THINGS YOU SHOULD KNOW AT THIS POINT… Twinning: zebra stripes describes what Polysynthetic/Albite twins look like in Plagioclase, but this is NOT the proper name for this type of twinning Color/Pleochroism vs. Birefringence/Interference Colors Color and Pleochroism are seen in Plane Polarized light (PPL) o nly . Color will be the true color of the mineral. Pleochroism is an optical phenomenon in which mineral grains within a rock display a change in color as the stage is rotated in plane light. It is produced because the two rays of light are absorbed differently as they pass through the colored mineral and therefore have different colors Interference colors and birefringence are seen in cross-Nichols/with the analyzer in/cross polars (XPL/XN) . Interference colors are produced because light is split into two rays on passing through the mineral. Birefringence is a number that is derived from the difference between the max and min RI (n) of a mineral and interference colors should be listed by color and order. The maximum birefringence is a characteristic of each mineral. Retardation (∆) is the distance that the slow ray is behind the fast ray after both have exited the crystal (from Nesse, Optical Mineralogy). **On the Michel Levy chart below, the birefringences are on the top and left sides, while the retardation (path differences) are listed on the bottom. When looking at birefringence, you always want to look for the MIN and MAX birefringence because generally minerals have a range of interference colors.
Transcript
THINGS YOU SHOULD KNOW AT THIS POINT…
Twinning: zebra stripes describes what Polysynthetic/Albite twins look like in Plagioclase, but this is NOT the proper name for this type of twinning
Color/Pleochroism vs. Birefringence/Interference Colors
Color and Pleochroism are seen in Plane Polarized light (PPL) o nly . Color will be the true color of the mineral. Pleochroism is an optical phenomenon in which mineral grains within a rock display a change in color as the stage is rotated in plane light. It is produced because the two rays of light are absorbed differently as they pass through the colored mineral and therefore have different colors
Interference colors and birefringence are seen in cross-Nichols/with the analyzer in/cross polars (XPL/XN). Interference colors are produced because light is split into two rays on passing through the mineral. Birefringence is a number that is derived from the difference between the max and min RI (n) of a mineral and interference colors should be listed by color and order. The maximum birefringence is a characteristic of each mineral. Retardation (∆) is the distance that the slow ray is behind the fast ray after both have exited the crystal (from Nesse, Optical Mineralogy).
**On the Michel Levy chart below, the birefringences are on the top and left sides, while the retardation (path differences) are listed on the bottom.
When looking at birefringence, you always want to look for the MIN and MAX birefringence because generally minerals have a range of interference colors. The max birefringence will have the higher number…so a third order color is higher than a second order color (this was a problem on the lab exam).
For example: Plagioclase has birefringence up to 0.013 (Anorthite), so you should see first order black, white and gray. The HIGHEST birefringence is first order white.
Pyroxene: birefringence varies from 0.018-0.033 (Augite). So for the max birefringence of 0.033 it is second order yellow and the minimum birefringence is first order red.
***Also of note and a problem on the exam: if a mineral is colorless or has some color in PPL (thus it is not opaque), then if the analyzer is in/polars crossed and the stage is rotated a full 360° and the color does not change from black,
then you are looking at an isotropic mineral. Thus there is no interference color – first order black is not acceptable. The mineral is in complete extinction in XPL – this is the correct answer.
Crystal Habit vs. Form
From About.com
Habits are the distinctive form that minerals may take in different geologic settings, for instance when growing in a free space or in a particular environment. Often a mineral's habit is a strong clue to its identity. Here are examples of some of the most useful mineral habits.
Examples of different crystal habits and what they look like in hand sample:
Reniform Habit Rhombohedral Habit Rosette Habit ***Also need to have an idea of what you would see in thin section.
Form
Euhedral crystals are those that are well-formed with sharp, easily-recognised faces. Normally, crystals do not form smooth faces or sharp crystal outlines. Many crystals grow from cooling liquid magma. As magma cools, the crystals grow and eventually touch each other, preventing crystal faces from forming properly or at all.
