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6.1: Cleavage - Geosciences

6.1: Cleavage - Geosciences


Cleavage is the splitting of a gemstone along the direction of its crystal faces where atoms have weaker bonding. The result of cleavage is a more or less flat plane with often a silky luster.
Cleavage is a reproducible property of a gemstone and can be done at any point of the cleavage direction.

There are several directions of cleavage.

  • Prismatic cleavage
  • Basal cleavage
  • Pinacoidal cleavage
  • Octahedral cleavage
  • Rhombohedral cleavage

The quality of cleavage is expressed with a few simple phrases.

  • Perfect
  • Good (or imperfect)
  • Fair (or moderate)
  • Poor (or weak)
  • None

Figure (PageIndex{1}): Prismatic cleavage

Figure (PageIndex{2}): Basal cleavage


Stones and their cleavage directions:

Table (PageIndex{1})

CleavageDirectionsStonesQuality
Basal1TopazPerfect
BerylImperfect
Prismatic2PeridotPerfect
SpodumenePerfect
ChrysoberylWeak to Moderate
DiopsidePerfect
Cubic3HalitePerfect
Rhombohedral3CalcitePerfect
RhodochrositePerfect
Octhahedral4FluoritePerfect
DiamondPerfect
Dodecahedral6SphaleritePerfect

Sources

  • Gemmology 3rd edition (2005) - Peter Read

Labradorite

Labradorite: Photograph of labradorite gemstones exhibiting a beautiful labradorescent play of iridescent colors. Photo copyright iStockphoto / Joanna-Palys.

Labradorite: A specimen of labradorite feldspar rough about four inches across exhibiting a beautiful play-of-color. Collected near Nain, Labrador, Canada.


Geometry

Bruce Hobbs , Alison Ord , in Structural Geology , 2015

2.4 Distortion and Volume Change

Many deformed rocks consist of localised regions of volume loss and/or gain. Examples include ‘spots’ (porphyroblasts), linear regions (lineations) and layers (foliations) where locally the metamorphic mineral reactions have been dominantly volume increasing or decreasing, zones where material has been removed due to ‘pressure solution’, metamorphic differentiation during the formation of crenulation cleavage ( Figure 2.9(a) ) and leucosomes where melt has left the system ( Figure 2.9(b) ). We will discuss the processes involved in these volume changes in Chapter 14 and Volume II but for now we present an example of deformations that might reproduce some of these situations. These consist of inhomogeneous shears with periodic distributions of volume changes ( Figure 2.9(c) ):

Figure 2.9 . Non-affine deformations associated with volume changes. (a) Differentiated crenulation cleavage from the Picuris Range, New Mexico, USA. (Photo: Ron Vernon.) View about 1.8 mm across. In some models for the formation of this structure, it is proposed that quartz is removed from the muscovite (Mu) rich layers and either removed from the system or added to the quartz (Q) rich layers ( Vernon, 2004, pp. 393–389 ). (b) Differentiated foliation in granulite facies rocks from Round Hill, Broken Hill, Australia. Scale: view about 0.5 m across. Models for the development of this foliation involve removal of melt from the lighter coloured garnet bearing areas ( Powell &amp Downes, 1990 ). (c) A non-affine deformation with periodic distribution of volume change. (d) The displacement field associated with the deformation in (c). (e) Zoom into part of (d). (f) Divergence of the displacement vector field superimposed on the vector field. On the left hand side the Jacobian of the deformation gradient is plotted as an indication of the volume change.

Various representations of the displacement field for this deformation are given in Figure 2.9(d, e and f) which are to be compared and contrasted with Figure 2.4 . The deformation in Figure 2.4 is a modification of an isochoric pure shear, whereas that in Figure 2.9 is a modification of a pure shear with local volume changes. Both produce deformations that are superficially similar in appearance yet the displacement fields are quite different. For the deformation in Figure 2.9 the volume change is given by the Jacobian:

The value of the Jacobian is plotted on the right side of Figure 2.9(f) which represents the divergence of the displacement field the volume change ranges from 0.4 to 1.4 times the initial volume. We discuss the concept of the divergence of a vector field in Chapter 3 but for the moment it is sufficient to grasp that zero divergence means that there are no sources or sinks for the vector field, whereas positive or negative values for the divergence mean that there are sources or sinks, respectively. Other examples of non-affine deformations involving volume changes are given in Sections 2.7.2 and 2.7.3 .

