Search results
Results from the WOW.Com Content Network
Greywacke or graywacke (German grauwacke, signifying a grey, earthy rock) is a variety of sandstone generally characterized by its hardness (6–7 on Mohs scale), dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or sand-size lithic fragments set in a compact, clay-fine matrix.
The types and abundance of minerals in a rock are determined by the manner in which it was formed. Most rocks contain silicate minerals, compounds that include silica tetrahedra in their crystal lattice, and account for about one-third of all known mineral species and about 95% of the earth's crust. [6] The proportion of silica in rocks and ...
Sylvinite – Sedimentary rock made of a mechanical mixture of sylvite and halite; Tillite – Till which has been indurated or lithified by burial; Travertine – Form of limestone deposited by mineral springs; Tufa – Porous limestone rock formed when carbonate minerals precipitate out of ambient temperature water
Limestone forms when these minerals precipitate out of water containing dissolved calcium. This can take place through both biological and nonbiological processes, though biological processes, such as the accumulation of corals and shells in the sea, have likely been more important for the last 540 million years.
Shelly limestone is a sedimentary rock because it is made up of fragments. To be shelly, it is full of broken shells which are "glued" together with calcite. Calcium carbonate often makes up around 10% of the volume, whilst many varied sized shells from granular to very large pebbles. [clarification needed] Its color is gray.
Talc dominantly forms from the metamorphism of magnesian minerals such as serpentine, pyroxene, amphibole, and olivine, in the presence of carbon dioxide and water. This is known as "talc carbonation" or "steatization" and produces a suite of rocks known as talc carbonates .
These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals.
The chemistry of clay minerals, including their capacity to retain nutrient cations such as potassium and ammonium, is important to soil fertility. [15] Clay is a common component of sedimentary rock. Shale is formed largely from clay and is the most common of sedimentary rocks. [11] However, most clay deposits are impure.