However, when snowflakes crystallize, they do not touch each other. Thus, snowflakes form euhedral, six-sided twinned crystals. In rocks, the presence of euhedral crystals may signify that they formed early in the crystallization of a magma or perhaps crystallized in a cavity or vug, without hindrance from other crystals.
By contrast, a rock with an anhedral texture is composed of mineral grains that have no well formed crystal faces or cross-section shape in thin section. Anhedral crystal growth occurs in a competitive environment with no free space for the formation of crystal faces. An intermediate texture with some crystal face formation is termed subhedral.
Etymology: Euhedral is derived from the Greek eu meaning true and hedron meaning shape.
(from Wikipedia) Euhedral pyrite crystals
Crystal habitFrom Wikipedia, the free encyclopedia
Pyrite sun (or dollar) in laminated shale matrix. Between tightly spaced layers of shale, the aggregate was forced to grow in a laterally compressed, radiating manner. Under normal conditions, pyrite would form cubes or pyritohedrons
Crystal habit is an overall description of the visible external shape of a mineral. This description can apply to an individual crystal or an assembly of crystals or aggregates.
In mineralogy, shape and size give rise to descriptive terms applied to the typical appearance, or habit of crystals. Each crystal can be described by how well it is formed, ranging from euhedral (perfect to near-perfect), to subhedral (moderately formed), and anhedral (poorly formed to no discernable habit seen).
The many terms used by mineralogists to describe crystal habits are useful in communicating what specimens of a particular mineral often look like. Recognizing numerous habits helps a mineralogist to identify a large number of minerals. Some habits are distinctive of certain minerals, although most minerals exhibit many differing habits (the development of a particular habit is determined by the details of the conditions during the mineral formation/crystal growth). Crystal habit may mislead the inexperienced as a mineral's internal crystal system can be hidden or disguised.
Goethite replacing pyrite cubes
Factors influencing a crystal's habit include: a combination of two or more crystal forms; trace impurities present during growth; crystal twinning and growth conditions (i.e., heat, pressure, space); and specific growth tendencies like growth striations. Minerals belonging to the same crystal system do not necessarily exhibit the same habit. Some habits of a mineral are unique to its variety and locality: For example, while most sapphires form elongate barrel-shaped crystals, those found in Montana form stout tabular crystals. Ordinarily, the latter habit is seen only in ruby. Sapphire and ruby are both varieties of the same mineral; corundum.
Some minerals may replace other existing minerals while preserving the original's habit: this process is called pseudomorphous replacement. A classic example is tiger's eye quartz, crocidolite asbestos replaced by silica. While quartz typically forms prismatic (elongate, prism-like) crystals, in tiger's eye the original fibrous habit of crocidolite is preserved.
The names of crystal habits are derived from:
Predominant crystal faces (prism - prismatic, pyramid - pyramidal and pinacoid - platy). Crystal forms (cubic, octahedral, dodecahedral). Aggregation of crystals or aggregates (fibrous, botroidal, radiating, massive). Crystal appearance (foliated/lamellar (layered), dendritic, bladed, acicular, lenticular, tabular (tablet shaped)).
List of crystal habits
Habit[1][2][3] Image Description Common Example(s)
Tourmaline in a muscovite-biotite schist. Tourmaline is unusual among common elongate minerals in having its strongest absorption when the plane of polarization is perpendicular to the crystal length. This is the opposite of micas and most amphiboles. Plane polarized light, 200x (East Clairindon, VT)
Tourmaline in a muscovite-biotite schist. Tourmaline is pale-colored when the plane of polarization is parallel to its length. Plane polarized light, 200x (East Clairindon, VT)
From [http://minerva.union.edu/hollochk/c_petrology/met_minerals.htm]
How to get an Interference Figure (on any microscope):
1. Locate the mineral in the lowest objective (that way as you focus in on it, it will remain in your field of view) and move up to the highest objective power. E.g. on the Zeiss, 40x.
2. Put in the condenser.3. Put in the analyzer.4. Put in the Bertrand Lens.5. Lastly, then you can put in the accessory plate to determine the sign.