We note that for finite deformations the Hencky strain tensor is the only strain measure whose trace gives the volumetric strain:

and so the Hencky tensor is particularly useful for deformations involving a volume change.


Streak

In the context of minerals, “colour” is what you see when light reflects off the surface of the sample. One reason that colour can be so variable is that the type of surface is variable. It may be a crystal face or a fracture surface or a cleavage plane, and the crystals may be large or small depending on the nature of the rock. If we grind a small amount of the sample to a powder we get a much better indication of its actual colour. This can easily be done by scraping a corner of the sample across a streak plate (a piece of unglazed porcelain) to make a streak . The result is that some of the mineral gets ground to a powder and we can get a better impression of its “true” colour (Figure 2.6.2).

Figure 2.6.2 The streak colours of specular (metallic) hematite (left) and earthy hematite (right). Hematite leaves a distinctive reddish-brown streak whether the sample is metallic or earthy.


Contents

The word cleavage was first used in the early 19th century in geology and mineralogy to mean the tendency of crystals, minerals, and rocks to split along definite planes. By the mid-19th century, it was generally used to mean splitting along a line of division into two or more parts. [2] [7] In the 1940s, Joseph Breen, head of the U.S. Production Code Administration, applied the term to breasts in reference to actor Jane Russell's costumes and poses in the 1943 movie The Outlaw. The term was also applied in the evaluation of the British films The Wicked Lady (1945), starring Margaret Lockwood and Patricia Roc Bedelia (1946), also starring Lockwood and Pink String and Sealing Wax (1945), starring Googie Withers. This use of the term was first covered in a Time article titled "Cleavage & the Code" on August 5, 1946, as a "Johnston Office (the popular name for Motion Picture Association of America office at the time [8] ) trade term for the shadowed depression dividing an actress' bosom into two distinct sections." [2] [3] [9] [10] The word cleavage is made of the root verb cleave 'to split' (from Old English clifian and Middle English clevien cleft in the past tense) and the suffix -age 'the state of, the act of'. [7] [11]

While the division of the breasts is a cleavage, the opening of a person's garments to make the division visible is called a décolletage, a French word that is derived from décolleter 'to reveal the neck'. [12] The term was first used in English literature before 1831 [13] and was the preferred term among educated people in the English-speaking world before cleavage became the popular term. [9] Décolletage (or décolleté in adjectival form) refers to the upper part of the female torso, consisting of the neck, shoulders, back and chest, which is exposed by the neckline, the edge of a dress or shirt that goes around the neck, especially at the front of a woman's garment. [14] The neckline and collar are often the most attention-grabbing parts of a garment, effected by bright or contrasting colors, or by décolletage. [15] [16] The term is most commonly applied to a neckline that reveals or emphasizes cleavage [17] and is measured as extending about two hand-breadths from the base of the neck down both in the front and the back. [18] In anatomical terms, the cleft in the human body between the breasts is known as the intermammary cleft or intermammary sulcus. [19]

While there has been much work done to classify breasts based on their shapes, contours and sizes there has not been much work done to classify the cleavage, [20] [21] despite its prominence in aesthetic determination. [21] [22] According to a paper by British surgeon Muhammad Adil Abbas Khan et al., there are eight common types of cleavage from a frontal and a bird's eye view. [21]


Hardness

One of the most important diagnostic properties of a mineral is its hardness. In practical terms, hardness determines whether or not a mineral can be scratched by a particular material.

In 1812 German mineralogist Friedrich Mohs came up with a list of 10 minerals representing a wide range of hardness, and numbered them 1 through 10 in order of increasing hardness (Figure 5.34, horizontal axis). While each mineral on the list is harder than the one before it, the measured hardness (vertical axis) is not linear. Notice that apatite is about three times harder than fluorite, and diamond is three times harder than corundum.

Figure 5.34 Minerals and reference materials in the Mohs scale of hardness. The measured hardness values are Vickers Hardness numbers. Source: Steven Earle (2015) CC BY 4.0 view source

Some commonly available reference materials are also shown on this diagram, including a typical fingernail [1] (2.5), a piece of copper wire (3.5), a knife blade or piece of window glass (5.5), a hardened steel file (6.5), and a porcelain streak plate (7). These are tools that a geologist can use to measure the hardness of unknown minerals: if you have a mineral that you can’t scratch with your fingernail, but you can scratch with a copper wire, then its hardness is between 2.5 and 3.5. The minerals themselves can be used to test other minerals.


