ROCK FORMING MINERALS

Mineral Pembentuk Batuan

Menurut W.T Huang (1962) komposisi mineral pembentuk batuan dikelompokkan menjadi tiga kelompok mineral, yaitu:

1.MINERAL UTAMA (Essensial Mineral): 2.MINERAL TEMBAHAN (Accessory Minerals)3.MINERAL SEKUNDER (Secondary Minerals)

MINERAL UTAMA (Essensial Mineral):

• Mineral yang terbentuk langsung dari kristalisasi magma dan kehadirannya sangat menentukkan dalam penamaan batuan. mineral utama dapat dilihat dari deret bowen series(1928).

…then click the placeholders to add your own pictures and captions.

Choose a layout…

Deret Bowen menggambarkan secara umum urutan kristalisasi suatu mineral sesuai dengan penurunan suhu [bagian kiri] dan perbedaan kandungan magma [bagian kanan], dengan asumsi dasar bahwa semua magma berasal dari magma induk yang bersifat basa.Bagan serial ini kemudian dibagi menjadi dua cabang; kontinyu dan diskontinyu.

On the Picture Tools | Format tab, you can create your own frames and make picture corrections such as adjusting contrast and brightness or cropping the picture for just the right look.

CONTINUOUS BRANCH [DERET KONTINU]

• Deret ini dibangun dari mineral feldspar plagioklas. Dalam deret kontinu, mineral awal akan turut serta dalam pembentukan mineral selanjutnya.

• Dari bagan, plagioklas kaya kalsium akan terbentuk lebih dahulu, kemudian seiring penurunan suhu, plagioklas itu akan bereaksi dengan sisa larutan magma yang pada akhirnya membentuk plagioklas kaya sodium.

• Demikian seterusnya reaksi ini berlangsung hingga semua kalsium dan sodium habis dipergunakan. Karena mineral awal terus ikut bereaksi dan bereaksi, maka sangat sulit sekali ditemukan plagioklas kaya kalsium di alam bebas.

Picture Quick Styles give you great looking “frames” in a single click.

• Bila pendinginan terjadi terlalu cepat, akan terbentuk zooning pada plagioklas [plagioklas kaya kalsium dikelilingi plagioklas kaya sodium].

Contoh struktur Zooning pada Plagioklas

Discontinuous branch [deret diskontinu]• Deret ini dibangun dari mineral ferro-

magnesian sillicates. Dalam deret diskontinu, satu mineral akan berubah menjadi mineral lain pada suhu tertentu dengan melakukan reaksi terhadap sisa larutan magma.

• Bowen menemukan bahwa pada suhu tertentu, akan terbentuk olivin, yang jika diteruskan akan bereaksi kemudian dengan sisa  larutan magma, membentuk pyroxene.

• Jika pendinginan dilanjutkan, akan dikonversi ke pyroxene,dan kemudian biotite [sesuai skema].

• Deret ini berakhir ketika biotite telah mengkristal, yang berarti semua besi dan magnesium dalam larutan magma telah habis dipergunakan untuk membentuk mineral.

• Bila pendinginan terjadi terlalu cepat dan mineral yang telah ada tidak sempat bereaksi seluruhnya dengan sisa magma, akan terbentuk rim [selubung] yang tersusun oleh mineral yang terbentuk setelahnya. Rim tersusun atas mineral yang telah terbentuk sebelumnya, misal Olivin dengan rim Pyroxene.

Apabila kedua jalur reaksi tersebut berakhir dan seluruh besi, magnesium, kalsium dan sodium habis, secara ideal yang tersisa hanya potassium, aluminium dan silica. Semua unsur sisa tersebut akan bergabung membentuk Othoclase Potassium Feldspar. Dan akan terbentuk mika muscovite apabila tekanan air cukup tinggi. Sisanya, larutan magma yang sebagian besar mengandung silica dan oksigen akan membentuk Quartz (kuarsa).

Dalam kristalisasi mineral-mineral ini tidak termasuk dalam deret reaksi karena proses pembentukannya yang saling terpisah dan independent.

MINERAL UTAMABerdasarkan warna mineral, dapat dikelompokkan

menjadi dua kelompok yaitu:1. Mineral Felsik ( mineral-mineral berwarna terang ) Kelompok Plagioklas ( Anortit, bitownit, Labradorit,

Andesin, oligoklas, Albit) kelompok Alkali Feldspar (ortoklas, Mikrolin,

Anortoklas, Sanidin) Kelompok Feldspatoid (Leusit, Nefelin, Sodalit) Kuarsa Muskovit

• Kelompok plagioklas dan kelompok alkali feldspar sering disebut kelompok feldspar.

