Amphibole in Cheviot Igneous Rocks
Amphiboles are a complex family of hydrous silicate minerals. They take the general formula X7Si8O22(OH)2where X can be a combination of calcium (Ca), Iron (Fe), magnesium (Mg) or sodium (Na). The commonest group of amphiboles are often given the generic name ‘hornblende’ where the chemical formula approximates to Ca2(Mg,Fe)5Si8O22(OH)2, and some of the silicon (Si) is replaced by aluminium (Al). The sodium rich amphiboles such as glaucophane or riebeckite are much less common.
Composition diagram for the amphibole group
Composition diagram for Cheviot actinolite
Al-Hafdh’s analyses of Cheviot amphiboles revealed there was little aluminium (1.47%) present but plenty of calcium (12.39%) magnesium(15.77%) and iron (13.03% ).
His electron probe analyses of eight samples of the Cheviot green amphibole revealed that the relative cation content of calcium, magnesium and iron was 27.%, 49.0% and 23% respectively.
The tertiary diagram below plots these relative amounts and confirms that the amphibole in Al-Hafdh’s samples were actinolite.
Identification
Most, if not all actinolite in Cheviot igneous rock occurs as a secondary mineral associated with hydrothermal alteration. In the hand specimen, actinolite is dark green to black.
In thin section and plain polarised light, it has a light grey to light green in colour and sometimes shows a feint pleochroism (colourless to pale green).
It occurs as bladed or acicular crystals and in fibrous masses that have high relief and, if the sections are basal, distinctive 124o intersecting cleavages. It often develops at the margins of pyroxene crystals through a process termed uralitisation. Some Cheviot actinolite is itself mantled or replaced or pseudomorphed by chlorite.
When viewed under the microscope with crossed polarising filters, actinolite’s has moderate to mid-second order interference colours with inclined extinction.
Photomicrographs of Cheviot amphibole
Location: Hedgehope Hill, Northumberland
Section viewed in plane polarised light revealing the distinctive 124 degree cleavage.
Location: Coldlaw Burn, Northumberland
In the centre field, a pair of pale green slightly pleochroic actinolite crystals together with biotite feldspars and quartz.
Location: Coldlaw Burn, Northumberland
Under crossed polars, the same crystals reveal twinning and high second order green interference colours with inclined extinction. These may be two actinolite pseudomorphs of pyroxene.
Return to list of minerals in Cheviot rocks