Transportation

One of the key principles of sedimentary geology is that the ability of a moving medium (air or water) to move sedimentary particles, and keep them moving, is dependent on the velocity of flow. The faster the medium flows, the larger the particles it can move. This is illustrated in Figure 6.4. Parts of the river are moving faster than other parts, especially where the slope is greatest and the channel is narrow. Not only does the velocity of a river change from place to place, but it changes from season to season.

During peak discharge [3] at this location, the water is high enough to flow over the embankment on the right, and it flows fast enough to move the boulders that cannot be moved during low flows.

Figure 6.4 Variations in flow velocity on the Englishman River near Parksville, B.C. When the photo was taken the river was not flowing fast enough anywhere to move the boulders and cobbles visible here, but it is fast enough when the discharge is higher.

Clasts within streams are moved in several different ways, as illustrated in Figure 6.5. Large bedload clasts are pushed (by traction) or bounced along the bottom (saltation), while smaller clasts are suspended in the water and kept there by the turbulence of the flow. As the flow velocity changes, different-sized clasts may be either incorporated into the flow or deposited on the bottom. At various places along a river, there are always some clasts being deposited, some staying where they are, and some being eroded and transported. This changes over time as the discharge of the river changes in response to changing weather conditions.

Other sediment transportation media, such as waves, ocean currents, and wind, operate under similar principles, with flow velocity as the key underlying factor that controls transportation and deposition.

Figure 6.5 Transportation of sediment clasts by stream flow. The larger clasts, resting on the bottom (bedload), are moved by traction (sliding) or by saltation (bouncing). Smaller clasts are kept in suspension by turbulence in the flow. Ions (depicted as + and – in the image, but invisible in real life) are dissolved in the water.

Clastic sediments are deposited in a wide range of environments, including glaciers, slope failures, rivers — both fast and slow, lakes, deltas, and ocean environments — both shallow and deep. Depending on the grain size in particular, they may eventually form into rocks ranging from fine mudstone to coarse breccia and conglomerate.

Lithification is the term used to describe a number of different processes that take place within a deposit of sediment to turn it into solid rock. One of these processes is burial by other sediments, which leads to compaction of the material and removal of some of the intervening water and air. After this stage, the individual clasts are all touching one another. Cementation is the process of crystallization of minerals within the pores between the small clasts, and also at the points of contact between the larger clasts (sand size and larger). Depending on the pressure, temperature, and chemical conditions, these crystals might include calcite, hematite, quartz, clay minerals, or a range of other minerals.

The characteristics and distinguishing features of clastic sedimentary rocks are summarized in Table 6.2. Mudrock is composed of at least 75% silt- and clay-sized fragments. If it is dominated by clay, it is called claystone. If it shows evidence of bedding or fine laminations, it is shale otherwise it is mudstone. Mudrocks form in very low energy environments, such as lakes, river backwaters, and the deep ocean.

Table 6. 2 The main types of clastic sedimentary rocks and their characteristics.
Group Examples Characteristics
Mudrock mudstone >75% silt and clay, not bedded
shale >75% silt and clay, thinly bedded
Coal dominated by fragments of partially decayed plant matter, often enclosed between beds of sandstone or mudrock
Sandstone quartz sandstone dominated by sand, >90% quartz
arkose dominated by sand, >10% feldspar
lithic wacke dominated by sand, >10% rock fragments, >15% silt and clay
Conglomerate dominated by rounded clasts, pebble size and larger
Breccia dominated by angular clasts, pebble size and larger

Most coal forms in fluvial or delta environments where vegetation growth is vigorous and where decaying plant matter accumulates in long-lasting swamps with low oxygen levels. To avoid oxidation and breakdown, the organic matter must remain submerged for centuries or millennia, until it is covered with another layer of either muddy or sandy sediments.

It is important to note that in some textbooks coal is described as an “organic sedimentary rock.” In this book, coal is classified with the clastic rocks for two reasons: first, because it is made up of fragments of organic matter and second, because coal seams (sedimentary layers) are almost always interbedded with layers of clastic rocks, such as mudrock or sandstone. In other words, coal accumulates in environments where other clastic rocks accumulate.