• catatan: Tidak semua mineral felsik berwarna terang tetapi ada mineral felsik yang berwarna gelap yaitu, obsidian.

• Mineral yang berwarna terang disebabkan banyaknya kandungan SiO2 dan jarang mengandung Fe dan Mg

2. Mineral Mafik (mineral yang berwarna gelap) Olivin (Forsterite dan Fayalite) Piroksen, dibagi menjadi dua kelompok yaitu

Orto Piroksen (Piroksen tegak) dan klino piroksen (piroksen miring). Orto piroksen antara lain; Enstatite dan Hypersten. Klino piroksen antara lain; Diopsit, Augit, Pigeonit, Aigirin, Spodemen, Jadeit.

Amfibol (Hornblande, Labprobolit, Riebeokit, Glukofan)

Biotit.

MINERAL TEMBAHAN ( ACCESSORY MINERALS)

Adalah mineral-mineral yang terbentuk oleh kristalisasi magma, terdapat dalam jumlah yang sedikit (kurang dari 5%). kehadirannya tidak menentukan nama batuan. Contoh dari mineral tambahan ini antara laian : ZIRKON, MAGNESIT, HEMATIT, PYRIT, RUTIL APATIT, GARNET,SPHEN.

MINERAL SEKUNDER (SECONDARY MINERALS)

• Merupakan mineral-mineral ubahan dari mineral utama, dapat dari hasil pelapukan, reaksi hidrotermal maupun hasil metamorfisme terhadap mineral utama. contoh dari mineral sekunder antara lain; SERPENTIN, KALSIT, SERISIT, KALKOPIRIT, KAOLIN, KLORIT, PIRIT.

PENGELOMPOKKAN MINERALA. BERDASARKAN FUNGSINYA• Mineral pembentuk batuan (rock forming minerals),

yaitu kelompok mineral yang keterdapatannya dalam jumlah banyak pada batuan dapat menentukan nama batuan terutama batuan beku. Di antara mineral pembentuk batuan itu adalah Kwarsa, Feldspar, Plagioklas, felspatoid (foida), Olivine, Biotot, Hornblenda dll.

• Feldspar merupakan mineral silica yang paling banyak terdapat dalam batuan (lebih dari 50 %). Dalam batuan beku mineral ini merupakan bagian terbesar yaitu sekitar 60 %, 30% terdapat dalam batuan malihan, sisanya dalam batuan endapan seperti batu pasir (arkose dan graywacke) dan konglomerat.

BERDASARKAN SUSUNAN/KOMPOSISI Mineral silica, yaitu mineral yang mengandung

gugusan silica (mengadung unsur Si dan O) atau alumosilika (Si, O dan Al) seperti sebagian mineral pembentuk batuan. contohnya adalah Ortoklas, Plagioklas, Hornblenda, Biotit dls.

Mineral Oksida, yaitu mineral yang mengadung gugusan oksigen, seperti Kwarsa, Kasiterit, Magnetite, Hematite, Khromit dls.

Magnetite sulfide, yaitu mineral yang mengandung gugusan sulfur, seperti Pirit, Khalkopirit, Sfalerit dls.

Mineral karbonat, yaitu mineral yang mengadung gugusan karbonat, seperti Kalsit dan Magnetis dls.

Mineral sulfat, yaitu mineral yang mengandung gugusan sulfat.

Mineral-mineral Sulfosalt, Fosfat Molibdat, Borat, Tungstat, Vanadad, Halide dll.

BERDASARKAN PEMBENTUKKANNYAMenurut genesanya mineral dapat di

kelompokkan menjadi mineral primer dan mineral sekunder.

Mineral primer, yaitu mineral yang terbentuk pada awal pembentukkan magma atau pembentukkan batuan. Mineral primer ini biasanya menyusun batuan beku. Contoh Feldspar, Plagioklas, Biotit, Kwarsa, Galena, Pirit, Khalkopirit, dls

Mineral sekunder, yaitu mineral yang terbentuk karena adanya proses ubahan atau pengaruh perubahan temperature, tekanan atau kedunya yang di alami batuan. Mineral macam sekunder ini dapat terbentuk karena pengaruh intrusi magma atau kegiatan atmosfir.

Contoh mineral lempung (Kaolin, Monmorilonit, Smektit, illite, Serisit dls), Serisit, Biotit (sekunder).

Olivine is a simple orthosilicate in which the SiO4 tetrahedra are independent of each other. It is a solid solution of the end-members forsterite (Mg2SiO4) and fayalite (Fe2SiO4), although most examples are closer to the forsterite end-member.Olivine is very susceptible to alteration and often has a brownish weathering rind of assorted clay minerals.Olivine is most commonly found in igneous rocks of low silica content, such as basalts and gabbros, and is occasionally found in metamorphic rocks.

