Physical Properties of Minerals PDF

2024 Physical Properties of Minerals Color Hardness (Moh’s scale) Luster Fluorescence Transparency Magnetism...

2024 Physical Properties of Minerals Color Hardness (Moh’s scale) Luster Fluorescence Transparency Magnetism Habit Reactivity or solubility Crystal morphology/symmetry Radioactivity Cleavage, fracture, parting Odor, feel, taste Streak color Density Inclusions, exsolutions, alteration 51 Properties – Transparency Transparent: Visible light passes through the material Translucent: Visible light passes through, though not all light is transmitted. Opaque: No light goes through. Transparent calcite Translucent ulexite Opaque hematite showing double refraction 52 26  2024 Properties – Color Color in minerals is caused by the selective absorption of certain wavelengths of visible light. The major factors responsible for the production of color in minerals are: 1. The presence of a color-causing (chromophore) element that is essential to the mineral composition 2. The presence of a minor color-causing (chromophore) chemical impurity 3. Crystal structure 4. Crystal structure defects 5. Inclusions of very fine impurities of another mineral or substance 6. Directional optical effects caused by thin film interference due to fine lamellar intergrowths (e.g., labradorite) First two are most common. Chromophore: An atom of group whose presence is responsible for the color of a compound. 53 Color – Crystal Field Transitions Transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, Cu) have partly filled inner 3d orbitals The electrons in these orbitals can be excited by energy in the visible spectrum. The corresponding energy change equals the energy of light absorbed. Fritsch & Rossman 1987 (Color in Gems; Gems & Gemology) 54 27  2024 Color – Charge Transfers Transfers of electrons between O2- and metal cation, or between two metal ions of different charge separated by O2- can also cause color (e.g., purple in amethyst, blue in sapphire). 55 Color – Color Centers Caused by structural defects from a missing ion. 56 28  2024 changingmineral folor Color – Chromophore (Essential element) “Olivine” (Forsterite-Fayalite series) Fe2+ causes green color in lower concentrations, dark brown in high concentrations 80% Typical mantle olivine ~ Mg1.6Fe0.4SiO4 “End-member” Forsterite “End-member” Fayalite Mg2SiO4 Fe2SiO4 (J. McAvoy photo) (R. Luetcke photo) 57 olivineisnot amineralseries Te Color – Chromophore (Minor/trace element) Single elements: For example Cr3+ substituting for Al3+ in red spinel (MgAl2O4) 901 Khan irocks.com 58 29  2024 Color – Chromophore (Minor/trace) Single elements: For example Co2+ substituting for Mg2+ in blue spinel (MgAl2O4) color causes colorless Belley & Groat 2019 59 Color – Chromophore (Minor/trace) Chromophore combinations: E.g., Co2+ + Cr3+ in purple spinel (red + blue are transmitted) Hausing Cr3+ Cr3+ Co2+ Belley & Palke 2021 60 30  2024 Color – Chromophore (Minor/trace) Ion pair transitions: Fe2+ - O – Ti4+ (coupled Fe2+ Ti4+ substitution in sapphire Al2O3) GIA.edu 61 Color - Structure In some cases, structure can affect the color of a mineral. A mineral may appear different in colors or color intensities when viewed at different crystallographic orientations – this is called pleochroism Pleochroism in cordierite 62 31  2024 Color - Impurities Nano to micro-scale particle inclusions of a different material strange Rare Red hematite in amethyst Cu nanocrystals in plagioclase (irocks.com) (C.Rose) 63 Color f - Optical effects Moresodium calcium Thin film interference in labradorite and “peristerite” plagioclase. Lamellar intergrowths of plagioclase with slightly different An:Ab content produces thin layers where this can take place. Weidlich & Wilkie 2009 64 Labradorite Notspecies is afeldspar Feldspars movetolayers