carbonate - calcite group Flashcards
calcite chemical formula
CaCO3
crystal form of calcite
- Hexagonal
- Three important habits: prismatic, rhombohedral, scalenohedral
- usually in crystals or in coarse- to fine-grained aggregates
- compact, earthy (as in chalk), and stalactitic (as in cave deposits).
calcite cleavage
rhombohedral perfect 74*55’
calcite hardness
3
calcite specific gravity
2.71
calcite color
- Usually white to colorless. Transparent to translucent
- Iceland spar is a chemically pure and optically colorless variety of calcite
- may be variously tinted gray, red, green, blue, yellow
- when impure, brown to black
- Luster vitreous to earthy
Calcite occurence
- one of the most common rock forming minerals
- igneous: a primary mineral or carbonatites and nepheline syenite; late crystallization product in the cavities in lava; in hydrothermal veins associated with sulfide ores
- meta: main constituent in marble
- sedimentary: main constituent in limestone; as stalagmites, stalactites, incrustations; travertine/tufa (cold/hot calcareous springs)
how to differentiate calcite from dolomite and aragonite
from dolomite: coarse calcite freely effervesces in cold dilute HCl
from aragonite: lower specific gravity and rhombohedral cleavage
magnesite formula
MgCO3
magnesite crystal form
- Rarely in rhombohedral crystals
- Usually cryptocrystalline in white, compact, earthy masses
- also in cleavable granular masses, coarse to fine.
magnesite cleavage
rhombohedral perfect
magnesite hardness
3.5 to 5
magnesite specific gravity
3 to 3.2
magnesite color
- Luster vitreous. Transparent to translucent.
- white gray yellow brown
magnesite occurence
- Common in veins and irregular masses derived from the alteration of Mg-rich metamorphic and igneous rocks; as such, a constituent of serpentinites and altered peridotites
= Such magnesite is compact and cryptocrystalline and may be associated with opaline silica. - Cleavable magnesite is found in talc, chlorite, and mica schists and in dolomitic limestones.
how to distinguish magnesite from chert and dolomite?
from chert: massive variety is softer than chert
from dolomite: has higher specific gravity
almost nonreactive to cold hcl, effervesces and dissolves with hot hcl
siderite formula
FeCO3
siderite crystal form
- Commonly in rhombohedral crystals, which may show curved faces
- in globular concretions and
- cleavable masses
- May be botryoidal, compact, and earthy.
siderite cleavage
rhombohedral perfect
siderite hardness
3.5 to 4
siderite specific gravity
3.96 (pure)
siderite color
- Luster vitreous. Transparent to translucent.
- light to dark brown
siderite occurence
- As a vein mineral, in well-crystallized form, it is associated with metallic ores containing silver minerals, pyrite, chalcopyrite, and galena.
- It also occurs as clay ironstone, with admixed clay minerals, in concretions with concentric layers.
- Also found in shales and coal measures as blackband ore, contaminated with carbonaceous material.
- It is also a common constituent of sedimentary Precambrian iron-formations, in association with chert and magnetite.
how to distinguish siderite from other carbonates? and specifically from sphalerite?
from other carbonates: brownish color and high specific gravity
from sphalerite: rhombohedral cleavage
in powder soluble by hot hcl, in fragments by hot hcl
rhodocrosite formula
MnCO3
rhodocrosite crystal form
- rarely rhombohedral maybe with cruved faces
- granular to compact
- cleavable, massive
rhodocrosite cleavage
rhombohedral perfect
rhodocrosite hardness
3.5 to 4
rhodocrosite specific gravity
3.5 to 3.7
rhodocrosite color
- shades of rose red, light pink to dark brown
- vitreous, transparent to translucent
rhodocrosite occurence
- hydrothermal veins with ore minerals of silver, lead, copper and of manganese deposits
how to distinguish rhodocrosite, specifically from rhodonite
in general: pink color and rhombohedral cleavage, soluble in hot HCl
from rhodonite: hardness of 4 vs rhodonite’s 6
smithsonite formula
ZnCO3
smithsonite crystal form
- usually reniform, botryoidal, stalactitic
- rarely in small rhomboheral or scalenohedral crystals
- crystalline incrustations, granular to earthy
smithsonite cleavage
rhombohedral perfect
smithsonite hardness
4 to 4.5
smithsonite specific gravity
4.3 to 4.45
smithsonite color
- vitreous, translucent to colorless, white
- dirty brown, pink, green, blue
- yellow variety is called turkey-fat ore
smithsonite occurence
(is a zinc ore of supergene origin)
- in zinc deposits in limestone
- associated with limonite, cerussite, calcite, galena, sphalerite, hemimorphite
smithsonite remarks
- its reniform, botryoidal and stalactitic habit
- powdered form reacts with hot HCl
- distinguish from other carbonates with its 4 hardness and high specific gravity