• Chemical composition - (Mg, Fe)2SiO4Hardness - 6.5-7Specific gravity - 3.2-4.4Transparency - Transparent to translucentColour - Yellowish green to green, also colourless, greenish brown to blackStreak - WhiteLustre - VitreousCleavage/fracture - Imperfect / conchoidalCrystal habit/mode of occurrence - Prismatic (equant to elongate) / granular, massive

Orthoclase is a member of the feldspar group (like plagioclase) and is a framework silicate. Orthoclase, also known as alkali feldspar or K-feldspar, is one end-member of a solid solution between orthoclase and albite.Orthoclase is found in silica-rich igneous rocks such as granite, and in high grade metamorphic rocks.

• Chemical composition - KAlSi3O8Hardness - 6Specific gravity - 2.6Transparency - Translucent to opaque (rarely transparent)Colour - Pinkish white, off-white, yellow, or shades of red, orange to brownStreak - WhiteLustre - VitreousCleavage/fracture - Perfect in two directions, seldom twinned / hackly, conchoidalCrystal habit/mode of occurrence - Prismatic, tabular

Plagioclase is a member of the feldspar group (like orthoclase) and is a framework silicate. Plagioclase consists of a solid solution between the albite and anorthite end-members, and together with quartz is the most common of the rock forming minerals.The twinning in plagioclase produces stacks of twin layers that are typically fractions to several millimetres thick. These twinned layers can be seen as striation like grooves on the surface of the crystal and, unlike true striations, these also appear on cleavage surfaces.Plagioclase is found in almost all igneous rocks and most metamorphic rocks, but is less common in sedimentary rocks where it usually weathers to clay minerals or a fine grained variant of muscovite (sericite).

• Chemical composition - CaAl2Si2O8 (anorthite), NaAlSi3O8 (albite)Hardness - 6-6.5Specific gravity - 2.6-2.8Transparency - Translucent to opaque (rarely transparent)Colour - Usually white, grey or colourlessStreak - WhiteLustre - VitreousCleavage/fracture - Perfect in two directions, commonly twinned / hackly, conchoidalCrystal habit/mode of occurrence - Prismatic, tabular

Muscovite is a member of the mica group of silicate minerals (sheet silicates) in which the base of all of the SiO4 tetrahedra lie in one plane and three corners of the base are shared with the neighbouring tetrahedra. This creates a strongly layered sheet-like structure, hence the term sheet silicate (the sheets are weakly bound together by layers of potassium ions). Muscovite, biotite and chlorite are all common mica group minerals.

Muscovite is commonly found in metamorphic rocks such as schists and gneisses, sedimentary rocks (as the fine grained variety sericite), and in igneous rocks such as granite.

Although muscovite has perfect cleavage, individual sheets are quite durable and are often found in sands that have undergone erosion and transport that would have destroyed most other minerals. Sheets of muscovite have high heat and electrical insulating properties and are used in the manufacture of many electrical components. Muscovite sheets were used for kitchen oven windows before synthetic materials replaced them.

• Chemical composition - KAl3Si3O10(OH)2Hardness - 2-2.5Specific gravity - 2.8Transparency - Transparent to translucentColour - White, silver, yellow, green and brownStreak - WhiteLustre - Vitreous to pearlyCleavage/fracture - Perfect in one direction producing thin sheets or flakes / unevenCrystal habit/mode of occurrence - Tabular (sheets or flakes)

Quartz is a complex silicate in which all the oxygen atoms of the SiO4 tetrahedra are shared between two tetrahedra, leading to complex 3-dimensional frameworks. For this reason, quartz is referred to as a framework silicate.Quartz is among the most common of all rock forming minerals and is found in many metamorphic rocks, sedimentary rocks, and those igneous rocks that are high in silica content such as granites and rhyolites. It is a common vein mineral and is often associated with mineral deposits.

• Cryptocrystalline varieties are used as semi-precious stones and for ornamental purposes. These varieties are divided more by character than by colour. Chalcedony, or agate, is divided into innumerable types that have been named for locally common varieties. Some of the more beautiful types have retained their names while other names have faded into obscurity. Some of the more common are chrysoprase (a pure green agate), sard (a yellow to brown agate), sardonyx (banded sard), onyx (black and white agate), carnelian (a yellow to orange agate), flint (a colourful and microscopically fibrous form), jasper (a colourful impure agate) and bloodstone (a green with red speckled agate).

