Amygdaloidal Basalt

Amygdaloidal basalt is a variety of extrusive igneous rock which has a fine crystalline texture to it and an often black or very dark brown colouration and sometimes with a hint of green. Like regular basalt, it forms by rapid cooling of lava which often has a low viscosity (runny/fluid). They are normally basic in composition and rich in dense, iron and pyroxene minerals but is often low in silica minerals such as quartz, giving the rock a dark colour and density. However, in some gaseous, violent eruptions, dissolved gases in the magma can form bubbles and as the pressure is released upon eruption, the  bubbles can become trapped within the solidified rock making it partially porous. Overtime, water or other liquid solutions produced by volcanic activity can percolate (pass) through the rock due to the porosity and result in the formation of crystals in said pores (cavities). These minerals are often hydrothermal in nature and include quartz, calcite and various species of zeolites, these infilled cavities are known as amygdales, hence the name, amygdaloidal basalt and give the often dark rock a white speckled appearance. These specimens come from Ireland, which formed during the Tertiary period approximately 60 million years ago and formed part of the Antrim Lava group which also includes the famous Giant’s Causeway. They are suitable for collectors and higher level education to show the fact that not all rocks of a similar composition will look the same and will vary depending on type of eruption, chemistry and other variable factors. The samples are available in 3 sizes and come supplied in a card tray with an information label.