Apatite in Cheviot igneous rocks
Apatite is an early mineral to crystallise and its well-formed, prismatic crystals often appear as an accessory mineral in Cheviot rocks as inclusions in other minerals that crystallised at the same time or later in the magma’s history.
Al-Hafdh analysed two grains of apatite by electron microprobe, one from a sample of medium-grained porphyritic, and one from a course-grained porphyritic. Although both contained the expected amount of phosphorus and calcium only one contained chlorine.
His results do not contain data for fluorine or hydroxide content, so the composition diagram below shows the average relative amounts of phosphorus, calcium and chlorine in his samples of apatite.
Composition Diagram for Cheviot Apatite
Identification
The small apatite crystals in Cheviot rocks cannot be seen in hand-specimen.
In thin section, apatite crystals show moderate relief. They appear as long, colourless crystals which tend to show a round or hexagonal outline in cross-section. Cleavage is usually absent or poor although cracks can be seen across the length of crystals
With crossed polars, apatite has a birefringence lower than both quartz and feldspar presenting a mid-grey maximum interference colour. Crystals seen in a basal view are virtually isotropic, and appear black.
Long crystals show straight extinction.
Photomicrographs of Cheviot Apatite
Locality: Hedgehope Hill, Northumberland.
This euhedral, elongate apatite crystal has been included in a much larger feldspar crystal. Its low to moderate relief outlines its colourless interior against the surrounding mineral.
Locality: Hedgehope Hill, Northumberland.
Here the low birefringence of the same crystal results in a mid-grey interference colour.
Locality: Hedgehope Hill, Northumberland.
Two colourless grains of apatite, again included in feldspar. Each shows one of the random cracks that commonly cut across the length of these crystals.
Locality: Hedgehope Hill, Northumberland.
With polars crossed, the low interference colour of apatite is very evident in contrast to the adjacent biotite.
Return to list of minerals in Cheviot rocks