• Chemical composition - SiO2Hardness - 7Specific gravity - 2.65Transparency - Transparent to translucentColour - Clear is most common (pure quartz), also white or cloudy (milky quartz); but can be very variable - purple (amethyst), pink (rose quartz), grey or brown to black (smoky quartz) are also common; yellow to orange (citrine) are more rare; cryptocrystalline varieties can be multicolouredStreak - WhiteLustre - VitreousCleavage/fracture - Non-existent / conchoidalCrystal habit/mode of occurrence - Prismatic (hexagonal prism terminated with a six sided pyramid) / cryptocrystalline, massive

QUARTZ ROSE QUARTZ

Hornblende is a member of the amphibole group of more complex silicates, in which the tetrahedra are linked to form a continuous chain twice the width of the pyroxene chains. For this reason they are often referred to as double chain silicates. Like the pyroxenes, they can be subdivided into those with orthorhombic symmetry (orthoamphiboles) and those with monoclinic symmetry (clinoamphiboles). Hornblende is the most common of the clinoamphiboles.

• Hornblende is commonly found in metamorphic rocks such as schists and gneisses, and igneous rocks such as diorites and dacites.

• Chemical composition - Ca2(Mg, Fe, Al)5(Al, Si)8O22(OH)2Hardness - 5-6Specific gravity - 2.9-3.4Transparency - OpaqueColour - Dark green to blackStreak - Dark greenLustre - VitreousCleavage/fracture - Imperfect in two directions at 56° and 124° / unevenCrystal habit/mode of occurrence - Prismatic, acicular, fibrous / massive, granular

Biotite is a member of the mica group of silicates (sheet silicates), like chlorite and muscovite. It occurs in more geological environments than any of the other micas. It is a common rock forming mineral, being present in at least some percentage in many igneous rocks (e.g. granite and rhyolite), and metamorphic rocks (e.g. schist, gneiss).

• Chemical composition - K(Fe, Mg)3AlSi3O10(F, OH)2Hardness - 2.5-3Specific gravity - 2.9-3.4Transparency - Transparent to translucentColour - Brown to blackStreak - Very pale brownLustre - Vitreous to pearlyCleavage/fracture - Perfect in one direction producing thin sheets or flakes / unevenCrystal habit/mode of occurrence - Tabular (sheets or flakes) / granular

Garnet is a more complex orthosilicate (than olivine, for example) in which the SiO4 tetrahedra are still independent.Garnets have the general chemical formula A3B2Si3O12, where A is a divalent cation (Fe2+, Ca2+, Mg2+, Mn2+) and B is a trivalent cation (Fe3+, Al3+, Cr3+). The end-members pyrope, almandine, and spessartine form one solid solution series, while the end-members grossular, andradite and uvarovite form another.

• Although valued as a gem stone, garnet is generally of low monetary value because of its relatively common occurrence.

• Garnet is commonly found in highly metamorphosed rocks and in some igneous rocks. They form under the same high temperatures and / or pressures that form those types of rocks. Garnets can be used by geologists to gauge the temperature and pressure under which a particular garnet-bearing rock formed.

• Chemical composition - Fe3Al2Si3O12 (almandine)Hardness - 6.5-7.5Specific gravity - 3.6-4.3Transparency - Transparent to opaqueColour - Variable - most commonly red, reddish brownStreak - WhiteLustre - Vitreous to resinousCleavage/fracture - Non-existent / conchoidalCrystal habit/mode of occurrence - Prismatic (12-sided rhombic, 24-sided trapezoidal) / granular, massive

GARNET IN SCHIST GARNET

Talc is the softest mineral, demonstrated by its position at the bottom of Mohs' Scale of Hardness with a relative hardness value of 1. It has a soapy, greasy feel.Talc is formed by the hydrothermal alteration of ultrabasic rocks, or low grade thermal metamorphism of siliceous dolomites.

• Most people know talc as the primary ingredient in talcum powder. However, talc is an important industrial mineral. Its resistance to heat, electricity and acids make it useful for lab counter tops and electrical switchboards. It is an important filler in paints, rubber and insecticides.

• Talc often replaces other minerals atom by atom to form pseudomorphs, taking the form of the replaced mineral. Thus, a specimen of what appears to be milky quartz would actually be talc, having a soapy feel and being able to be scratched with a fingernail.

• Chemical composition - Mg3Si4O10(OH)2Hardness - 1Specific gravity - 2.7-2.9Transparency - Crystals translucent, masses opaqueColour - Green, grey and white to almost silverStreak - WhiteLustre - Pearly, greasyCleavage/fracture - Perfect in one direction / unevenCrystal habit/mode of occurrence - Tabular (thin flakes, never large crystals) / granular, cryptocrystalline