Name/Title
Agate (Fire)Entry/Object ID
2013.2.5Description
Chemical Composition: SiO2 *nH2O
Crystal System: Orthorhombic System
Description: Common Name: Agate (Fire)
Chemistry: SiO2
Class: Silicates
Location: Arizona
Description: Small, irregular convex with light tan matrix with convex polishred face. Brown translucent with a narrow tan surrounded on outside of brown.
Physical Characteristics:
•Color is as variable as the spectrum, but clear quartz is by far the most common color followed by white or cloudy (milky quartz). Purple (Amethyst), pink (Rose Quartz), gray or brown to black (Smoky Quartz) are also common. Cryptocrystalline varieties can be multicolored.
•Luster is glassy to vitreous as crystals, while cryptocrystalline forms are usually waxy to dull but can be vitreous.
•Transparency: Crystals are transparent to translucent, cryptocrystalline forms can be translucent or opaque.
•Crystal System is trigonal; 3 2.
•Crystal Habits are again widely variable but the most common habit is hexagonal prisms terminated with a six sided pyramid (actually two rhombohedrons). Three of the six sides of the pyramid may dominate causing the pyramid to be or look three sided. Left and right handed crystals are possible and identifiable only if minor trigonal pyramidal faces are present. Druse forms (crystal lined rock with just the pyramids showing) are also common. Massive forms can be just about any type but common forms include botryoidal, globular, stalactitic, crusts of agate such as lining the interior of a geode and many many more.
•Cleavage is very weak in three directions (rhombohedral).
•Fracture is conchoidal.
•Hardness is 7, less in cryptocrystalline forms.
•Specific Gravity is 2.65 or less if cryptocrystalline. (average)
•Streak is white.
•Other Characteristics: Striations on prism faces run perpendicular to C axis, piezoelectric (see tourmaline) and index of refraction is 1.55.
•Associated Minerals are numerous and varied but here are some of the more classic associations of quartz (although any list of associated minerals of quartz is only a partial list): amazonite a variety of microcline, tourmalines especially elbaite, wolframite, pyrite, rutile, zeolites, fluorite, calcite, gold, muscovite, topaz, beryl, hematite and spodumene.
•Notable Occurrences of amethyst are Brazil, Uraguay, Mexico, Russia, Thunder Bay area of Canada, and some locallities in the USA. For Smoky Quartz; Brazil, Colorado, Scotland, Swiss Alps among many others. Rose Quartz is also wide spread but large quantities come from brazil as do the only large find of Rose Quartz prisms. Natural citrine is found with many amethyst deposits but in very rare quantities. Fine examples of Rock crystal come from Brazil (again), Arkansas, many localities in Africa, etc. Fine Agates are found in, of course, Brazil, Lake Superior region, Montana, Mexico and Germany.
•Best Field Indicators are first the fact that it is very common (always assume transparent clear crystals may be quartz), crystal habit, hardness, striations, good conchoidal fracture and lack of good cleavage.
Fracture: Concoidal
Hardness: 5 Apatite
Luster: Vitreous
Rock Type: Sedimentary
Specific Gravity: 2-2.5
Streak: WhiteCollection
RAGM Mineral CollectionDimensions
Width
1-1/4 inDepth
1/4 inLength
3/4 inLocation
Location
Cabinet
Case# 70Wall
CenterBuilding
Freida Smith HallCategory
Permanent
