Igneous rocks:
Igneous rocks are formed from the solidification of magma deep in the earth. They contain 45 to 66 percent silica (quartz). The remaining minerals are mostly feldspar, mica and iron ores. Granite is the most abundant igneous rock found on Earth.
Rocks exhibit a crystalline form with grain size ranging from very small to several inches. The large crystal granites are formed when the magma cools slowly. The smaller crystals are formed with rapid cooling. All this takes place before the magma reaches the Earth’s surface. If it does reach the surface, it’s called lava. Hardness ranges from six and higher on the Mohs’ scale. Igneous rocks can be identified very easily by performing a scratch test with an ordinary knife blade. If it is difficult to scratch, it is most likely igneous. Igneous rocks will generally not react with acids. However Hydrofluoric Acid (HF) will react with a polished surface. The minerals contained in igneous rocks are usually dense and packed tightly. They lack bedding or foliation. However, be aware of a granite look-a-like known as gneiss. Gneiss looks like an igneous rock but is a transitional material between metamorphic and igneous. It is a brittle material and does have a bedding plane. Color key for igneous rocks:
- White granites are rich in potassium.
- Combinations of mica and quartz result in gray.
- Pink contains sodium and calcium rich feldspar.
Sedimentary rocks:
Sedimentary rocks are produced from erosion of other rocks, compression and underground water erosion. The sedimentary rocks cover nearly 75 percent of the Earth’s surface. For our purposes, sedimentary rocks can be classified into two types: limestones and sandstones. Limestones are formed in shallow waters of the sea shelf. They consist of calcite, but maybe mixed with other minerals depending on the water clarity when it was formed. Many limestones contain a mineral known as magnesium carbonate (dolomite). Dolomite does not react with acid unless it is first crushed. Any rock with more than 50 percent carbonate mineral is classified as a limestone. Some common limestones are travertine, slate, coral, shell stone or coquina. Limestones have a bedding plane, and because they are formed in the sea, they often contain fossils and shell fragments that make them more easily identifiable. Sandstones are also sedimentary rocks that are primarily quartz minerals loosely cemented together with calcite, iron oxides and/or mud. Sandstones are frequently used as a dimensional building stone. Major cities throughout the world contain buildings made with sandstone. Some common sandstone is brown stone and shale. Sandstone can be recognized by its distinct sand-like appearance. It fractures very easily around the individual grains, and exhibits a distinct bedding plane.
Metamorphic rocks:
Metamorphic rocks are limestones that have been exposed to high temperature and pressure over a long period of time. This change is known as metamorphoses and causes the minerals to go through a molten phase. For this reason, most marbles have distinct veins, swirls or bands, but there are exceptions. Light veins may contain quartz and feldspar, while dark veins may contain hornblend or biotite. All marbles will have calcite and/or dolomite and will react with acid.
Marble is very heavy, but relatively soft with a Mohs scale hardness of 3-4. It can be cut by a steel knife. Granite is much harder and cannot. If you cannot cut it with a knife, it is probably granite.
Vinegar will make bubble fizz, whereas granite will be unaffected. The bubbles can be very small so use your magnifying glass to see.
Splash water on the material surface . Marble does not absorb water and water will pool.
If you put an acid liquid like vinegar or lemon juice, marble will react producing fizzing bubbles. The bubbles are very small, so you might need a magnifying glass to se it. Granite does not react.
Polished marble is smooth and silky with a highly reflective surface. You can often see your face reflected. If the polished and clean surface has roughness or bumps, it is not marble.
Marble is at least two colors or tones and has a distinct pattern. The pattern grain that runs through it, made up of lines and waves that create a jumbled pattern. Some of the lines may smudge and swirl. If the marble has a speckled or crystal-like markings, then it is more likely to be granite. The colors in marble include pink, black, white, greenish, red and cream.
The colors are soft shades, rather than bright.
In order to identify the types of stone you may encounter, you need to be able to identify the minerals in the rocks. Certain groups of rocks contain specific types of minerals. We know granites are igneous and contain quartz, marble is metamorphic and contains calcite, and sedimentary materials such as limestone can contain dolomite. The minerals contained in the stones we deal with have some physical and chemical properties. Some of these properties can be tested and will tell us which mineral we are dealing with. The following are those properties: Luster — Luster is the way a mineral reflects light from its surface. A mineral can be metallic, which means it looks shiny, or nonmetallic, which has no shiny reflection but is described as glassy or vitreous, pearly, silky, dull, greasy or soapy. Hardness — Frederick Mobs in 1822 developed the hardness test which measures how resistant a mineral is to abrasion. We commonly call this the “scratch test.” Taking minerals and scratching one against the other developed the test. The one that scratched was softer then the one that caused the scratch. This test was continued until 10 minerals were discovered. The minerals were arranged in order of hardness and given a number. “1” was the softest and “10” was the hardest. Cleavage — Cleavage is how the mineral breaks or fractures. Some minerals, when they break, will have one, two or three cleavages. This is perhaps the hardest property to determine and will require the use of a field microscope or hand lens. Color — Color is an important clue to identifying certain minerals. Stone gets its color from many of these minerals. Some minerals only need to be present in very small amounts to lend color to a stone. For example, many of the red granites and marbles are the result of the mineral iron. Acid Test — The acid test is an important test and can immediately tell you if you are dealing with a calcium-based stone. A simple drop of a dilute solution of hydrochloric acid will tell you if you have marble, limestone or granite. If the acid bubbles and/or fizzes, you have calcium. No reaction means there is no calcium and you may have granite or possibly a dolomitic limestone. Those are the most common properties used to identify a mineral. Geologists and others may also use specific gravity; magnetism, electrical properties, optics and taste for further identification.
STEPS FOR IDENTIFICATION PROCESS:
Touch the surface of the material. Stroke it and feel its texture. Marble is very smooth and silky to touch. If you experience any roughness, then the material is not marble.
Look at the material from a different angle to identify a glossy, almost translucent quality on the surface. Marble is shiny and very reflective when polished. Moving your head will allow light to reflect off the marble at different angles for you to check this characteristic. Inspect the rock closely. Granite has large, tightly fitted crystalline grains. Classic granite has a “salt and pepper” appearance, with grains of black and white.
Assess the type of pattern the material has. Marble has a grain that runs through it, consisting of fluid lines that create a disorganized pattern in the stone. Some of the lines smudge and swirl. If the marble has speckles or crystal-like markings, then it is more likely to be granite stone. Look for quartz in the rock. Granite always consists of quartz and feldspar, which usually give granite a light, almost glittery color, ranging from almost translucent white (from the quartz) to a pale pink (from the feldspar). This lighter color is mixed with grains of other darker minerals, creating the “salt and pepper” look. Look at the color of the marble. Marble consists of at least two tones of color, since the grain is a different color than other parts of the marble. The colors can be cream, red, black and white, or gray. The colors will be subtle pastel shades, rather than bright green, pink or yellow.
Questions:
1. Identify (from top to bottom) the colour sequence of the stones of the statue
2. Associate each colour with a rock type, using the 3 easy steps above to identify the rock types
3. Name two other process that can be used to identify marble/granite
Sources:
http://marble.com/articles/geology-and-stone-identification
http://www.intertile.com/pages/stoneloc/stoneloc_3faqs.html#faqs_granite
https://cycleback.wordpress.com/2013/04/17/identifying-standard-stone-in-sculptures-and-construction/