Unlike the feldspars or hornblende, pyroxenes show mediocre cleavages that are hard to spot. Augite is most easily confused with hornblende. Hornblende has highly reflective cleavages whereas augite cleavages are dull. Hornblende tends to form elongate rectangular crystals whereas augite crystals tend to be blocky.
Q. Which type of igneous rock would have the least amount of silica?
Ultrabasic: rocks that are usually less than 45% silica (mostly mafic minerals such as olivine and pyroxene), e.g. peridotite.
Q. Is mafic light or dark?
Chemically, mafic rocks are enriched in iron, magnesium and calcium and typically dark in color. In contrast, the felsic rocks are typically light in color and enriched in aluminium and silicon along with potassium and sodium. The mafic rocks also typically have a higher density than felsic rocks.
Q. What causes some igneous rocks to have a glassy texture?
Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian.
Q. What is the most important visual characteristic of a glassy texture?
If a rock looks like a block of (colored) glass, with no visible mineral crystals, it has a glassy texture. Superficially, a glassy texture suggests cooling that was so extremely fast that no crystals could form. However, composition is also vitally important.
Q. What is the name for a glassy igneous rock?
A glassy igneous rock consists of dense volcanic glass. The very porous, foamy variety is called Pumice. The most important rock type belonging to this group is: obsidian.
Q. What are the factors that affect texture of igneous rocks?
Texture of igneous rocks defines the shape, size and mutual relationship of the constituent minerals. The texture in igneous rocks depends on the following four factors: i) Viscosity of magma ii) Rate of cooling iii) The order of crystallization of the constituent minerals.
Q. What are the factors explaining texture?
Explanation: The term texture is defined as the mutual relationship of different mineralogical constituents in a rock. It is determined by the size, shape and arrangement of these constituents within the body of the rock.
Q. What does a porphyritic texture mean?
A porphyritic texture displays minerals in two distinct size populations: one or more minerals are consistently larger than the rest of the minerals in a rock. The extra large mineral grains are called phenocrysts.
Q. What factors can affect the texture of a sedimentary rock?
The Rock Cycle, Igneous Rock
| Question | Answer |
|---|---|
| What do the size, shape, and positions of the grains that make up a rock determine? | the rock’s texture |
| What factors can affect the texture of a sedimentary rock? | the size of the grains that make up the rock |
