PERTEMUAN KE II: SIFAT FISIK MINERAL

MODUL 2

IDENTIFIKASI MINERAL

• Warna

• Bentuk Kristal

• Cerat

• Kilap

• Kekerasan

• Belahan dan Pecahan

• Sifat lain

Warna

Streak (Cerat)• Black - Graphite • Black - Pryite • Black - Magnetite • Black - Chalcopyrite • Gray - Galena • Limonite - Yellow-

brown • Hematite - Red-brown

LUSTER (KILAP):refleksi mineral dalam menangkap sinar: ada dua

yaitu metalik dan non-metalik

Non metalik: kilap kaca, tanah (earthy) dan tidak memantulkan sinar (dull)

Cleavage (Belahan)

• Mengikuti bentuk kristal

Fracture (Pecahan)• Yaitu bentuk permukaan

antar atom penyusun mineral saat mineral pecah atau hancur

• Pecahan: – beraturan (regular)

– tidak beraturan (irregular)

• Permukaan pecahan: – Kasar (splinterry)

– Halus (smooth)

– berbentuk kurva (conchoidal): seperti gelas saat pecah.

Kekerasan

Crystal Shape (Bentuk Kristal)

• Kubik

• Prismatik

• Tabular (berlembar)

• Menjarum

• Hexagonal

• Piritohedron

Sifat Lain

• Magnetism - A few minerals are attracted to a magnet or are themselves capable of acting as magnets (the most common magnetic mineral is magnetite). Because these are so rare, this property helps narrow the possibilities drastically when trying to identify an unknown specimen.

• Feel - Some minerals, notably talc and graphite, feel greasy or slippery when you rub your fingers over them. The greasiness occurs because bonds are so weak in one direction that your finger pressure alone is enough to break them and to slide planes of atoms past neighboring atomic layers

• Taste - Geologists use as many senses as possible in describing and identifying minerals. Taste is one of the last tests to be conducted, because some minerals are poisonous. Some minerals taste salty-most notably halite (salt). Sylvite, a mineral similar in all other properties to halite, tastes bitter. Taste is thus a diagnostic property because it distinguishes between these minerals. NEVER TASTE A MINERAL UNLESS INSTRUCTED TO!

• Reaction with Dilute Hydrochloric Acid - This is actually a chemical property rather than a physical attribute of a mineral. Minerals containing the carbonate anion (C03)2- effervesce ("fizz") when a drop of dilute hydrochloric acid is placed on them. Carbon dioxide is liberated from the mineral and bubbles out through the acid, creating the fizz. This test is best performed on powdered minerals. Calcite (calcium carbonate) will effervesce readily in either massive or powdered form, but dolomite (calcium-magnesium carbonate) reacts best as a powder.

Olivine

Pyroxene

Peridotite- Mostly Olivine

Gabbro- Pyroxene and Olivine

Andesite- Hornblend and Biotite? and Plagioclase Feldspar

Potassium Feldspar

Quartz

Pink Granite

Mineral Fractionalization in a Magma Chamber

Most rock-forming minerals are…

The percent composition of Silicate minerals in a rock determines the type of rock it is.

Rocks that have a high percentage of Silicates are light in color, and

are called Felsic rocks.

Rocks that have a low percentage of Silicates are dark

in color, and are called Mafic rocks.

Rocks are often made of several different minerals.

Minerals are most often identified by inspecting their

Physical and Chemical properties.

Color is the most easily observed, but least reliable property of a mineral for identification.

This is because the color of many minerals varies with the kind of impurities in the mineral.

Luster is the way in which a mineral shines in the light.

Types of Luster

Streak is the color of the powder left on a streak plate when a mineral is rubbed on it.

Streak is much more reliable than color because, although the color of a mineral changes, its streak does not.

Cleavage is the tendency of a mineral to split in a particular direction.

“Sheety” Cleavage

Feldspars always cleave readily at or near right angles.

Fracture is the tendency of a mineral to break in directions other than along crystal faces or cleavage surfaces.

Conchoidal or shell-like fracture

Fibrous facture

Hardness is the resistance of a mineral to being scratched.

The Mohs Hardness Scale is one of the most effective tool for identifying minerals.