alternatemoreaffite Pearinsterite albite.is 32  2024 Luster Description of surface-light interaction Crystallinity and refractive index affect luster Somewhat subjective descriptive terms 65 Luster Adamantine – sparkles like diamond (high refractive index! Zircon, diamond) Metallic - the look of metals Submetallic - a poor metallic luster, opaque but reflecting little light Vitreous/glassy - the most common luster, it simply means the look of glass Dull - just a non-reflective surface of any kind Earthy - the look of dirt or dried mud Greasy - the look of grease Pearly - the look of a pearl Resinous - the look of resins such as dried glue or chewing gum Silky - the look of silk, similar to fibrous but more compact Waxy - the look of wax 66 33  2024 Habits External shape of an individual crystal or group of crystals Dependent on crystal structure, growth conditions, mineral grain to grain contacts, etc. Many descriptive terms! welldevelopedboundrys notPerfect shapepresentbut norealshapebloblike 67 Individual crystal habit terms Acicular (needle-like) Prismatic Bladed Platy Tabular Blocky Equant (equally dimensioned) 68 34  2024 Acicular Bladed 69 Tabular Prismatic Fiberous 70 35  2024 cubicsystem 71 Eavant Blockey Aggregate habit terms Botryoidal Dendritic (tree-like) Granular (like sugar granules stuck together) Peridite crystal Lamellar clusterofbladed Massive Nodistinthabit at Point Radiating conected one 72 36  2024 Dendritic Maginise Botryoidal gothite 73 milleride Lameller radiating Nickelsulfide 74 37  2024 Massive Serpintine Granular Peridite 75 Habits - Twinning Twin: Two or more crystals grown in symmetrical but opposite orientations Notstriations Polysynthetic twinning catnaotwinning (Plagioclase Feldspar) Carlsbad twin (K-Feldspar) lesscommenir.am 76 38  2024 Habits - Twinning Fluorite Staurolite Twinned cassiterite (Tin oxide) Aniron twins cross 2Fusedtogether Penetration Twins 77 Cleavage Smooth, flat breaks that occur when a mineral is broken. Cleavage is preferential breaking that occurs on planes of structural weakness. These planes are inherent to the atomic structure of the mineral. Cleavage is: 1. Regular and reproducible 2. Parallel to actual or possible crystal faces 3. Match mineral symmetry Cleavage is not always present! 78 39  2024 Cleavage We can describe how easily cleavage is produced in a certain mineral species: Perfect Imperfect Good Distinct Indistinct Poor 79 Cleavage Cleavage that occurs perpendicular to the c axis of a crystal is called basal cleavage. Basal cleavage in sheet silicates like mica 80 40  2024 Cleavage Cleavage that occurs parallel to the c axis of a crystal is called prismatic cleavage. Typical in amphiboles and pyroxenes 81 Cleavage Multiple cleavages can produce geometric polygons Octahedral cleavage (e.g., fluorite) Cubic cleavage (e.g., halite) Rhombohedral cleavage (e.g., calcite, dolomite, siderite) 82 41  2024 Cleavage Cleavage can be crucial to differentiating fairly similar looking mineral groups, like pyroxenes vs. amphiboles Pyroxene (~90 degree cleavage) Amphibole (~60/120 degree cleavage) 83 Parting NOT cleavage! Can look like cleavage when present. Most common in pyroxenes. You may not encounter it often. Mineral breakages along specific planes in specimens due to twinning, exsolution lamellae, structural defects, or chemical alteration. 84 42  2024 Fracture Texture and shape of a mineral surface formed when it is broken. Not the same as cleavage (i.e., not along specific crystallographic planes). All minerals exhibit fracture, but it can be difficult to see in minerals with strong cleavage. 85 Fracture Conchoidal fracture: Shell-shaped fracture similar to that in glass. Common in quartz. 