It’s worth taking a closer look at the different types of sandstone because sandstone is a common and important sedimentary rock. Typical sandstone compositions are shown in Figure 6.6. The term arenite applies to a so-called clean sandstone, meaning one with less than 15% silt and clay. Considering the sand-sized grains only, arenites with 90% or more quartz are called quartz arenites. If they have more than 10% feldspar and more feldspar than rock fragments, they are called feldspathic arenites or arkosic arenites (or just arkose). If they have more than 10% rock fragments, and more rock fragments than feldspar, they are lithic [4] arenites. A sandstone with more than 15% silt or clay is called a wacke (pronounced wackie). The terms quartz wacke, lithic wacke, and feldspathic wacke are used. Another name for a lithic wacke is greywacke.

Some examples of sandstones, magnified in thin section are shown in Figure 6.7. (A thin section is rock sliced thin enough so that light can shine through.)

Clastic sedimentary rocks in which a significant proportion of the clasts are larger than 2 mm are known as conglomerate if the clasts are well rounded, and breccia if they are angular. Conglomerates form in high-energy environments where the particles can become rounded, such as fast-flowing rivers. Breccias typically form where the particles are not transported a significant distance in water, such as alluvial fans and talus slopes. Some examples of clastic sedimentary rocks are shown on Figure 6.8.

Figure 6.6 A compositional triangle for arenite sandstones, with the three most common components of sand-sized grains: quartz, feldspar, and rock fragments. Arenites have less than 15% silt or clay. Sandstones with more than 15% silt and clay are called wackes (e.g., quartz wacke, lithic wacke). /> Figure 6.7 Photos of thin sections of three types of sandstone. Some of the minerals are labelled: Q=quartz, F=feldspar and L= lithic (rock fragments). The quartz arenite and arkose have relatively little silt-clay matrix, while the lithic wacke has abundant matrix. Figure 6.8 Examples of various clastic sedimentary rocks.

Exercise 6.2 Classifying Sandstones

The table below shows magnified thin sections of three sandstones, along with descriptions of their compositions. Using Table 6.1 and Figure 6.6, find an appropriate name for each of these rocks.


6.2 Classification of Metamorphic Rocks

There are two main types of metamorphic rocks: those that are foliated because they have formed in an environment with either directed pressure or shear stress, and those that are massive (not foliated) because they have formed in an environment without directed pressure or relatively near the surface with very little pressure at all. Some types of metamorphic rocks, such as quartzite and marble, which can form whether there is directed-pressure or not, tend to be massive because their minerals (quartz and calcite respectively) do not tend to show alignment (see Figure 6.2.1).

When a rock is squeezed under directed pressure during metamorphism it is likely to be deformed, and this can result in a textural change such that the minerals appear elongated in the direction perpendicular to the main stress (Figure 6.2.1). This contributes to the formation of foliation.

Figure 6.2.1: The textural effects of squeezing during metamorphism. In the original rock (left) there is no alignment of minerals. In the squeezed rock (right) the minerals have been elongated in the direction perpendicular to the squeezing.

When a rock is both heated and squeezed during metamorphism, and the temperature change is enough for new minerals to form from existing ones, there is a strong tendency for new minerals to grow with their long axes perpendicular to the direction of squeezing. This is illustrated in Figure 6.2.2, where the parent rock is shale, with bedding as shown. After both heating and squeezing, new minerals have formed within the rock, generally parallel to each other, and the original bedding has been largely obliterated.

Figure 6.2.2: The textural effects of squeezing and mineral growth during regional metamorphism. The left diagram is shale with bedding slanting down to the right. The right diagram represents schist (derived from that shale), with mica crystals orientated perpendicular to the main stress direction and the original bedding no longer easily visible.

Figure 6.2.3 shows an example of this effect. This large boulder has bedding visible as dark and light bands sloping steeply down to the right. The rock also has a strong slaty foliation, which is horizontal in this view (parallel to the surface that the person is sitting on), and has developed because the rock was being squeezed during metamorphism. The rock has split from bedrock along this foliation plane, and you can see that other weaknesses are present in the same orientation.

Squeezing and heating alone (as shown in Figure 6.2.1) can contribute to foliation, but most foliation develops when new minerals are formed and are forced to grow perpendicular to the direction of greatest stress (Figure 6.2.2). This effect is especially strong if the new minerals are platy like mica or elongated like amphibole. The mineral crystals don’t have to be large to produce foliation. Slate, for example, is characterized by aligned flakes of mica that are too small to see.

