Conversations with the Earth

Endapan mineral di Finlandia dan Swedia

Perjalanan saya ke lingkaran kutub utara

Atlas of ore minerals: my collection

Basic information of ore mineralogy from different location in Indonesia

Sketch

I always try to draw a sketch during hiking

Apa itu inklusi fluida?

Inklusi fluida adalah istilah yang digunakan untuk menjelaskan adanya fluida yang terperangkap selama kristal tumbuh. Gas dan solid juga bisa terperangkap di dalam mineral.

Situ Cisanti di Pengalengan, Bandung

50 km dari Bandung, Situ Cisanti terkenal karena menjadi sumber mata air sungai Citarum

Showing posts with label article in english. Show all posts
Showing posts with label article in english. Show all posts

Monday, September 4, 2017

Regional Aspects of Economic Geology (North America-South America-Africa-Australia)

NORTH AMERICA

Major geological unit:
a. North American (Canadian) Shield (Precambrian)
b. Appalachian orogeny (paleozoic)
  - Mineralization in Paleozoic orogens (Pb/Zn)
c. North american cordillera (Mesozoic-cenozoic)
d. North American platform
a. North American (Canadian) Shield (Precambrian)
- Consolidation of Archean nuclei (2,7-2,5 Ga)
- Late Archean to early Proterozoic basic and ultrabasic (2,6 Ga)
- Hudsonian/ Penokean orogeny (1,9-1,75 Ga)
- Paleoproterozoic sediments (e.g. Huronian supergroup)
- Sudbury impact (1,85 Ga)
- Anorogenic orogeny (1,6-1,2 Ga)
- Greenvillian orogeny (1 Ga) 

A. Consolidation of Archean Nuclei Deposit:
- stratiform massive Zn-Cu sulphides (VMS)
--> ore types: massive, laminated, breccia, stringer-type
ex. Abitibi belt : Kidd Creek deposit (hydrothermal black smoker) Kidd Creek is source of indium
- Orogenic gold ex. Ontario (Porcupine district, Hemlo)
- Archean BIF (syn-genetic)
ex. Homestake type (commonly located along the contacts of ultrabasic-basic volcanic rock to sediments)
ex. Algoma type (keypoints: associated with submarine volcanic rock, siderite-hematite BIF, less-extensive than superior type)
- BIF-superior type --> up to 200 m (chemical carbonate sediments)
=iron ranges related with Dulluth gabbro (Cuyuna, stromatolitic), Biwabik Iron Formation
=textures in Superior BIF
==Cherty (oxide facies, shallow water, thick bedded, granular, massive)
==Slaty (carbonate-silicate facies, deep water, thin bedded, non-granular, laminated)
- Ni,Cu (sulphides) --> in komatiite rocks within greenstone belts
- Rare-element pegmatites Sudbury district: Ni up to 50% world production

B. Mineralization in paleoproterozoic (1,9 Hudson orogeny- Churchill province-Transhudson orogen) -massive polymetallic
-uranium/Ag in Great Bear Lake

 C. Sudbury complex (1.85 Ga)
- meteorite impact, almost near to the mantle, resulted in the lake of magma
= stratigraphy from top to the bottom:
a. Post-impact sedimentary rock (top)
b. Suevite (Onaping sediment)
c. Impact-melt layer (granophyre - quartz gabbro - norite)
d. Brecciated crater floor (bottom)
== differentiation of layered complex
== formation of Ni-Cu-PGE mineralization in sublayer and offset dikes

D. Mineralization in Middle Proterozoic clastic sedimentary basin: U, Cu, Ni
= native Cu
= sedimentary U
= thick clastic sediments, basalts and native Cu (Ni-Cu-Co Thompson,Voisey's, Duluth Complex)

E. Anorogenic magmatic province (1.5 - 1.4 Ga)
=Titanium deposits (rutile, ilmenite) in anorthosites

Appalachian-Ouchita orogen, Innuit-Ellesmere Orogen (Arctic), Antler orogen (Rocky mountains)
=Pb/Zn, baryte (VMS, SEDEX)
ex. Brunsswick no.12
=Iron ores
=Asbestos (ophiolites from ultramafic rocks around Quebec, biggest in the world)
=Rare element (Li) pegmatites


b. Mineralization in the forelands of Paleozoic orogens: 

