Sedimentary Rock Textures:
Clastic rocks consist of the solid particles from preexisting rocks. These type of rocks originate and are transported as solid particles from both mechanical and chemical weathering. When lithified, they form clastic sedimentary rocks. This sandstone rock (left photo) has a clastic texture. Sandstone is the name given to rocks in which sand-sized grains (1/16 mm to 2 mm) predominate. Two common types of sandstone include regular and arkose. If abundant feldspar grains are present within the rock, then it is referred to as arkose. If all of the grains in a sample of sandstone, like the one to the left, are about the same size, the sand is considered to be well-sorted.
Bioclastic rocks are those primarily composed of skeletal fragments of organisms that died and settled down as sediment in marine or terrestrial environments. They can be particularly useful for relative age dating purposes. A type of limestone composed almost entirely of fragmented seashells is known as coquina (left photo). Most modern shells are composed of aragonite. Like calcite, aragonite has a mineral composition of calcium carbonate. As the sediments become partially buried, a unique transformation begins to take place where aragonite chemically alters to calcite. This process readily occurs because calcite is a more stable mineral than that of aragonite. This type of rock will react with hydrochloric acid.
Chemical texture is derived from mineral precipitation. Such rocks form when mineral constituents in solution become saturated and inorganically precipitate. One example of a deposit resulting from chemical processes is the salt left behind as a body of seawater evaporates. Rock salt (left photo) has a chemical texture. Evaporates, such as rock salt, can also form in regions where higher elevations are near desert floors. The snow in the higher elevations begin melting in the warmer months and flow downward to the desert floors. During the daytime when temperatures are higher on the desert floor, the heat evaporates the salt-rich, snow-melt water, thus leaving behind deposits of salt.
Biochemical texture is derived from solutions and minerals present in water by organisms to build their shells. Representing about 10 percent of the total volume of all sedimentary rocks, limestone (left photo), is the most abundant sedimentary rock. It is composed chiefly of the mineral calcite and forms either by inorganic (chemical) or organic (biochemical) processes. All limestone rocks dissolve readily and effervesce when hydrochloric acid (HCL) is applied because they contain an abundance of the mineral calcite (calcium carbonate). Specks of calcite crystals can be seen shining in the limestone rock, especially in sunny, outside conditions. Limestone rocks can come in a varity of colors, but the most common color is medium gray.
Igneous Rock Textures:
Slow cooling beneath the Earth's surface produces large crystals which are visible to the unaided eye. This is because the atoms in the magma have time to arrange themselves in a regular pattern. In such cases, igneous rocks with phaneritic texture are always intrusive. These igneous rocks, diorite (left photo) and granite (right photo), formed as a result of magma cooling slowly beneath the Earth's surface. They both have crystal grains roughly of the same size in their matrixes, and they both have crystal grains with random orientation.
Fast cooling above the Earth's surface produces small crystals, which are difficult to see with the unaided eye. During fast cooling, such as in lava flows, the rate in which mineral nuclei form exceeds the rate of growth and an aggregate of many small mineral grains is formed. The result is a fine-grained or aphanitic texture, in which individual minerals are too small to be seen without magnification. Fast cooling prevents the atoms in the magma from making large crystals. Some rocks, such as rhyolite (left photo) may contain small shards of obsidian glass. Although visible, glass is not a mineral; therefore, it is not included when determining a rock's texture. Rhyolite is a volcanic igneous rock and is equivalent to its counterpart granite, which is an intrusive rock.
Porphyritic texture is represented by igneous rocks which have minerals grains of different sizes. The larger minerals are called phenocrysts. The smaller ones are collectively known as groundmass, which is simply the grains between the phenocrysts. The groundmass can be aphanitic or phaneritic; therefore, the only requirement for this type of texture is that the phenocrysts be considerably larger than the minerals in the groundmass. Note the large crystals (phenocrysts) of potasium- (K)-feldspar which is surrounded by a matrix of amphibole and quartz (groundmass). Igneous rocks with porphyritic texture are designated as porphyry, as in granite porphyry or basalt porphyry.
Lava may cool so fast that its constituent atoms do not have enough time to become arranged in the ordered, three-dimensional frameworks of minerals. As a consequence, natural glass, such as obsidian forms. Obsidian cools very fast, therefore having a glassy texture. Even though obsidian is not composed of minerals, geologists nevertheless classify it as an igneous rock. Depending on the presence of iron, obsidian may be black, dark gray, red, or brown, and in some cases, green, yellow, and rainbow. Obsidian breaks with the conchoidal (smooth curved) fracture which is typical of glass. Obsidian has a very high silica content and is compositionally similar to the extrusive igneous rock, rhyolite and the intrusive rock, granite. Obsidian often forms on the bottom layer of pyroclastic flows travelling down a volcano at speeds of up to 100 mph.
