The material that would normally form mineral crystals does not have time to form a crystalline structure because of very rapid cooling after volcanically erupting into the air or onto the surface. Obsidian is a volcanic glass.
Obsidian cools so quickly the mineral grains do not have time to form.
Obsidian is a type of volcanic glass that lacks a crystalline structure, so it does not contain mineral grains. It forms from quickly cooling lava rich in silica, resulting in a smooth, glassy texture without distinct mineral crystals.
The material that would normally form mineral crystals does not have time to form a crystalline structure because of very rapid cooling after volcanically erupting into the air or onto the surface. Obsidian is a volcanic glass.
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Obsidian, which is volcanic glass, is a good example.
Granite has large mineral grains compared to basalt, obsidian, and pumice. It is an intrusive igneous rock formed from the slow crystallization of magma beneath the Earth's surface, allowing large crystals to develop. In contrast, basalt has smaller grains due to its rapid cooling, while obsidian is volcanic glass with no visible grains, and pumice is a light, porous rock with small, frothy bubbles.
Glassy textuered rocks, such as Obsidian.
Actually, obsidian doesn't have grains at all. Technically obsidian is a glass, which means it has no internal structure. When geologists refer the the grain size of a rock, they mean how big the crystals that make it up are. Since obsidian has no crystals, it therefore has no grain size.
Granite has large mineral grains compared to obsidian, basalt, and pumice. Granite is an intrusive igneous rock that forms deep within the Earth's crust, allowing for the growth of large mineral crystals due to slow cooling. Obsidian, basalt, and pumice are extrusive igneous rocks that cool quickly at the Earth's surface, resulting in smaller mineral grains or a glassy texture.
Pumice normally doesn't have any grains at all. It would be very unusual for it to have grains. Just like Obsidian, Pumice is cooled too fast to have any grains in it. One of the biggest differences between them is that Pumice is cooled with pockets of air while Obsidian has a glassy look.
Grains.
most rocks from what i know contain minerals especially sedimentary rocks. and don't all rocks eventually become sedimentary rocks so really don't they all contain minerals to some extent :) hope it helped (by the way i am only a gcse student)