Pb-Zn (MVT)
=forebulge, basinal brines
=major exploration target: forebulge unconformities, syncollisional faults and strike slip zones
==metal-bearing fluids migrate towards the marginal zones of the orogenic foreland due to hydrostatic and thermal gradients, compaction of fine-grained basin sediments and squeezing-out of basinal brines (by tectonic pressure)
==uptake of hydrocarbons and metal precipitation along permeability, boundaries, mainly in carbonates
==ore minerals: galena, sphalerite (Ag-poor),fluorite,barite,pyrite,marcasite,chalcopyrite,siegenite, bravoite. Carbonate occur as a "pod" for sulphide
==hydrocarbons and coal
==uranium (roll-front and colorado type)

c. Mineralization in the North American Cordillera

- part of the global circum-Pacific belt (Cu,Mo,Au,Ag)
==Deposits in the Lower Precambrian (>2500 Ma) 
ex. Stillwater complex (Montana),
==Deposits in the Middle and Upper Proterozoic (1500-600 Ma)
ex. SEDEX
-PGE, chromite, layered ultramafic-mafic intrusion (44 km strike length), 2.7 Ga
==Deposits in the Paleozoic (Cambrian to Carboniferous)
ex. Red Dog (Alaska)
==Deposits in the Mesozoic
ex. Stratiform VMS in volcanic rocks, porphyry copper deposits, Juneau
ex. Orogenic: Juneau, Klondike
==Upper Paleogene to recent
ex. Bingham (Utah), Climax (Colorado), epithermal vein-type Au-Ag ores

Deposit
==Disseminated Cu/Mo-porphyry (by products: Au,Ag,Pb,Zn,Mn,Fe,Mo)
==Epithermal Au-Ag
==Carlin deposit (polycyclic)
==Massive and banded VHMS (Cu-Pb-Zn) (ex: Red Dog, Selwyn)

AFRICA

3,5-3,0 Ga : greenstone belts : Nickel sulphide
  • Granite (Sn) 
  • Sediments 
  • Nickel in komatiite (most important) in Australia 
  • Zn-Cu in VMS (Barbara Mt, south Africa) + Sb+Au 
  • Banded-iron-formation
----------- break (granite intrusion) do not have much economic importances (Sn, Ta, Li) 
ex. Wodgina in Australia
3,0-2,5 Ga : greenstone belts
  • Witwatersrand gold (paleoplacer) Au
-----------Limpopo events associated with great dyke
  • LIC (chromium) PGE, REE, Sn-Ta
2,5-2,0 Ga : shelf sediments
  • Banded-iron-formation (most important), Mn
-----------big orogeny event in west aftrica (ebunean/ebundian) collision granite Sn-Ta
  • Bushveld LIC 
  • Vredefort impact (Transvaal)
2,0-1,0 Ga : anorogenic event (boring event) = Proterozoic
  • Kibaran belt 
  • SEDEX in Ghammsberg Zn-Pb-(Cu)
1,0 Ga       : big event granite Sn-Ta-W
----------- erosion causes mollase
1,0-0,5 Ga : Congo copper belt
  • Clastic: sediment-hosted kupferschiefer type (Cu,Co,U)
  • Carbonate: tsumeb type (Cu,Zn,Pb,As,Ag,…)
  • Carbonate: MVT (Pb-Zn)


0,5 Ga          : Pan African orogeny (major granites Ta-Nb-Sn)

1. Deposits of the African Precambrian platform and orogenies1.1 Deposits of the Guinean metallogenic province
- Fe ores (BIF; Archaean to Upper Proterozoic)
- Mn: Nsuta (Ghana) - upper Birimian gondites; 50-55% MnO2
- Cu: Guelv Moghrein (Mauretania)
- Au: in qz veins: Obuasi, Bibiabi, Prestra, Bondaie, Konongo, Nangodi (Ghana)- Birimian bearing conglomerates
- Diamonds: Ghana and Ivory coast. In Guinea: Mesozoic kimberlites. In Liberia: placers
1.2. Deposits of the Sahara-Cameroon metallogenic province
- Bou Azzer Co-(Ni-As-Au): ore shoots 40m thick in dolomitized serpentinite, amphibolite, chlorite schist
- Jos plateau, Nigeria Sb, Nb, W in younger granites

