The majority of crystal occurrences, however, are not part of well-formed single crystals but are found as crystals grown together in aggregates. The external shape (habit) of well-developed crystals can be visually studied and classified according to the various crystal systems that span the 32 crystal classes. SpaceNext50 Britannica presents SpaceNext50, From the race to the Moon to space stewardship, we explore a wide range of subjects that feed our curiosity about space!.Learn about the major environmental problems facing our planet and what can be done about them! Saving Earth Britannica Presents Earth’s To-Do List for the 21st Century.Britannica Beyond We’ve created a new place where questions are at the center of learning.100 Women Britannica celebrates the centennial of the Nineteenth Amendment, highlighting suffragists and history-making politicians.COVID-19 Portal While this global health crisis continues to evolve, it can be useful to look to past pandemics to better understand how to respond today. Student Portal Britannica is the ultimate student resource for key school subjects like history, government, literature, and more.Britannica Explains In these videos, Britannica explains a variety of topics and answers frequently asked questions.Demystified Videos In Demystified, Britannica has all the answers to your burning questions.This Time in History In these videos, find out what happened this month (or any month!) in history.#WTFact Videos In #WTFact Britannica shares some of the most bizarre facts we can find.Britannica Classics Check out these retro videos from Encyclopedia Britannica’s archives.Agate-quartz crystallizing around wall rock - fine-grained volcanic rock with small infillings, amygdules, of agate. Penn State News – an interview with Peter Heaney. A piece of host rock – a fine-grained volcanic(?) – hosts first generation agate crystallization.īrown, N.M., 2001, How do agates form?. The pyramidal terminations of largest quartz crystals are infused a golden brown hue suggesting citrine. This final image shows a more coarsely crystalline fragment (crystal length to 3 cm) from a second fragment of the same rock. Top showing multiple generations of quartz crystals and dark iron oxide basal agate-quartz. The mineral mass ends with terminating 6-sided pyramids 1- to 5-mm in diameter. Some quartz crystal have a light purple hue suggesting amethyst. The overlying layer has more of a cloudy, milky quartz appearance with some hint of purplish amethyst and even yellow citrine. The lower or first-formed layer comprises clear crystalline quartz. The coarse crystal zone comprises two generations of hexagonal quartz crystal with an aggregate thickness of 2-3 cm thick. The agate layer and coarser quartz crystals are separated by an irregular surface. Agate-Quartz with basal goethite-hematite(?). 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. Precipitation of this exceedingly fine-grained mass probably occurs rapidly. It piled up about 6-9 mm of featureless agate locally punctuated by needles of coarser crystals growing perpendicular to the wall oxides. The cryptocrystalline agate formed first on a thin bed of goethite/hematite(?). The specimens shown here illustrates a complex crystallization history beginning with the formation of agate and terminating in coarse, hexagonally-terminated quartz crystals. 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. 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.Įxamining agate under a powerful microscope shows the presence of fibrous crystals that typically nucleate on the outer wall and radiate inward. Agate is prized for its lustrous variegated banding. Agate is a cryptocrystalline mineral comprising quartz (90%+) and moganite (~10%), whose composition is identical to quartz with a different crystal structure.
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