General Mineralogy and Petrology
Isotropic Minerals
|
Relief |
Mineral | Refractive Index | BH250 Sample |
| Very Low | Fluorite | 1.434 | BH250-12, 12b, 173b |
| Very Low | Analcime | 1.484 | BH250-85 |
| Very Low | Sodalite | 1.483-1.487 | BH250-33, 33b |
| Very Low | Hauyne | 1.496-1.510 | BH250-60, 60b |
| Very Low | Volcanic Glass | 1.480-1.610 | BH250-244, 231b, 227d |
| Low | Leucite | 1.509 | BH250-83, 84, 131 |
| Moderate to Strong | Periclase | 1.738-1.760 | BH250-102, 103 |
| Moderate to Strong | Garnet Group | 1.736-1.887 | BH250-1, 2, 3, 10, 20, 22, 26, 34, 45, 51, 52, 58, 59, 67, 68, 110, 121, 137, 149, 162, 221 |
| Very High | Spinel | 1.720-1.780 | BH250-18, 18b, 112c, 222 |
| Very High | Chromite | 2.070-2.160 | BH250-203, 203b, 136 |
| Very High | Perovskite | 2.340-2.380 | BH250-207, 208a, 208b, 208c |
Table modified after Paul F. Kerr, Optical Mineralogy, 4th edition, pg. 196
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Uniaxial Minerals
|
Uniaxial Negative |
nε | nω | Birefringence | BH250 Samples |
| Calcite | 1.486 | 1.658 | 0.172 | BH250-55, 79, 103b, 175e, 179a |
| Dolomite | 1.500-1.526 | 1.680-1.716 | 0.180-0.190 | BH250-43, 44 |
| Nepheline | 1.527-1.543 | 1.530-1.547 | 0.003-0.004 | BH250-16, 184, 185, 209 |
| Beryl | 1.564-1.590 | 1.568-1.598 | 0.004-0.008 | |
| Stilpnomelane | 1.612-1.634 | 1.700-1.745 | 0.030-0.119 | BH250-149 |
| Tourmaline | 1.613-1.628 | 1.632-1.655 | 0.019-0.025 | BH250-57, 58, 150b |
| Melilite | 1.626-1.629 | 1.632-1.634 | 0.005-0.006 | BH250-18 |
| Apatite | 1.630-1.651 | 1.633-1.655 | 0.003-0.004 | BH250-16, 90, 131c |
| Corundum | 1.759-1.763 | 1.767-1.772 | 0.008-0.009 | BH250-59, 211a |
|
Uniaxial Positive |
nω | nε | Birefringence | BH250 Samples |
| Quartz | 1.5442 | 1.5533 | 0.009 | BH250-10, 21, 27, 49, 66, 72, 144b, 175g, 183, 237 |
| Brucite | 1.566 | 1.585 | 0.019 | BH250-102b |
| Alunite | 1.572 | 1.592 | 0.020 | BH250-47 |
| Willemite | 1.691 | 1.719 | 0.010-0.280 | BH250-194, 214 |
| Zircon | 1.925-1.931 | 1.985-1.993 | 0.060-0.062 | BH250-27, 132, 220, 224 |
| Rutile | 2.603-2.616 | 2.889-2.903 | 0.286-0.287 |
Table modified after Paul F. Kerr, Optical Mineralogy, 4th edition, pg. 199
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Common Igneous Textural Terms
Crystallinity
|
Terminology |
Definition | Sample Number |
| Holocrystalline | Consisting entirely of crystals | BH250-41, 50, 113, 189 |
| Hypocrystalline | Containing both crystals and glass | BH250-19, 89 |
| Holohyaline, Vitric | consisting entirely of crystals | BH250-231a, 231b |
Grain Size
|
Terminology |
Definition | Sample Number |
| Aphanitic | Most minerals are too fine-grained to see with the naked eye | BH250-60, 74, 89, 219 |
| Phaneritic | Minerals are coarse enough to see with the naked eye | BH250-11 |
| Cryptocrystalline | Too fine-grained to distinguish the minerals microscopically | |
| Fine-grained | Average crystal diameter is less than 1 mm | BH250-49 |
| Medium-grained | Average crystal diameter is 1-5 mm | BH250-11, 46 |
| Coarse-grained | Average crystal diameter is greater than 5 mm | BH250-50 |
| Pegmatitic | Very coarse grained | BH250-211c |
| Aplitic | Fine- to medium-grained xenomorphic and equigranular | |
| Equigranular | Grains are all appoximately the same size | BH250-41 |
| Inequigranular | Grains vary considerably in size | BH250-50 |
Porphyritic Textures
| Terminology | Definition |
Sample Number |
| Porphyritic | Approximately bimodal size distribution | BH250-86 |
| Megaporphyritic | Porphyritic texture that can be seen in hand specimen | BH250-86 |
| Microporphyritic | Porphyritic texture that is visible only under the microscope | BH250-98 |
| Phenocryst | Large crystal set in a fine matrix | BH250-8, 60, 72, 82, 83, 86 |
| Microphenocryst | Microscopic crystals that are still larger than the remainder of the groundmass | BH250-60 |
| Megacryst | An unusually large crystal, either a phenocryst or a xenocryst | BH250-8 |
| Poikilitic | The host phenocryst (oikocryst) contains many inclusions of other minerals | BH250-11, 50, 83 |
| Oikocryst | The host phenocryst in poikilitic texture | BH250-11 |
| Groundmass | The glassy or finer grained element in the porphyritic texture (matrix) | BH250-15, 60, 80, 82, 83, 85, 88, 243 |
| Cumulophyric | Phenocrysts of the same or different minerals occur in clusters | BH250-85 |