AUSTRALIA

Archean Yilgarn-Pilbara Craton
1.1. Kambalda Ni subprovince (komatiites)
1.2 Norseman-Wiluna goldbelt
1.3 Volcanogenic Cu-Pb-Zn-Ag
1.4 Archean Fe-ores(Algoma BIF)
1.5 Pegmatites(Ta, Sn, Li)
Wodgina (Pilbara)
- pegmatite
Greenbushes (Yilgarn)
- spodumene pegmatite (LCT) 2,5 Ga
- world largest hard rock lithium mine

Proterozoic Arunta Gawler Province2.1. Hamersley Range BIF
Hammersley BIF
- Mt Tom Price
mineralogy: Mt, Hm, mpHm
2.2. BangemallPb-Cu-Ba(Abra)
2.3. Kimberley Fe-Ni, diamonds
2.4. PineCreek Au-U, basemetal
2.5. Central Australia. TennantCreek Au-Cu-Bi
2.6. Mount Isa basemetal(SMS)
Mt Isa (1,54-1,45 Ga)
- Zn-Pb-Ag-(Cu)
- syngenetic base metal accumulation in the basin

2.7. McArthur basin base metal(SMS)
2.8. BrokenHill base metal(SMS)
- Zn-Pb, New South Wales
2.9. Gawler Range-Stuart Shelf: BIF, IOCG
Olympic Dam-IOCG
- Hm-qz breccia
- Hm-granite-breccia
- Volcanic rock diatreme
Keypoints:
o   1. Olympic dam granite
o   2. Extensional fault, volcaniclastic sediments preserved in fault basin
o   3. Breccia complexes emplaced along faults
2.10. Adelaide base metal
2.11. Georgetown, Yambo, CoAu-Cu

3. Paleozoic Tasman metallogenic province3.1. Delamere subprovince
= Mount read volcanic
= Au-Cu-Pb-Zn-Ag
= syngenetic
3.2. Lachlan subprovince
= Bendigo, gold-quartz saddle veins (orogenic Au)
= In addition to Au-Sn+Cu-W-Sb-U-Th-mineralization
= Saddle reefs
3.3. New England-Queensland subprovince

ex. Ridgeway PCD, Cadia District

4. Secondary deposits
5. Older Alpine province
6. Australasian Alpine province

-Related to Mesozoic-Cenozoic subduction along the eastern margin of the Australasiatic plate.
-Older and younger Alpine provinces
-Deposit: Au-Cu porphyry, Ni-laterites and lateritic Ni/Co ores



South America

Brazil 

Sketch after Vanecek 1994 (modified from the lecture note given by Frank Melcher-regional economic geology)


- Fe (3), Nb (1), Ta (3), Bauxite (4)
- Asbestos (3), graphite (3), kaolin (6), talc (4), vermicculite (3)

BIF in Brazil (2,8-1,6 Ga)
grade: primary Fe: 30-50%
a. Priasov type
ex. Sierra de Imataca
mineralogy: original magnetite ores upgraded to secondary "crustal ores"
b. Algoma type 
ex. Minas Gerais, Raposos
keypoints: Itabirite-Archean granitic gneisses and migmatites (metamorphosed at 2,75 Ga), phyllite
lithology: quartzite-phyllite-itabitite-dolomite
mineralogy: hematite rich, oxide
c. Superior type
ex. Serra dos Carajas, Para
mineralogy: hem-mag-martite-goethite
d. Uncertain genesis
ex. Urucum/ Mato Grosso

Bauxite
producer: Brazil, Venezuela, Guyana, Suriname
-HR: silicate rock with high-Al and low-Fe, SiO2 removal produces Al2O3 ranging from 28-60%
-formation condition:
humid to dry and warm climate
plateau
open anticlinal

Lowland type-bauxite: developed on kaolin-rich young sediments
Plateau type-bauxite: weathering of Precambrian rocks

Fe-Ti-V-Cr
associated with metabasite rocks in greenstone belts and LIC
mineralogy: scheelite, cassiterite
locality: Northern Brazil, Pitinga (1,8 Ga) with Sn, Nb, Ta, REE, Zr, Y

Gold
Serra Pelada
IOCG Carajas
host: granite, gabbronorite, metavolcanics, BIF, gabbro (age 2,8-1,6 Ga)

Pegmatite
commodity: gemstone, quartz, rare metal, elbaite (multicolor gemstones), beryl
related to Brasiliano orogeny (600-500 Ma, same event as Pan African orogeny)
Amethyst: basalt lavas, degassing lava at 1150 degree, filling the crack (epigenetic)

