Cleavage and FractureoCleavage is a crystal's ability to break along weak bonds within the crystal structure. The break results in a smooth surface. Calcite exhibits rhombic cleavage, which means that it breaks along three planes of weakness that create a rhombic shape for the crystal. Quartz does not have strong cleavage but can fracture across the crystal, leaving a rough surface on the broken crystal. Quartz fractures are described as conchoidal when the fractured surface exhibits a swirl pattern on the stone.Chemical CompositionoCalcite is a calcium carbonate mineral while quartz is a silicon dioxide crystal. Visually, you cannot tell the difference in the mineral composition, but you can perform a test to determine if the crystal you have is calcite. Calcium carbonate reacts with an acid to produce bubbles on the surface of the crystal. To test your sample, drop dilute hydrochloric acid, lemon juice or vinegar onto the sample and watch for bubbles. Quartz does not react to a dilute acid.In general you distinguish them by oColoroStreak: The streak of a mineral refers to the color of a powder produced by pulverizing the mineral. You can obtain a streak by scraping the mineral against an unglazed ceramic plateoLuster: Luster refers to the way a mineral surface scatters light. Geoscientists describe luster by comparing the appearance of the min- eral with the appearance of a familiar substance.oHardnessoSpecific gravity: Specific gravity represents the density of a mineral, as represented by the ratio between the weight of a volume of the mineral and the weight of an equal volume of water at 4°C.For example, one cubic centimeter of quartz has a weight of 2.65 grams, whereas one cubic centimeter of water has a weight of 1.00 gram. Thus, the specific gravity of quartz is 2.65.oCrystal habit: The crystal habit of a mineral refers to the shape of a single crystal with well-formed crystal faces, or to the character of an aggregate of many well-formed crystals that grew together as a group
oSpecial propertiesoFracture and cleavage: Different minerals fracture(break) in different ways,depending on the internalarrangement of atoms. IIgneous intrusive structures (dykes, sills,plugs, necks, etc.) and Mt. RoyalIntrusive igneous rocks—cool outof sight, undergroundoMuch greater volume than extrusive igneous rocksoCooling rate is slower than for extrusives.oLarge volume magma chambersoSmaller volume tabular bodies or columnsIntrusive settingsoMagma invades preexisting wall rock bypercolating upward between grains.forcing open cracks.oThe wall rock—magma-intrusive contact reveals high heat.Baked zone—rim of heat-altered wall rockChill margin—rim of quenched magma at contactoMagma invades colder wall rock, initiatingthermal (heat) metamorphism and melting.