Sandstone is a sedimentary rock formed over time as sand deposits, sand sized minerals and rock particle deposits. Rocks are continually re-cycled over millions of years. The source rocks are weathered by wind, water or ice and erode and carried away. The greater distance the rock travels, the smaller the grains are likely to be. The sand sediments mix with minerals such as quartz, calcites and feldspars and fill a bowl shaped basin. The pressure of the sediments above and the water passing through, turn the resulting grains into sandstone. The finer grains mixed with the new mineral growth of natural silica or carbonate, fill in the gaps and cement them together. These sediments are linked by one factor. Water.
Sandstone is common in beach and desert environments. Sedimentary environments such as streams, lakes, rivers, coral reefs, beaches and glaciers; even deserts where they are later cemented when water percolates through the sand. Two contrasting climatic conditions, water such as an ocean compared to the heat from the sun and the wind, both have an effect on weathering and erosion, creating sand particles.
The colour of sandstone varies due to the minerals and elements in the stone. The rock type and cement will also have a bearing on the colour and the grain size of the rock, as well as the durability. It has an overall factor in the usefulness of the stone as demonstrated in the structure of Queen Mary's Bath House. Minerals have differing properties as cementing agents in sandstone. Sandstone with calcite is prone to erode when in contact with acid. One such acid is 'rain'. Sandsones containing clay or gypsum are poor building stones as the sand rubs off easily. Silica based sandstone is durable and hard.
Impurities such as iron oxides, manganese oxides and other trace elements cause contrasting colour and layering within the rock. These can be red, yellow, orange, purple, grey, black and even green. In rock containing iron, the oxygen reacts in the rock causing the range of red colours forming Fe3 (iron oxide). This is an oxidation process. The same applys to manganese, causing manganese oxide, creating a stone with a purple hue. Large nodules of 24% manganese have been found on many ocean floors.
Identifying Clastic Sedimentary Rocks
A grain size less than 1/16mm, if smooth, is refered to as mudstone. if gritty, it is siltstone. The mineral composition of both is clay. If there are fossils present the rock name is fossilerous mudstone or siltstone.
If the grain is between 1/16mm - 2mm, the name of the rock is sandstone. The mineral composition, a combination of: quartz, sandstone,, rock particles, mica and feldspar. If fossils are present, the mineral composition is fossilerous sandstone.
If the grain size is greater than 2mm, coarser than sand, with roundd grains, the name of the rock s conglomerate. If the grains are angular, the name of the rock is breccia. The mineral composition for both would be quartz, feldspar and rock fragments. If fossils were present the rock would be called fossilerous conglomerate or breccia.
The grain size will have an effect on the texture of the stone. It is the most common type of sedimentary rock and is a useful building material.
1. Choose a sample stone and describe how it looks and feels. What is the grain size?
2. On the structure, immediately above the black window frame, what mineral has influenced the curved purple stone? One row below and again in the third row below the window frame, what is the mineral responsible for the red colour?
3. Given the information in the text and the hardness of the stone, would you say the sandstone contained gypsum, calcite or silica?
4. What is a sandstone?
5. What date is written on the blue sign adjacent to the railings?
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