Argentina

Producer: Li (4), B (4), diatomite (3)
low- to intermediate- sulfidation Cu-Pb-Zn-Ag-Au-In
mineralogy: sphalerite with elevated In
- mainly mesozoic-cenozoic deposits (part of Circum Pacific megaprovince, subduction Nazca plate with South America plate)
Chile
Producer: Mo (3), As (2), Cu (1), Li (1), B (4)
IOCG: Archean to tertiary
setting:
- within magmatic arc (Jurassic to Lower Cretaceous)
- related to intrusions of gabbro-granodiorite
- primitif mantle signature, tholeiitic-calc-alkaline
- extensional tectonic and formation of orogen-parallel fault system
- east of the arc: volcanosedimentary rock in back-arc setting
- alteration: pervasive feldspar destructive
- forms of IOCG: Veins in magmatic rocks(gabbro, diorite), hydrothermal breccia, mantos, skarn, composite deposits in volcano sedimentary settings

Cordilleran Mineral Province
porphyry Cu-Au

Manto deposit
stratabound, associated with andesites

Colombia
Platina di Pinto (platinum) from 17th

Bolivia
Producer: W (4), Sb (4), Sn (4)
Peru
Producer: Mo (4), Cu (3), Pb (4), Zn (3), Sn (3)

> 30% world porphyry-Cu
shape: elongated, ovoid, mineralized zones

Pb/Zn deposits
- vein system in shallow and peripheral parts of PCD systems
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Wednesday, July 26, 2017

Hiking with Kid, why not?


July 17th, 2017. We packed our lunch, children carrier, water and took a bus at about 8.40 from Leoben Annaberg. The bus costs 4 Euro/ person, because we have zwei und mehr discount card (Steirische Familienpass), which allow reduced price (38%). Unfortunately, the OeBB Vorteilscard Family cannot be used oin Postbus. It tooks for about 40 minutes from Leoben to Praebichl. 
At around 9.30, we arrived at Praebichl bus stop. Although it is a summer time, the wind blows mildly. It was too cold for Vidya, so she tooks her jacket, but Aqila insisted that it was not cold. From the bus stop, we searched for the hiking route and Aqila, our 3 years old toddler was very excited. We followed the hiking signature "Erzwanderweg."
 
From there, the hiking trails was surrounded by high trees (pine?) and moss. Aqila can still manage and did not want to sit in the bike carrier. After 1 hours walked, she was tired and wants to sit in the bag. I was surprised that she could managed to hiked by herself. We took the shortest way to reach Polster from Praebichl, which means, the steepest route. After high trees, the scenery is more opened and we can saw the Polsterschutz huette. From that point, the route split into two. To the left is Polster and the other side is the "knappensteig" to Leobnerhuette. We took the left and no more high trees surrounding us. 
 It was 11.00, and Vidya got tired and was hungry. We decided to stop in a beton under the "sessellift", where scenery to Reichenstein, Erzberg and Kaiserschild are clearly be seen. Aqila was excited and I offered her pen and paper to draw. It was one hour break. Some other hiker was behind us, some brought their "flying bags" (i do not know how to say). 
It was 12.00, we continued until Polsterschutz huette. It tooks less than 1 hour and we met Ludwig, the owner of those huette. We ordered a cup of coffee and black tea. It was only 6 people on that huette. We were three, Ludwig and another couples above. Ludwig said, that the polster lift will be renovated soon. He seemed happy, because more people will come on winter, but he said, he cannot depending only from those huette. It was a hobby for pensionist, he said.
 
From huette, we continued our steps to Polster. In half an hour, we arrived at the top of Polster. Hurraaay, I celebrated my 29th birthday in Polster....
We stayed for about 20 minutes and enjoyed the scenery, and follow the routes into the Leobner huette, and tooked the zigzag trails until back to Praebichl. It was longer than I expected, and we arrived back at bus halte at 16.25. In totals, it tooks 6 hours of walked (1 hour Aqila walked by herself) and 5 hours in my bag. My shoulder was getting red, because then I realized that the bag was a little bit inclined. Anyway, I am pleased that Vidya, Aqila enjoyed this trip, and asked me "where will we hike after this?".  I just smiled to her. 
 




Video 3 - Hiking bersama buah hati 


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Saturday, July 22, 2017

I found my stolen bike!

22.07.2017

13.30: Me, Vidya and Aqila went to buy some stuff in a store. We park our bike side by side and lock it together.



