Laboratorium Bahan Galian

Sie. Petrologi2015

ARTIKEL 1 :

Definition:

As metamorphic rocks change under heat and pressure, their ingredients

recombine into new minerals that are suited to the conditions. The concept

of metamorphic facies is a systematic way to look at the mineral suites in

rocks and deduce the pressure and temperature (P/T) conditions that

made them. There are seven widely recognized metamorphic facies,

ranging from the zeolite facies at low P and T to eclogite at very high P

and T. Geologists determine a facies in the lab after examining many

specimens under the microscope and doing bulk chemistry analyses.

Metamorphic facies is not obvious in a given field specimen. To sum up, a

metamorphic facies is the set of minerals found in a rock of a given

composition. That mineral suite is taken as a sign of the pressure and

temperature that made it. Here are the typical minerals in rocks that are

derived from sediments (pelitic rocks). That is, these will be found in slate,

schist and gneiss. The minerals shown in parentheses are "optional" and

don't always appear, but they can be essential for identifying a facies.

ARTIKEL 2 :

Definition:

Just as index fossils are pointers to a specific period of geologic time,

index minerals are minerals that point to particular conditions of

temperature and pressure. Index minerals are essential to studies of

metamorphic rocks.

The concept of index minerals relies on the chemical stability of minerals.

Most minerals are actually metastable at the Earth's surface, ready to be

degraded by the processes ofweathering and turn into a small number

of surface minerals.

Index minerals mark the reverse process, appearing at a particular

minimum of pressure and/or temperature as sediments are turned into

rock by lithification and then further transformed by diagenesis and

metamorphism to become metamorphic rocks.

The most widely used index minerals

are biotite, chlorite, garnet, glaucophane,hornblende, prehnite, sillimanite, 

staurolite, and the zeolites. In rough ascending order of

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Laboratorium Bahan Galian

Sie. Petrologi2015

pressure/temperature, they are zeolites, chlorite, prehnite, biotite,

hornblende, garnet, staurolite, sillimanite, glaucophane. But which index

mineral appears also depends on the composition of the protolith, or

original source rock. For instance, glaucophane is found only in

metamorphosed mafic rocks like basalt, and sillimanite appears only in

metasedimentary rocks. There are dozens of additional minerals that

supplement the major ones, and all of them together produce typical

assemblages, or metamorphic facies.

Index minerals are generally used to mark the lower end of a metamorphic

facies; that is, we look for rocks that have undergone a simple upward

progression in pressure and temperature.

Many index minerals are stable well above the conditions where they first

form, so their value is limited only to marking the point where they first

appeared. Garnet is an example: once it forms, it remains throughout all

higher stages of metamorphism. The calcite found in limestone has no

value as an index mineral because it never changes. The opposite casean�

index mineral that exists exclusively in one metamorphic faciesis called a�

critical mineral. Retrograde metamorphismchanges in minerals that�

happen as pressure and temperature decreasemay add confusion by�

changing some high-grade minerals while leaving others in a metastable

state.

ARTIKEL 3:

Observe across the top of the chart the ranges of the rocks slate, phyllite,

schist, and gneiss. But note that with a few exceptions (e.g. chlorite,

garnet) the minerals composing these rocks are not the same as for a

mafic parent. The chemistry of the parent rocks are different, so the

metamorphic minerals that can be produced are different.

     Chlorite is the one common mineral for the greenschist facies (low

grade), but after that the mineralogies are different. Quartz, feldspar, and

muscovite are relatively universal minerals, and so can not tell us much

about the intensity of metamorphism.

     Other minerals produced during the metamorphism are special

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however, because they each form at a specific set of

temperature/pressure conditions, which we know, and are thus index

minerals (red in the above chart). They are index minerals because they

index when, during the metamorphic processes, a particular set of

temperature and pressure conditions have been achieved, and they appear

in a set order: Chlorite, Biotite, Garnet, Staurolite, Kyanite, and sillimanite.

And because they each form at a higher temperature and pressure, when

we first find one of the index minerals in the rock we know a particualr

temperature/pressure condition has been achieved. 

     Imagine walking across a country side where metamorphic rocks are

exposed. As you walk you begin in a shale, but bit by bit the texture of

the rocks changes into slate, phyllite, schist, and finally gneiss. At the

same time the mineralogy is changing, so as we walk we notice the first

presence of the index mineral chlorite. The chlorite tells us that

metamorphism has begun at this point. 

     So, we keep walking, and soon the index mineral biotite appears; now

we know the metamorphic grade has reached the biotite grade. If we

were drawing a map as we went we would mark on the map this first

appearance of biotite. In fact, we would wander around finding the first

appearance of biotite everywhere, and plotting the points on the map. A

line connecting all these first appearances of biotite is an isograd (equal

grade), and tells us where in this country side, this big body of

metamorphic rock, the temperature and pressure finally reached biotite

grade. 

     We continue this process for each index mineral - garnet, staurolite,

kyanite, sillimanite - plotting out the isograd concentric circles that close

in tighter and tighter onto the source of the heat causing the

metamorphism.

     Note on the chart that the ranges of the index minreals overlap, so it is

possible to have more than one of these in a rock at the same time. But it

is always the higher temperature/pressure index mineral we want

because it tells us the highest grade of metamorphism the rock has

reached.

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Mineral Indeks Batuan Metamorf

Batuan metamorf merupakan batuan yang terbentuk akibat batuan asal

mengalami perubahan suhu dan tekanan. Perubahan suhu dan tekanan yang terjadi

pada batuan metamorf, memberikan pengaruh kepada mineral dari batuan asal

sehingga mineral berubah menjadi mineral baru yang juga stabil dan ideal pada

kondisi barunya.

Melalui konsep fasies batuan metamorf dapat diketahui mineral-mineral

hasil dari perubahan batuan asal dan pada suhu berapa mineral tersebut

terubahkan menjadi mineral baru. Mineral indeks menjadi penanda batas bawah

dari suatu fasies metamorphism.

Dalam pengertian sederhana mineral indeks batuan metamorf berarti

mineral yang menjadi penanda untuk kondisi suhu dan tekanan mineral tersebut

terbentuk pada batuan metamorf. Mineral indeks terbentuk pada satuan suhu dan

tekanan tertentu. Sehingga, ketika kita menemukan mineral indeks tersebut pada

batuan metamorf dapat diketahui suhu dan tekanan yang membentuk batuan

tersebut.

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Mineral-mineral indeks itu sendiri antara lain sebagai berikut berdasarkan

urutan dari suhu dan tekanan yang rendah hingga suhu dan tekanan yang paling

tinggi:

Zeolites

1. Chlorite

2. Prehnite

3. Biotite

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4. Hornblende

5. Garnet

6. Staurolite

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7. Sillimanite

8. Glaucophane

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