86 43  2024 Walastnite Fracture Splintery 87 Fracture Uneven pyrite 88 1 NotFracturecrystalface Subconcordial causedbyfusingof crystals 44  2024 Streak Color of the mark left behind when abrading the mineral against an object (ceramic streak plate, paper if soft) Can be useful in discerning between hematite (reddish brown streak) and Mn oxides (black streaks) effectscolor Or molybdenite (bluish streak) and graphite (dark grey streak) canbehardtotelllighting Molybdenite 89 1. Talc Hardness 2. Gypsum 2.5 Fingernail 3. Calcite 3.5 copper Moh’s hardness scale: Relative scale from 1 to 4. Fluorite 10. 5. Apatite 5.5 window glass Can test using pocket knife manmade objects or 6. Orthoclase on smooth mineral 6.5 Steel file surfaces. 7. Quartz Easier with a hand lens or microscope! 8. Topaz 9. Corundum 9.5 Silicon carbide abrasive 10. Diamond 90 45  2024 Magnetism A magnet pen is your friend 91 Magnetism Mayhave amixofboth Magnetite – Fe3O4 Hematite – Fe2O3 92 46  2024 Magnetism Pyrite - FeS2 Pyrrhotite - Fe1-xS 93 Reactivity / Solubility Calcite: Vigorous reaction to 10 % HCl Dolomite, siderite, ankerite: Noticeable but slower reaction to 10 % HCl Halite: Water soluble. 94 47  2024 Density Some minerals can be recognized rapidly by their unusual density. The most common example is barite (BaSO4), which may vaguely appear like calcite or feldspar, but is extremely dense. Barite specific gravity ~ 4.5 g/cm3 Quartz 2.65 Feldspar ~2.6 Calcite 2.7 95 Density Density can also be measured by weighing the samples in a liquid of known density – however the sample must be pure and non-porous. 96 48  2024 Fluorescence Exposure to ultraviolet light can excite electrons in a mineral, which then releases part of that energy as visible light. A mineral species may be fluorescent sometimes, always, or never. Some minerals can fluoresce different colors. Minerals may react to different UV wavelengths “Shortwave” (UVC – dangerous) “Midwave” (UVB – like tanning lamps) “Longwave” (UVA – like those at a nightclub) 97 notcauseflouresence may upmistakingforflorences causesamplestolight cheepormay Fluorescence Fluorescence can be used as a prospecting tool. Example: Fluorescent scapolite associated with sapphire mineralization. Nunavut. 98 49  2024 Fluorescence Fluorescence can be used as a prospecting tool. Example: Scheelite (tungsten ore, sometimes abundant in ore zones of orogenic gold deposits). Gold in scheelite, Val D’Or, 14 cm high Photo courtesy of Michael Bainbridge (theoccurrence.ca) 99 Fluorescence Fluorescence has been used in automated ore processing of rubies (UV fluorescence) and diamonds (X- ray luminescence) Gemfields ruby sorting facility 100 50  2024 Fluorescence Fluorescent Zinc silicate ore (willemite, green) with calcite (red) Sterling Hill Mine, New Jersey, USA Now a tourist attraction 101 Radioactivity Minerals containing highly radioactive elements (U, Th) can be detected using scintillometers or geiger counters. 102 51  2024 Exsolutions Mixture of minerals which formed from the separation of a single homogeneous phase into two or more solid solution phases under subsolidus conditions. “Perthite” texture typical of microcline or orthoclase (both K-feldspars) which have exsolved albitic plagioclase lamellae. 103 Odor, Feel, Taste Extremely soft minerals (talc, graphite, molybdenite) feel greasy to the touch Sulphides may smell like sulphur when broken, or if in the process of weathering Halite (NaCl) and sylvite (KCl) have distinctive flavors. However it is generally not a good idea to taste minerals. One rare halide is rather toxic – villiaumite, NaF! 104 52  2024 When physical properties don’t help enough Stage 4: Advanced analytical methods – to be discussed in future lectures. Use of petrographic thin sections and optical properties to identify the mineral. Use of special analytical equipment to measure the chemical composition or internal crystallographic structure of the mineral. 105 53