15.40: We finished shopping. Unfortunately, someone opened the lock. Vidya's bike was lost and another bike was not locked.

15.42: I decided to report this cases to the local police officer.

15.43: I MET THE THIEF ON MY WAY TO POLICE OFFICE! +-20-25 years old guy riding Vidya's bike. He was trying his new bike and did not recognize me. I try to hit the bike and he run. I cannot hit him, unfortunately I felt down. He drove away using Vidya's bike.


15.44: I drove back and try to chased at the back of him. Fortunately, I can still see him, going through the main square, and so on, and he went to his apartment.

15.48: I was too late, the door is locked, but I am 100% sure that the thief has entered the apartment with Vidya's bike. I made an emergency call: 122 (Remember this number!:EURO emergency call).

15.55: Officer came, 2 cars, 4 officer (2 men, 2 women). They opened the door. I was waiting outside.

15.58: Officer asked me to come in. I found my bikeeeeee.....

15.58 - 17.00: I was interrogated in Police office. They were very kind and professional. The report will be submitted to the Justizezentrum Leoben (court). I HOPE THEY FOUND THE THIEF.

Thank you for quick response and cooperation. The officer said, locks with number combination were easy to be opened (although I know that my current lock is perfect-number combination, but then, after today, I changed my mind). They suggest to change with a locks+key.

Congratulations AmazonPrime, you have new customer for bike accessories. Anyway, thank you officer for quick response. Vielen Dank LPD (Landes Polizei Direktion) Leoben!

To Joel and Anung, I am sorry for this matter. I will make sure your bike is saved with me. Thank you for your trust.

Alhamdulillah, masih diberi kepercayaan dari Allah buat menjaga amanah. Barang dikunci saja bisa hilang, apalagi ga dikunci. Abus juga buatan manusia. Waspadalah, waspadalah!


18.45: article started

19.13: article finished



Abus is still man made. Nothing is perfect.



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Tuesday, April 25, 2017

My Sketchbook

My wife  knows that: I hate details. I know my hand sketch always looks like unfinished sketch, but no matter how bad my sketch is, I enjoy this process. Time for relaxation, time for oneself, it is priceless. Everyone has their own problems, but one should divine our happiness. For me, sketching makes me happy. Please excuse my unrealistic sketch.

1. Der Dachstein
Dachstein (Hoher Dachstein-DE, Dach=roof, stein=rock), part of North Calcaerous Alps (NCA). It consists of Dachstein Limestone and strongly karstic. Due to the nature of karst, cave can be easily developed through the reaction of water with limestone, resulted by the rich cave system, including the Mammuthhöhle (the biggest ice cave in the world). At the back of Dachstein, you will find an oldest salt mine in a history, and may be, the one of the most of the picturesque village in the world. Hallstatt.



2. Großglockner Hochalpenstrasse
It was pretty sudden, when I decided to biking in the highest road in Austria. I packed all my stuff a week before, then, everything was so smooth until I can see the highest peak of Austria (3,798 m). The hohe Tauern (the mountain range where Großglockner is located) is the result of the subduction of the African plate towards the Eurasian plate. This mountain range formed as a product of subduction around 100 millions years ago. 
Grossglockner from andyyahya on Vimeo.

3. Amperstein and Marchreisenspitze
I started this tour when my Boss (the so called Professor) offering me to went hiking in the middle of conference. We drove to Axamer Lizum (Tirol) and started to climb up to reach Nockspitze (or some others call Saile). During our trip, my Boss described everyyyyything about the geology in this area, because he did a mapping in this area almost 30 years a go. He was totally different to me. He loves details. :p 

4. Hafelekarspitze
If I were asked, what is the best city to visit in Austria, then I will answer "Innsbruck." The combination of an old city, modernity and Karwendel Mountain is totally awesome. Innsbruck is located in the Inns valley and looking to the North, a huge dolomite mountain (Wetterstein dolomite) is just astonishing.

5. Gosowong gold mine
I visited this gold mine on 2014, when I have to accompanied two master students from Kyushu University to take a sample. The gosowong is a good example of high-grade low sulfidation epithermal deposit, also notable for a good mine closure. As far as I know, this mine has been closured and the gold project were focussed on another location, some of them are the Kencana and Toguraci.  

