Examining agate growth pattern

Agate is a cryptocrystalline mineral comprising quartz (90%+) and moganite (~10%), whose composition is identical to quartz with a different crystal structure. Agate is prized for its lustrous variegated banding. Every bit as interesting is the silica solution from which the agate is derived and the manner in which agate precipitates on the host rock.

Examining agate under a powerful microscope shows the presence of fibrous crystals that typically nucleate on the outer wall and radiate inward. Agates form as silica-rich fluids percolate though porous rocks – most commonly volcanic rocks -  typically precipitating on the void walls – e.g., vesicles in basalt lavas or a void in a sedimentary rock.

The specimens shown here illustrates a complex crystallization history beginning with the formation of agate and terminating in coarse, hexagonally-terminated quartz crystals. The cryptocrystalline agate formed first on a thin bed of goethite/hematite(?). It piled up about 6-9 mm of featureless agate locally punctuated by needles of coarser crystals growing perpendicular to the wall oxides. Precipitation of this exceedingly fine-grained mass probably occurs rapidly. Formation of the coarse quartz crystals is a slower process permitting crystal habit to establish and grow into terminating pyramids characteristic of well-formed quartz.

The agate layer and coarser quartz crystals are separated by an irregular surface. The coarse crystal zone comprises two generations of hexagonal quartz crystal with an aggregate thickness of 2-3 cm thick. The lower or first-formed layer comprises clear crystalline quartz. The overlying layer has more of a cloudy, milky quartz appearance with some hint of purplish amethyst and even yellow citrine.  Some quartz crystal have a light purple hue suggesting amethyst. The mineral mass ends with terminating 6-sided pyramids 1- to 5-mm in diameter.  Dark inclusions occur in some crystals.

This final image shows a more coarsely crystalline fragment (crystal length to 3 cm) from a second fragment of the same rock. The pyramidal terminations of largest quartz crystals are infused a golden brown hue suggesting citrine. A piece of host rock – a fine-grained volcanic(?) – hosts first generation agate crystallization.

Resources:
Brown, N.M., 2001, How do agates form?. Penn State News – an interview with Peter Heaney.