Some magma chambers contain large amounts of water vapor and other gasses. These gasses may be trapped in cooling lava where they form numerous small holes or cavities known as vesicles. Rocks, such as scoria (left photo) with many vesicles are referred to as having vesicular texture. Pumice, another extrusive igneous rock, can be incorrectly identified as scoria; however, it should be noted that scoria rocks are more crystalline and denser than pumice rocks. Scoria is commonly a dark-red to black color and is produced as slag is ejected out of a volcano during an eruption. Because of its red and vesicular uniqueness, scoria rocks are often used for landscaping purposes.
Pumice is a variety of volcanic glass mixed with ash containing numerous vesicles (small, large, or a mixture of the two) that develop when gas escapes through lava and forms a froth. Pumice has a frothy texture because of its appearance and light-weight characteristics. If pumice falls into water, it can be carried great distances because it is so porous and light. Pumice is often white or semi-white in color, and it is used for cosmetic purposes to exfoliate dry skin. Pumice cools fast to almost very fast above ground. Pumice forms from thick, slow-moving felsic magmas. Compared to mafic magmas, felsic magmas have a higher concentration of silica, higher viscosity, and lower temperature.
Any igneous rock, which forms from an explosive volcanic eruption, is referred to as a pyroclast. Such rocks have a pyroclastic texture. "Pyro" means fire and "clast" means rock particle. Pyroclastic rocks, such as welded-ash tuff, lapilli, volcanic bombs, and volcanic breccia all form from explosive volcanoes. The photo to the left represents a volcanic breccia. Such rocks (left photo) form as ash is blasted up into the atmosphere and settles back down onto the ground with shards of rock and/or glass. Once the hot material has cooled, it solidifies together forming as one rock. Layers of ash can accumulate on the ground up to 50 feet or more depending on the amount of ash and other debris ejected from the volcano.
Metamorphic Rock Textures:
Slaty (Platy) Texture:
Rock cleavage refers to closely spaced planar surfaces along which rocks split into thin, tabular slabs when hit with a hammer. Rock cleavage is developed in various metamorphic rocks but is best displayed in slates that exhibit an excellent splitting property called slaty (platy) cleavage. Slate (left photo) is a low-medium grade metamorphic rock which forms from the parent sedimentary rock called shale. Both shale and slate have mica minerals present within their matrix. However, as shale metamorphoses to slate, the mineral grains become larger and are more noticeable in the rock matrix. Muscovite mica is the most common form of mica present in these rocks.
Under high temperature and pressure conditions, the minute mica and/or chlorite mineral grains in slate begin to grow many times larger. When these platy minerals grow large enough to be discernible with the unaided eye and exhibit a planar or layered structure, the rock is said to exhibit a type of foliation called schistostic texture. A rock having such texture is called schist. In addition to the presence of platy minerals, schist rocks often contain deformed quartz and feldspar, or lens-shaped mineral grains among the mica grains. The rock represented in the left photo is a typical schist rock. Schist rocks often contain other minerals, such as garnets. In such cases, they are referred to as garnet schists.
During high-grade metamorphism, mineral ion migrations can result in the segregation of minerals. Notice that the dark amphibole crystals and light silicate minerals (quartz and feldspar) have separated, thus giving the rock a "banded" appearance called gneissic texture. Although foliated, gneiss rocks will not usually split as easily as slates and some schists. As with all metamorphic rocks, gneiss rocks (left photo) are very hard. Metamoprhic rocks, such as gneiss, can be generated from the transformation of sedimentary rock shale or igneous rock granite or diorite. Those formed from granite are often called granite gneiss, while those formed from diorite are called diorite gneiss.
Not all metamorphic rocks exhibit a foliated texture. Those that do not, are referred to as non-foliated, such as is the case for marble (left photo). Metamorphic rocks with non-foliated texture typically undergo minimal deformation and the parent rocks are composed of minerals that exhibit equidimensional crystals, such as calcite or quartz. The parent rock from which marble forms through metamorphism is limestone. Limestone is composed of calcite (calcium carbonate). Calcite has a hardness of 3 on the Mohs Hardness Scale. Because marble contains a slightly harder version of calcite, it too has a hardness of around 3. When viewed close-up, visible crystal grains of calcite can be seen interlocking one another, thus forming a massive rock like marble (left photo).
Another texture common to metamorphic rocks consists of particularly large grains, called porphyroblasts, that are surrounded by a fine-grained matrix of other minerals. During recrystallization, certain metamorphic minerals, such as garnet, staurolite, and andalusite, invariably develop a small number of very large crystals. In such cases, a porphyroblastic texture is observed. The photo to the left represents a garnet schist. The large pinkish-red garnets are the porphyroblasts which are surrounded by the mica-rich matrix of schist. Technically, schist rocks, which have smaller crystals of garnet or other minerals are not classified as having porphyroblastic texture.