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Rocks and Minerals

Gems and Minerals: Spectacular minerals

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Quartz variety Smoky Quartz

Satin Spar Gypsum

Spectacular Minerals

Galena (silver) and Calcite (white)Beautifully crystallized well-shaped crystals typically are exceptionally rare and occur only when crystal growth is unimpeded by growth of other crystals.  This is most likely to occur when mineral nucleation rates are low and the few crystals that do nucleate grow in gas- or liquid-filled cavities, fractures or spaces.  Most igneous minerals crystallize from liquid magma, but crystallization of other later formed minerals and continued chemical reaction with the surrounding melt typically interfere with the shape and stability of early well-formed crystals.  

Cavities or spaces are produced by expanding of gases or volatile-rich solutions as a result of decompression (decreasing pressure) in igneous rocks emplaced or erupted near or at the Earth's surface.  These holes are called miarolitic cavities in plutonic rocks.  Similar crystal-lined cavities or spaces also occur in lavas and pyroclastic volcanic rocks.  The cavities and fractures that occur in sedimentary rocks are typically caused by localized solution of the soluble carbonate or evaporite rock.  If the cavities are beneath the water table or partially filled with water, precipitation of well-formed Botryoidal Azurite minerals may occur.  Botyoidal mineral aggregates are typical of precipitation of minerals from evaporating aqueous solutions above the water table.  The botyoidal mass of the copper mineral azurite (shown on the left) probably formed in an air-filled cavity.  Solution and secondary recrystallization are important processes in the supergene enrichment of copper and other economic ores.  Solution of unstable minerals creates void space that may be partially filled in by well-formed or botryoidal aggregates of secondary minerals precipitated by low temperature aqueous solutions, depending upon whether the cavity is water- or air-filled.  This is most likely to occur in arid or semi-arid climates.   

Solution of metamorphosed carbonates can also occur, but the creation of open space in metamorphic rocks is more typically the result of small volumetric changes resulting through the replacement of the original mineral assemblage by a lower volume, denser mineral assemblage under conditions of increasing pressure or fracturing.  Although metamorphic minerals exclusively crystallize in the solid state, a small amount of water increases reactivity, diffusion rates, and crystal growth rates.  There are some metamorphic minerals such as garnet, staurolite, and andalusite that often crystallize in extremely well-formed crystals.

Replacement deposits of minerals may occur as a result of contact metamorphism or the action of hydrothermal Sphalerite (dark gray), Chalcopyrite (brass gold), Quartz (white) solutions infiltrating unstable rocks.  The beautiful crystals of benitoite, neptunite, and natrolite (upper left photograph) occur in veins in a brecciated (fractured) serpentinite near the San Benito River in California.  Benitoite (BaTiSi3O9) and neptunite (KNa2Li(Fe,Mn)2TiO2(Si4O11)2) only occur in a few other localities in the world, presumably because of their unusual compositions.  At low temperature this can occur in sedimentary rocks.  Mississippi Valley Type replacement mineralization occurred when metal-bearing brines from sedimentary basins react with carbonate melts.  The sphalerite, chalcopyrite, and quartz in the specimen to the right may have formed in this way.  They may also have precipitated from a higher temperature hydrothermal solution.

QuartzWell-formed crystals can occur in hydrothermal solution veins if vugs or open fluid-filled spaces exist after crystallization ceases.  The well-shaped smoky quartz crystal in the right top corner of this page precipitated from silica-bearing hydrothermal solutions.  The same is also likely to be the case for the well-formed transparent, colorless quartz crystals shown to the left.  The orange coloring on parts of this specimen is due to iron-oxide staining on the surface of the quartz.




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