A collection can be informative and interesting when rocks are identified and labeled.
A classification key can be a useful tool to determine rock types. Although after identifying hundreds of rocks, geologists and amateurs alike will be able to name a rock on sight, a key can still provide useful information regarding the diagnostic traits of rocks and their mineral content.
Each type of rock -- igneous, sedimentary and metamorphic -- has its own set of defined parameters. They can be identified using various characteristics such as texture, mineral content, grain type and size, and special features.
Instructions
1. Interlocking mineral crystals can be different shapes and sizes.
Define the texture of the rock by referring to it's general appearance. This will help determine if a rock is igneous, sedimentary or metamorphic.
The most fundamental reference is whether or not the rock is made up of interlocking crystals that have grown or fused together, or if the rock has a granular texture. A granular texture, where individual grains of sand or pebbles are seemingly glued together, is indicative of a sedimentary rock. However, this is only a generalization. Chemically precipitated sedimentary rocks will not display a granular texture. Rocks with interlocking crystals are identified as igneous. The texture of metamorphic rocks is determined by its parent rock. they can be fine- or coarse-grained and may display a foliated or banded texture.
2. Sandstone is classified as a coarse-grained sedimentary rock on a classification key.
Assess the grain size of individual mineral crystals. Grain size is indirectly related to texture. The grain size of a rock is an intrinsic quality in identifying a rock. A classification table will list rock types according to grain size and shape.
For example, sedimentary rocks with fine-grained quartz crystals may be a siltstone, while a coarser-grained rock composed of the same minerals may be a sandstone. Likewise, the grain size of an igneous rock indicates its cooling history. Igneous rocks that cool quickly are fine-grained, such as obsidian, while mountain ranges have millions of years to cool forming rocks with large grains, such as granite.
3. Quartz is easily identified in rocks because of its hardness -- it easily scratches glass.
Identify the mineral content of the rock. This can sometimes prove a bit problematic because the individual crystals may not be easily manipulated within a rock sample, but minerals, like rocks, can be identified by various characteristics.
Some of the parameters to evaluate when identifying the mineral composition include color, streak, luster, hardness and density. Special traits can also provide information on a mineral's identity, such as a mineral's reaction to hydrochloric acid -- calcitic minerals will fizz, or whether a mineral is magnetic.
4. Marble shows a smooth texture with veins of a darker mineral running throughout.
Check for preferential mineral banding or foliation. This banding of minerals in a rock, or lack thereof, happens only in metamorphic rocks, and the layering of minerals can be an important diagnostic characteristic.
When heat and directed pressure are applied to shale, minerals are allowed to migrate and move. Minerals tend to navigate to their closest relative neighbors and will form layers, giving those forms a foliated or banded texture. Nonfoliated metamorphic rocks, like marbles, will have a smooth texture, with veins of minerals that have migrated together during the metamorphic processes.
5. The presence of NaCl, or salt, identifies a sample as rock salt.
Look for other special characteristics that may give a clue to a rock's identification. Most classification keys will provide a list of unique traits that can provide insight. Some may be repetitive of the same traits looked at in mineral composition, simply because it is the chemical composition that gives rocks their uniqueness. Some of these qualities include a rock's reaction to hydrochloric acid or whether gas bubbles, called vesicles, are present. Fossils and organic matter pertain to sedimentary rocks. Some can even have a salty taste, like rock salt.
Tags: grain size, granular texture, metamorphic rocks, mineral content, sedimentary rocks, banded texture