|
Glomerocryst |
A cluster or aggregate of several crystals (phenocrysts) within an igneous rock. synonymous with cumulophric |
BH250-133 |
| Glomeroporphyritic |
A type of porphyritic texture in igneous rocks characterized by the presence of glomerocrysts (clusters of phenocrysts) embedded in a finer-grained groundmass. |
BH250-133 |
| Hiatial Porphyritic | There is a pronounced difference in size between the phyric and groundmass phases | |
| Seriate | There is a continuous gradation in size | |
| Aphyric | Non-porphyritic (phenocrysts absent) | BH250-124 |
Form of Individual Grains
|
Terminology |
Definition | Sample Number |
| Euhedral (idiomorphic) | Completely bounded by crystal faces | BH250-41 |
| Subhedral (subidiomorphic) | Crystal faces only partially developed | BH250-41 |
| Anhedral (allopriomorphic) | Crystal faces are entirely absent | |
| Corroded (embayed) | Subhedral or anhedral forms produced by partial melting (resorption) of phenocrysts by the melt | |
| Crystal Habits | Equaint, prismatic, columnar, accicular, fibrous, tabular, platy, micaceous, lath-shaped, etc. |
Forms of Grains in the Rock
|
Terminology |
Definition | Sample Number |
| Panidiomorphic | Most of the grains are euhedral | BH250-41 |
| Hypidiomorphic | Consisting predominantly of subhedral grains | BH250-11 |
| Allotriomorphic | Most of the grains are anhedral | |
| Sutured |
Characterized by articulation along highly irregular inter-penetrating boundaries Common in recrystallized de-formed rocks |
BH250-27, 13, 21 |
| Mosaic | A texture of polygonal equigranular crystals | BH250-132 |
Intergrowths
|
Terminology |
Definition | Sample Number |
| Host (Oikocryst) | The large mineral that includes others in poikilitic texture | BH250-11, 50, 86 |
| Guest | The included mineral in poikilitic texture | BH250-86 |
| Poikilitic |
One large crystal contains several small discrete crystals of another mineral Refers to growth phenomena, not exsolution or replacement |
BH250-11, 50 |
| Graphic |
An intergrowth in which the rock shows angular wedge-like forms Usually occurs with quartz in microcline |
BH250-92 |
| Micrograpic | Graphic texture visible only under the microscope | |
| Granophyric |
A texture of certain porphyritic rocks in which the groundmass minerals (usually quartz and alkali feldspar) penetrate each other as feathery irregular intergrowths Resembles micrographic texture, but is more irregular |
|
| Exsolution Lamellae | Lamellar bands of a phase exsolved from a host phase | BH250-9, 92, 81, 222 |
| Perthitic |
Irregular veins, patches, lenses, etc., of sodic plagioclase in an alkali feldspar host Usually results from exsolution |
BH250-81, 92, 132 |
| Antiperthitic |
Exsolution lamellae of alkali feldspar in a plagioclase host Usually much thinner than perthite |
BH250-81 |
| Symplectite | A replacement texture in which a mineral is replaced by intergrowth of one or more minerals | |
| Myrmekite | A secondary texture consisting of irregular "wormy" blebs or rods of quartz in a plagioclase host adjacent to alkali feldspar grains | BH250-13, 49 |
| Spherulitic |
A radial intergrowth of fibrous minerals Commonly alkali feldspar and quartz in devitrified silicic volcanics |
BH250-85 |
| Axiolitic | Similar to sperulitic, but the fibers occur in a layer and are oriented normal to its walls | BH250-85 |
| Variolitic | Spherulitic structures consisting of divergent plagioclase fibers in certain basalts | |
| Coalescent | Anhedral textures developed by simultaneous growth of two mineral grains in contact |
Textures of Mafic Igneous Rocks
|
Terminology |
Definition | Sample Number |
| Ophitic | Large pyroxene grains enclose small random plagioclase laths | BH250-11, 50 |
| Subophitic | The plagioclase laths are larger and only partially enclosed by the pyroxene | BH250-11, 50 |
| Nesophitic | Plagioclase is larger and the pyroxenes are interstitial | BH250-11, 50 |
| Intergranular | Small discrete grains of phyroxene, olivine, etc., fill the interstices in a random network of larger plagioclase laths | |
| Intersertal | Glass, cryptocrystalline material, or alteration products occupy the spaces between plagioclase laths | BH250-11 |
| Hyalo-ophitic | An intersertal texture in which a larger amounts of glass is present and less pyroxene | |