Figure 6.2.3: A slate boulder on the side of Mt. Wapta in the Rockies near Field, B.C. Bedding is visible as light and dark bands sloping steeply to the right (yellow arrows). Slaty cleavage is evident from the way the rock has broken (along the flat surface that the person is sitting on) and also from lines of weakness that are parallel to that same trend (red arrows).

The various types of foliated metamorphic rocks, listed in order of the metamorphic grade or intensity of metamorphism and the type of foliation are: slaty , phyllitic , schistose , and gneissic (Figure 6.2.4). As already noted, slate is formed from the low-grade metamorphism of shale, and has microscopic clay and mica crystals that have grown perpendicular to the stress. Slate tends to break into flat sheets. Phyllite is similar to slate, but has typically been heated to a higher temperature the micas have grown larger and are visible as a shiny sheen on the surface. Where slate is typically planar, phyllite can form in wavy layers. In the formation of a schist , the temperature has been hot enough so that individual mica crystals are big enough to be visible, and other mineral crystals, such as quartz, feldspar, or garnet may also be visible. In a gneiss , the minerals may have separated into bands of different colours. In the example shown in Figure 6.2.4d, the dark bands are largely amphibole while the light-coloured bands are feldspar and quartz. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable. Unlike slate and phyllite, which typically only form from mudrock, schist, and especially gneiss, can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

Schist and gneiss can be named on the basis of important minerals that are present. For example a schist derived from basalt is typically rich in the mineral chlorite, so we call it chlorite schist or greenschist. One derived from shale may be a muscovite-biotite schist, or just a mica schist, or if there are garnets present it might be mica-garnet schist. Similarly, a gneiss that originated as basalt and is dominated by amphibole, is an amphibole gneiss or, more accurately, an amphibolite .

Figure 6.2.4: Examples of foliated metamorphic rocks: (A) Slate, (B) Phyllite, (C) Schist, (D) Gneiss.

Rather than focusing on just the metamorphic rock types (slate, schist, gneiss, etc.), geologists also tend to look at specific index minerals within the rocks that are indicative of different grades of metamorphism. Some common minerals in metamorphic rocks derived from a mudrock protolith are shown in Figure 6.2.5, arranged in order of the temperature ranges over which they tend to be stable. The upper and lower limits of the ranges are intentionally vague because these limits depend on a number of different factors, such as the pressure, the amount of water present, and the overall composition of the rock.

Figure 6.2.5: Metamorphic grades, common metamorphic index minerals, and corresponding rock names for a mudrock protolith under increasing metamorphism (increasing temperature and pressure). [Image description]

If a rock is buried to a great depth and encounters temperatures that are close to its melting point, it may partially melt. The resulting rock, which includes both metamorphosed and igneous material, is known as migmatite (Figure 6.2.6).

Figure 6.2.6: Migmatite from Prague, Czech Republic

As already noted, the nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions. The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 6.1. Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees.

Table 6.1 A rough guide to the types of metamorphic rocks that form from different parent rocks at different grades of regional metamorphism. You are expected to know the rock names indicated in bold font.
Protolith Very Low Grade (150-300°C) Low Grade (300-450°C) Medium Grade (450-550°C) High Grade (Above 550°C)
Mudrock slate phyllite schist gneiss
Granite no change no change almost no change granite gneiss
Basalt greenschist greenschist amphibolite amphibolite
Sandstone no change little change quartzite quartzite
Limestone little change marble marble marble

Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated, and their texture is described as massive . In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust. This is contact metamorphism . Some examples of non-foliated metamorphic rocks are marble , quartzite , and hornfels .

Marble is metamorphosed limestone. When it forms, the calcite crystals recrystallize and tend to grow larger, and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone was pure calcite, then the marble will likely be white (as in Figure 6.2.7), but if it had various impurities, such as clay, silica, or magnesium, the marble could be “marbled” in appearance.

Figure 6.2.7: Marble with visible calcite crystals (left) and an outcrop of banded marble (right).

Quartzite is metamorphosed sandstone (Figure 6.2.8). It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Most sandstone contains some clay minerals and may also include other minerals such as feldspar or fragments of rock, so most quartzite has some impurities with the quartz.

Figure 6.2.8: Quartzite from the Rocky Mountains, found in the Bow River at Cochrane, Alberta.

Even if formed during regional metamorphism , quartzite (like marble) does not tend to look foliated because quartz crystals don’t align with the directional pressure.

