of the white and black minerals surrounding
it. The plagioclase feldspars are generally white with a porcelaneous
luster. The K-feldspars are generally the ones that give granite its
color variations from yellow to orange to pink or blue. Dark K-feldspars
can give granite its black varieties as well. The micas are generally
biotite (black or brown) or
lepidolite (violet or pink)
and provide the sparkle that some granites possess. The hornblende and
biotite provide granite with the black pepper portion of the famous and
distinctive "salt and pepper" look to classic granite.
Some accessory minerals include gemstones
such as tourmaline,
These minerals are generally scattered in the groundmass and generally
do not affect the overall appearance of the stone. Other accessory
minerals are important economically such as phosphates
and rare earth
oxides. Related to the rare earth elements is a significant concentration
in granite of the element uranium. Granite is actually rather radioactive
and has 5 to 20 times the concentration of uranium compared to other common rock
types. Some health concern exists in areas that are rich in granitic terrain,
as background radiation is enhanced by the presence of large granite bodies.
Although the uranium is generally not concentrated enough to make granite
a uranium ore, the leaching and erosion of granite has helped produce most
of the uranium ore deposits around the world.
Granite is a very general term and is applied to a whole host of different
rocks. Many different granites have been identified based on their varied
chemical and mineralogical compositions. Generally the term granite is used
as a suffix to indicate its textural and general composition. If a granite
is rich in lithium then the rock might be referred to as lithium granite.
Also, petrologists may choose to classify different but similar granitic
rock types by using the terms such as granodiorite or syeno-granite.
Some granites are named according to their unusual characteristics. In some
granites the feldspars had time to form rectangular crystals before quartz
and the other minerals crystallized. The resulting rock appears to
be a collection of unintelligible letters, numbers or figures and the rock
is called "Graphic Granite".
There are hundreds if not thousands of granites that are given local or
marketable names. Some of these name are descriptive such as indicating
an unusual color or indicate the source of the rock such as Pikes Peak Granite.
Many of these granites are distinctive and many rocksmiths can easily
distinguish where a sample came from.
Some rocks are incorrectly called granites. Several course grained metamorphic rocks are incorrectly referred
to as granites. Generally granite does not exhibit signs of metamorphism
such as a layered pattern, metamorphic minerals present such as garnet
or crystal orientation. Rocks showing these characteristics are probably
gneisses or schists,
some of which can be very attractive and durable like a granite. But really,
who wants a schistic countertop? Since the general public trusts a term like
granite, the name granite is often applied to these stones.
Many other igneous rocks are also incorrectly referred to as granite.
These rocks have granite's "salt and pepper" look, but have different
mineral assemblages that preclude them from being classified as a granite.
gabbros, monzonites and
anorthosites have little or no
quartz. If the rock has little or no
K-feldspar but generally equal
amounts of plagioclase feldspar and
quartz, then it is a
very rich quartz rock (>90% quartz) of igneous intrusive origin is called
a quartzolite, but will have a very different look from granite. Rocks
that have so little silica content that they can not produce decent feldspar
let alone quartz are classified as feldspathoid rocks and they also can look
like granite. Quartz is always a must and identification of quartz
in the rock is key. Basically, if there are randomly scattered crystals
of nearly equal amounts of quartz and plagioclase feldspar with some
and hornblende or
mica crystals that are melted into each other (an intrusive
igneous rock), then it is granite.
As an igneous rock, granite forms from melted or molten rock called magma.
As an intrusive rock, granite forms from molten rock that never reaches
the surface of the Earth. Granite forms from the melting of lighter materials
than is found in the deep crust or mantle. Where did this magma come from?
There are scientists that disagree on this subject. In general there is
agreement that most granite is derived from the melting of subducted crustal
rock (lighter weighted rock) that slipped into the mantle in subduction zones
such as those that are found ringing the Pacific Ocean today. If true then
granite is a "newer" rock type as it required the plate tectonic process to
have proceeded along before the first granites formed. It could have
formed from some other process that segregates the lighter aluminum/silica
material from more dense magnesium/iron material. It could also have been
produced from a process called granitization or the melting of a chemically
similar rock from intense metamorphism into a completely melted magma. However
most granite crystallization models require water to be in the magma and the
intense metamorphism scenario would not allow a lot of water to be present.
The great variation and abundance of granite suggest that there could be
many various formation models.
Granite is found in all continents around the world and is generally the
foundation of many orogenic belts or mountain chains. Most often granite
is the underlying rock upon which sedimentary and other continental rocks rest.
Granite is found in batholiths or large magma plumes that rose into
the continental rocks. But it can be seen in lots of other intrusive
features such as dikes, sills and lacoliths.