Chapter 3 Mineral naturally ocurring formed by geologic processes solid crystalline structure definable chemical composition inorganic An inorganic naturally occurring deposit of liquid mercury Hg not be considered a mineral Crystal single continuous piece of crystalline solid If a crystal grows in a liquid salt crystal growing in evaporating seawater olivine crystal grow ing in a basalt lava lake the solid will probably be surrounded completely by crystal faces un less a growing grain in the sedimentary environment evaporating seawater is resting on the seabed or or a crystal growing in a cooling magma docks with other nearby crystals that are floating in magma The angles between crystal faces can be used to infer something about the kinds of symmetry that a mineral structure contains Symmetry about a plane reflection operation mirror symmetry in which a left handed starting object reflects an equal distance on the other side of the plane to produce and equivalent but right handed version of the starting object If the reflected equivalent is shifted parallel to the plane translation operation instead of a mirror you have a glide plane Symmetry about an axis rotation operation a left handed starting object is rotated ei ther 360o 180o 120o 90o or 60o to form equivalent but rotated left handed objects These rotation axes called 1 fold 2 fold 3 fold 4 fold and 6 fold axes project all the way through a pattern so in order for a 2 fold axis to be legal at a particular location all aspects of the geometry have to have equivalents located 180o away from the starting positions Symmetry about a point inversion operation each point of a left handed starting ob ject is projected through an inversion point also called a center of symmetry along a straight line and ends up an equal distance away on the opposite side of the inversion point The equivalent object is upside down and right handed compared to a starting left handed object There are only 32 ways of combining mirrors rotation axes and center of symmetry in a single structure an impressively small number of options If you add in translation as an additional symmetry operation the number goes to 220 but still impressively small if you think that all of the 4 000 or so known minerals have a structure that can be described in one of a limited number of ways 1 For the black and white pattern on the left how many mirrors are there and where would you put it them Only consider mirrors that are perpendicular to the page Are there any rotation axes in this pattern or not 2 For the red and white pattern on the right do the same thing but also consider whether there are glide planes in addition to the mirrors and where it they are if so Are there any rotation axes in this pattern or not In this pattern find 2 fold and 4 fold rotations mirrors and glide planes Pay particular atten tion to the interior details of each flower because they will help you decide whether there can or can t be diagonal mirrors running across the pattern 3 Patterns that display an over and under or weave pattern may or may not have mirror symmetry but rotations are possible Formation and destruction of crystals o Growth from solution salt water evaporates to form crystals of halite gypsum o Growth from cooling magma basalt andesite rhyolite magmas cool to form pri marily silicate minerals refer to Bowen s Reaction Series in igneous chapter olivine pyroxene amphibole mica quartz alkali feldspar plagioclase feldspar o Growth from cooling gas sublimation hot gases from fractures at volcanoes can precipitate mineral crusts on the sides or in the crater of a volcano sulfur crystals at some volcanoes form this way o Growth in the solid state due to changes in P or T metamorphic rocks form when minerals that are stable at one set of P T conditions are changed to new structures that are more stable at higher or lower T or higher or lower P o Growth by living organisms life processes How do minerals break down dissolving atoms or ions separate from crystal face and are surrounded by solvent molecules melting heating at which thermal vibration of atoms or ions in lattice break chemical bonds metamorphic change or crushing in the sedimentary environment Color how a mineral interacts with light the color you see represents the wavelengths the min eral does not absorb Luster how a mineral surface scatters light metallic vs nonmetallic glassy pearly Hardness ability to resist scratching resistance of bonds to being broken Diamond is the hard est mineral Use Mohs hardness scale Specific Gravity density of a mineral ration between weight of a volume of the mineral and weight of an equal volume of water Crystal habit shape of a single crystal with well formed faces depends on internal arrangement of atoms Silicate silicate minerals make up most of continental crust and all of oceanic Most common minerals minerals like pyroxene and amphibole that crystallize as single chain or double chain struc tures form crystals that have a long dimension parallel to the chain and other dimensions of the crystal are smaller So there is a prismatic shape that reflects the underlying crystal architec ture Micas sheet silicates crystallize into flat book shaped grains whose shapes reflect the sheet structure that they crystallize in How do people use minerals that fall into the gemstone category What kinds of properties help to establish value o Geologic rarity o Size of an individual sample carat 0 2 grams when referring to weight o Freedom from small inclusions of other crystals and freedom from cleavage or other fractures diamond graders use terms like VVS1 VVS2 very very slight inclusions 1 2 etc to help describe this aspect of quality o Saturation of color for colored stones like amethyst ruby emerald sapphire o Purity of color ditto o Perfection of the facets or other shaping of a stone o Manufactured economic rarity diamond is not an especially rare mineral compared to many of the approximately 4 000 known minerals but if a few buyers control at least half the output of new diamonds mined annually they can control how slowly enough are re leased onto the world s markets to keep a high value in place o Fashion garnets were in great demand worldwide due to Queen Victoria s use of them the demand changed quickly when leadership of the British Empire changed following her death areas that specialized in mining and making garnet jewelry were very hard hit during that transition The 4 C s of diamond