| Hyalophilitic | Still more glass is present and plagioclase occurs only as tin random microlites | |
| Diktytaxitic | Texture of certain basalts containing abundant angular interstitial gas cavities between the plagioclase laths | |
| Cumulate | Interstitial growth of a mineral betwen earlier ones which are all in contact and give the distinct impression that they accumulated at the bottom of a magma chamber | BH250-41 |
| Orthocumulate | Cumulate texture with other minerals occupying the interstitial areas | |
| Adcumulate | Cumulate texture in which the early cumulate minerals grow to fill the pore space | |
| Mesocumulate | Intermediate between ortho- and adcumulate |
Replacement Textures
|
Terminology |
Definition | Sample Number |
| Pseudomorph | One or more minerals replace another, retaining the form of the original mineral | BH250-50, 60 |
| Symplectite | A replacement textures in which a mineral is replaced by an intergrowth of one or more minerals | BH250-48 |
| Uralitization | Replacement of pyroxene by amphibole | |
| Saussuritization | Replacement of plagioclase by epidote | |
| Biotitization | Replacement of pyroxene, amphibole, or garnet by biotite | |
| Chloritization | Replacement of any mafic mineral or garnets by chlorite | BH250-49 |
| Seritization | Replacement of feldspar or feldspathoids by fine white micas, miscellaneous terms | BH250-49 |
Miscellaneous Terms
|
Terminology |
Definition | Sample Number |
| Interstitial | One mineral fills the interstices between earlier crystallized grains | BH250-11 |
| Crystallites | Minute, inchoate crystals in earliest stages of formation. They are isotropic and cannot be identified under the microscope | |
| Microlites | Tiny needle- or lath-like crystals of which at least some properties are microscopically detainable | |
| Felty | Consisting of random microlite | |
| Pilotaxitic | Like felty | |
| Trachytic | Consisting of (feldspar) microlites aligned due to flow | |
| Embayed | Having embayments due to reaction with the melt (resorption) | |
| Skeletal | Crystals which grew as, or have been corroded to, a skeletal framework with a high proportion of internal voids | |
| Sieve | Crystals are filled with channelways (appearing as holds) due to resportion | |
| Epitactic | Oriented nucleation of one mineral on another of a different kind | |
| Rapakivi | Overgrowths of plagioclase on alkai feldspar | |
| Vesicular | Contains gas bubbles | BH250-60, 82, 84 |
| Scoriaceous | Highly vesicular | BH250-151, 151b |
| Pumiceous | Frothy vesicular structure characteristics of pumice | BH250-231a |
| Diktytaxitic | Containing vesicles into which microphenocrysts protrude | |
| Miarolitic | Gas cavities present in certain plutonic rocks into which euhedral minerals protrude | |
| Pipe Vesicles | Tube-like elongate vesibles that result from rising gases | BH250-151c |
| Amygdaloida | Vesicles completely or partially filled with secondary minerals | BH250-85, 168, 173, 250a |
| Lithophysae | Large ovoid structures representing gas bubbles in devitrified rhyolitic glass | |
| Flow | A parallel structure developed as the result of flow | BH250-98 |
| Foliation | Planar parallelism | BH250-26, 52, 66, 20, 21, 22, 67 |
| Banding | Alternating planar layers | |
| Lineation | Linear parallelism | |
| Xenolith | An inclusion of country rock | BH250-112, 136 |
| Xenocryst | A single-crystal foreign inclusion | BH250-112, 136 |
| Perlitic | Concentric fracture pattern resulting from contraction of some volcanic glasses upon cooling | BH250-92, 81 |
| Pyroclastic | Comprised of fragments | BH250-19 |
| Ocelli | Ovoid blobs created by liquid immiscibility, mingled magmas, or filled vesicles | |
| Orbicules | Ovoid masses of radiating crystals, commonly concentrically banded, found in some granites | |
| Spinifex | Centimeter-scale sub-parallel to dendritic growth of olivine crystals in some quenched ultramafics | BH250-87, 87b, 161 |
Proclastic Terms
|
Terminology |
Definition | Sample Number |
| Pele's Tears | Glassy lapilli | BH250-232a, 232b |
| Pele's Hair | Volcanic glass formed from cooled lava stretched into thin strands | BH250-232b |
| Reticulite | Type of pumice that is extremely vesicular | BH250-232a |
| Fiamme | Compressed pumice fragments in a tuff | BH250-243 |
| Bomb | A mass of partially molten rock larger than 64 mm | BH250-151d, 241, 243 |