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What's my rock

Welcome to What’s my Rock. You’ll need a few simple things to help you through this process so have them handy before you start:

  • A hand lens or magnifying glass
  • Some dilute acid (stone tile cleaner or vinegar will do)
  • A large steel needle

First off you need to know how to tell what your rock is made up of. This really helps to get an accurate picture of how your rock was made. Rocks are made up of three different kinds of bits, crystals or grains. Telling the difference between these rock-making materials is the key to successful rock identification.

What is a crystal? A crystal is something that has a definite structure. It often has shiny, light reflecting surfaces at lots of different angles. Crystals in rocks will form around each other making an interlocking pattern filling in any gaps. Crystals can be lots of different shapes, colours and sizes.

What is a grain? A grain is a recycled piece of rock, crystal or fossil. It is something that has already existed in a different form to the one that we see now.

 

Information about the three rocks featured in the photos.

The Rock Identification key – by Don Peck

What are Rocks

Rocks are what the crust of the earth is made of. They are the mountains and the bottom of the ocean. They are everywhere on earth, but often buried under soil. Rocks are made of minerals, like quartz, calcite, feldspars, and micas. Most rocks are made from more than one mineral, but there are quite a few kinds that are made from only one mineral. Minerals are not rocks, rocks are made of minerals. A car is made of steel, glass, and plastic. A rock is like the car, a mineral is like the steel, or glass, or plastic.

What types of rocks are there?

There are three different types of rock:

Igneous Rock
is formed when a magma cools underground and crystallizes or when it erupts unto the surface of the ground, cools and crystallizes. Magma that erupts onto the surface is called lava. When magma cools slowly underground the crystals are large enough to see. When it cools quickly on the surface, the crystals are very small and you would need a magnifier or a microscope to see them. Sometimes, when the magma cools very quickly, it forms a kind of black glass that you cannot see through.
Igneous rocks
Sedimentary Rock
forms from particles, called sediment, that are worn off other rocks. The particles are sand, silt, and clay. Sand has the largest particles while clay has the smallest. If there are a lot of pebbles mixed with the sand, it is called gravel. The sediment gets turned into rock by being buried and compacted by pressure from the weight above it. Another way it becomes rock is from being cemented together by material that has been dissolved in water. Often, both cementing and compaction take place together.
Sedimentary rocks
Metamorphic Rock
is formed by great heat, or pressure, or both. The pressure can come from being buried very deep in the earth’s crust, or from the huge plates of the earth’s crust pushing against each other. The deeper below the surface of the earth, the higher the temperature, so deep burial also means high temperatures. Another way that high temperatures occur is when magma rises through the earth’s upper crust. It is very hot and bakes the rock through which it moves. Hot liquids or gases from the magma also can cause chemical changes in the rock around the magma.
Metamorphic rock

What is the Rock Cycle?

Rocks, like mountains, do not last forever. The weather, running water, and ice wear them down. All kinds of rocks become sediment. Sediment is sand, silt, or clay. As the sediment is buried it is compressed and material dissolved in water cements it together to make it into sedimentary rock. If a great amount of pressure is exerted on the sedimentary rock, or it is heated, it may turn into a metamorphic rock. If rocks are buried deep enough, they melt. When the rock material is molten, it is called a magma. If the magma moves upward toward the surface it cools and crystallizes to form igneous rocks. This whole process is called the Rock Cycle.

Rock Cycle

What minerals form rocks?

The list of minerals that commonly form rocks is short. With a little practice you will recognize most of them when you see them. Descriptions of some of the minerals, as they look in rocks, follow: Quartz: Quartz is the last mineral to crystallize, so in igneous rocks it never has any definite shape. In rocks, it does not show flat faces. It is usually gray in igneous rocks; gray, white, yellow, or red in sedimentary rocks; and gray or white in metamorphic rocks. It has a glassy, or sometimes waxy, look to it.

Potassic Feldspars*: (microcline, orthoclase) Potassic feldspars are pink or tan, sometimes white. They show flat, shiny faces in igneous rocks. The crystal grains are usually blocky and nearly rectangular. They look like good china.

Plagioclase Feldspars*: (albite, labradorite) Look like the potassic feldspars, except they are white to dark gray, sometimes black. They may show flashes of blue or green.

Micas*: (muscovite, biotite, phlogopite) Micas have very thin layers that peel off (or cleave) very easily. In rocks they are usually flakes or layers of flakes. Muscovite is silvery to brown; biotite is black; phlogopite is a reddish brown. Phlogopite may be found in marble.

Chlorite*: Like mica, but the flakes are usually not as thin and do not peel apart as easily. The color is medium to dark green, sometimes almost black but with a greenish tint.

Hornblende: Hornblende is dark green to black. It shows nearly flat, shiny faces in almost rectangular or long thin needle like crystals in rock. Hornblende is usually found in dark colored metamorphic rocks; sometimes in igneous rocks.

Actinolite and Tremolite: Actinolite and tremolite are usually in long thin blades or needle like crystals. Actinolite is dark green; tremolite is white to gray. The crystals may be parallel to each other, or spread from a point. Actinolite is usually found in schists or gneisses. Tremolite may be found in marble.

Olivine*: Olivine in rocks is an olive green to greenish yellow. In rocks it is in rounded grains. If there is much of it, it is almost sugary. It is found mostly in dark colored igneous rocks.

Calcite and Dolomite: The color is usually white, but can be other colors when impure. Crystal grains show flat shiny faces, often shaped like parallelograms. Calcite and dolomite are both soft. They are easily scratched with a steel point. Powdered calcite will fizz in white vinegar; dolomite will not. The minerals are found in limestone or dolostone ( the rock is dolostone, the mineral is dolomite) and marble.

note: Names marked with an asterisk (*) are groups of related minerals.

Collecting rocks

Rocks are easier than minerals to collect. That is because they are found nearly everywhere. If you want to start a rock collection, try to find pieces of rock that are freshly broken off a ledge. A ledge is a bed of rock that is sticking out of the ground, or the side of a mountain. It is not loose, but is still part of the bedrock below the soil. Pieces of rock that have been buried in the soil, or rolled in a stream or river are not good to collect. It is difficult to see what they are or what they are made of and you really don’t know where they came from.

  • Collect clean fresh specimens.
  • Make a label that has the name of the rock and the location where it was collected.
  • Assign a number to each rock.
  • Record in a notebook the name, location where you found it, and number of the rock.
  • Paint a small white rectangle on each rock, and write the rock’s number on it.

Safety when collecting rocks

  • Always wear safety glasses or goggles when breaking rocks.
  • Use only hammers that are intended for breaking rocks. Do not use a carpenter’s claw hammer (a hard rock can break sharp steel splinters off a claw hammer).
  • Do not climb on dangerous ledges or on quarry walls. Stay away from quarry walls, they might collapse.
  • Never enter mine tunnels. They are very dangerous!
  • If possible, always collect with an adult.

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This page arose from the Shaping of Shropshire joint project between Shropshire Wildlife Trust and Shropshire Geological Society,
supported by the Heritage Lottery Fund.
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