6. Batu dinding (the wall of stone)
As I try to remember, I draw this sketch when I driving a boat towards Long Pahangai, Mahakam Ulu region. That was my first time I love sketching. Batu dinding is a huge limestone formation which has a lot of foraminiferra. Batu dinding is located close to Long Bagun area.

7. Batu ayau
It is named after a legend of Dayaknese people. I started to draw when I was boring during my trip from Long Pahangai to Long Bagun. No more extra battery on my camera, I just documented what I have seen.  


The basic things of this cartoon, is the replacement of the hematite (ferrous) ore body with the marbles, and remobilized, enriched by the iron-rich fluid and metamorphic fluid, and minor meteoric fluids. The chloritisation occur near the ore vein, and pegmatite occur older than the mineralization. The mineralisation tends to be epigenetic, means, the ore deposit mineralised later than the host rock.

9. Iron ore mine, Erzberg and Eisenerzer Reichenstein, North Alps
The biggest iron ore deposit in Central Europe can be seen in a city of Eisenerz. It is thirty kilometer to the north of my current city, Leoben. On my birthday, me, my wife and daughter went hiking to Polster mountain, and from Polster, we can clearly see the Erzberg iron mine and Reichenstein mountain. Both are located in the city of Eisenerz.

10. Gare du Palais, Québec City
11. Mont Royal, Montreal, Canada
12. Windsborn Krater, Eifel, Germany
13. Bernkastel-kues, Germany
14. Trier, Germany


16. Karwendel Mountain from Innsbruck. This is the city where i would always want to come back to.


17. After 10 days of field works in Northern Finland and Sweden, I fly back to Vienna from Rovaniemi. This city is on the border with polar circle
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Friday, February 10, 2017

And after all, it;s just a carbonate


1. Okay, today let's take a look of some polish-thin section using microscope.
2. Wow, this sample is colourful. Let's take some photograph of it.
3. Time to describe, what is inside. Carbonate? absolutely.... we can't missed from the birefringence color. 
4. Now, what else to describe. Habit? Fibrous, banding? I though fibrous is more correct.
5. What kind of carbonate is it? I think it's calcite, because usually calcite (CaCO3) has fibrous habit. Let's write calcite. (see explanation below this bullshit)

6. But, how about the red-brownish thin veinlet? I don't think it's homogenous. Let's coat it using carbon and identify using SEM (scanning electron microscope).
.....................
7. Vacuum is ready, let's find those spot with fibrous calcite.
8. Wow, cool... finally i found the spot. Lte's analyse using EDS (energy dispersive x-ray spectroscopy)
9. Dammmn, it's not even calcite. It's dolomite-ankerite (Ca,Mg(CO3)2). See, Ca-Mg are bright. Fe is not that bright. Now, we know, the red-brownish veinlet is siderite (FeCO3), and the fibrous is not calcite. It's dolomite-ankerite...

..................... 
10. Yes, and then, what does it mean? Let's make some story...
11. Mg in dolomite. Hmmm...where does the magnesium come from? Let's search some literature. 
.....................
This finding is of significance as earlier reports of Phanerozoic radiaxial fibrous carbonates are exclusively of calcite mineralogy. Dolomite concretions described here formed beneath marine transgressive intervals within palustrine coal seams. This is of significance as seawater was arguably the main source of Mg2 + ions for dolomite formation. (literature somewhere )
.....................
12. Yes, correct... okay, don't forget, that most of sample were altered into dolomite. So, probably it is dolomitization (dolomitization: alteration of limestone into dolomite)
13. Woooow, I spend almost an hour only to describe one spot for one thin section............
14. Wooooooooow, I still have another samples to identify.....................
15. Wooooooooooooow, someone just learn mineralogy...... congratulatioooooon...
16. And after all, it's just a carbonate












explanation number 5: 
limestone does not always classified as calcite. Other form can be dolomite, ankerite, siderite, magnesite, or rhodochrosite. See ternary diagram below)

explanation point 9: 
the brighter the phase, means, the higher it's density, and the more abundant it's element. In this picture, only, Ca-Mg-O and minor Fe were detected. So, we can eliminate "rhodocrosite (MnCO3) and calcite (CaCO3). Now, see thin veinlet of Fe in the middle of fibrous. It is dark. It means, the thin veinlet should contain more Fe, and now, we understand, it is siderite)



suggested references:
-Microscopic description of veins
http://www-odp.tamu.edu/publications/149_SR/chap_34/c34_6.htm

artikel mikroskopi lain bisa dilihat disini
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