Practice Exercise 6.3 Naming metamorphic rocks

Provide reasonable names for the following metamorphic rocks based on the description:

  1. A rock with visible crystals of mica and with small crystals of andalusite. The mica crystals are consistently parallel to one another.
  2. A very hard rock with a granular appearance and a glassy lustre. There is no evidence of foliation.
  3. A fine-grained rock that splits into wavy sheets. The surfaces of the sheets have a sheen to them.
  4. A rock that is dominated by aligned crystals of amphibole.

Image Descriptions

Figure 6.2.5 image description: Metamorphic index minerals for a mudrock protolith. As conditions change with increasing metamorphism, certain minerals become unstable and undergo solid-state changes to form new, stable minerals. For example, between

300-400°C, the elements in chlorite will be re-ordered to form the mineral biotite. Note that while garnet, for example, is a common mineral in schist, it is not present in all schists! The new minerals that form in a metamorphic rock are dependent upon the composition of the protolith and a wide variety of minerals are possible. Approximate temperature range of metamorphic index minerals: Chlorite, 50 to 450°C. Muscovite, 175 to 625°C. Biotite, 300 to 725°C. Andalusite, 300 to 650°C. Garnet, 375 to 900°C. Sillimanite, 575 to 1000°C. Not all minerals in a metamorphic rock are indicative of a particular metamorphic grade. Quartz, feldspar, and calcite (not shown), for example, are stable over the entire range of temperatures shown in Figure 6.3.1. [Return to Figure 6.2.5]

Media Attributions

  • Figures 6.2.1, 6.2.2, 6.2.3, 6.2.4abd, 6.2.8: © Steven Earle. CC BY.
  • Figure 6.2.4c: Schist detail © Michael C. Rygel. CC BY-SA.
  • Figure 6.2.5: © Siobhan McGoldrick. CC BY.
  • Figure 6.2.6: Migmatite in Geopark on Albertov © Chmee2. CC BY.
  • Figure 6.2.7 (right): An outcrop of banded marble by the USGS. Public domain.

the texture of a metamorphic rock with a foliation

the texture of a metamorphic rock that is not foliated

a descriptive term for the relative temperature and pressure conditions under which metamorphic rocks form e.g., low grade, intermediate grade or high grade

Type of foliation defined by closely spaced, flat surfaces along which a slate splits. Formed by the growth of microscopic mica minerals.

a foliated metamorphic rock and a sheen on the surface produced by aligned micas (phyllitic foliation).

Type of foliation defined by scaly layers of visible mica minerals or other platy or elongate minerals. Rocks with this texture appear shiny or sparkly, as the light glints off cleavage planes of the aligned minerals.

Type of foliation defined by segregation bands of light and dark coloured minerals in a gneiss.

A foliated metamorphic rock with visible aligned mica crystals.

a foliated metamorphic rock in which the mineral amphibole is an important component

specific metamorphic minerals indicative of a particular metamorphic grade or range of pressure and temperature conditions

a rock that is a mixture of metamorphic and igneous rock, formed at very high grades of metamorphism when a part of the metamorphic rock starts to melt

metamorphism that takes place adjacent to a source of heat, such as a body of magma

metamorphosed limestone (or dolostone) in which the calcite or dolomite has been recrystallized into larger crystals

a non-foliated metamorphic rock formed from the contact or regional metamorphism of sandstone

a fine-grained metamorphic rock that is not foliated

A purely physical process (no change in composition) that occurs in a solid-state during metamorphism. Atoms in a mineral are re-organized and typically grain size increases.

metamorphism caused by burial of the parent rock to depths greater than 5 kilometres (typically takes place beneath mountain ranges, and extends over areas of hundreds of km2)


Density

Density is a measure of the mass of a mineral per unit volume, and it is a useful diagnostic tool in some cases. Most common minerals, such as quartz, feldspar, calcite, amphibole, and mica, have what we call “average density” (2.6 to 3.0 g/cm 3 ), and it would be difficult to tell them apart on the basis of their density. On the other hand, many of the metallic minerals, such as pyrite, hematite, and magnetite, have densities over 5 g/cm 3 . They can easily be distinguished from the lighter minerals on the basis of density, but not necessarily from each other. A limitation of using density as a diagnostic tool is that one cannot assess it in minerals that are a small part of a rock with other minerals in it.


Watch the video: Mineral Identification- Cleavage, Fracture