carbonate - calcite group

Question 1

calcite chemical formula

Answer 1

CaCO3

Question 2

crystal form of calcite

Answer 2

• 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).

Question 3

calcite cleavage

Answer 3

rhombohedral perfect 74*55’

Question 4

calcite hardness

Answer 4

3

Question 5

calcite specific gravity

Answer 5

2.71

Question 6

calcite color

Answer 6

• 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

Question 7

Calcite occurence

Answer 7

• 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)

Question 8

how to differentiate calcite from dolomite and aragonite

Answer 8

from dolomite: coarse calcite freely effervesces in cold dilute HCl
from aragonite: lower specific gravity and rhombohedral cleavage

Question 9

magnesite formula

Answer 9

MgCO3

Question 10

magnesite crystal form

Answer 10

• Rarely in rhombohedral crystals
• Usually cryptocrystalline in white, compact, earthy masses
• also in cleavable granular masses, coarse to fine.

Question 11

magnesite cleavage

Answer 11

rhombohedral perfect

Question 12

magnesite hardness

Answer 12

3.5 to 5

Question 13

magnesite specific gravity

Answer 13

3 to 3.2

Question 14

magnesite color

Answer 14

• Luster vitreous. Transparent to translucent.
• white gray yellow brown

Question 15

magnesite occurence

Answer 15

• 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.

Question 16

how to distinguish magnesite from chert and dolomite?

Answer 16

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

Question 17

siderite formula

Answer 17

FeCO3

Question 18

siderite crystal form

Answer 18

• Commonly in rhombohedral crystals, which may show curved faces
• in globular concretions and
• cleavable masses
• May be botryoidal, compact, and earthy.

Question 19

siderite cleavage

Answer 19

rhombohedral perfect

Question 20

siderite hardness

Answer 20

3.5 to 4

Question 21

siderite specific gravity

Answer 21

3.96 (pure)

Question 22

siderite color

Answer 22

• Luster vitreous. Transparent to translucent.
• light to dark brown

Question 23

siderite occurence

Answer 23

• 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.

Question 24

how to distinguish siderite from other carbonates? and specifically from sphalerite?

Answer 24

from other carbonates: brownish color and high specific gravity
from sphalerite: rhombohedral cleavage

in powder soluble by hot hcl, in fragments by hot hcl

Question 25

rhodocrosite formula

Answer 25

MnCO3

Question 26

rhodocrosite crystal form

Answer 26

- rarely rhombohedral maybe with cruved faces - granular to compact - cleavable, massive

Question 27

rhodocrosite cleavage

Answer 27

rhombohedral perfect

Question 28

rhodocrosite hardness

Answer 28

3.5 to 4

Question 29

rhodocrosite specific gravity

Answer 29

3.5 to 3.7

Question 30

rhodocrosite color

Answer 30

- shades of rose red, light pink to dark brown - vitreous, transparent to translucent

Question 31

rhodocrosite occurence

Answer 31

- hydrothermal veins with ore minerals of silver, lead, copper and of manganese deposits

Question 32

how to distinguish rhodocrosite, specifically from rhodonite

Answer 32

in general: pink color and rhombohedral cleavage, soluble in hot HCl from rhodonite: hardness of 4 vs rhodonite’s 6

Question 33

smithsonite formula

Answer 33

ZnCO3

Question 34

smithsonite crystal form

Answer 34

- usually reniform, botryoidal, stalactitic - rarely in small rhomboheral or scalenohedral crystals - crystalline incrustations, granular to earthy

Question 35

smithsonite cleavage

Answer 35

rhombohedral perfect

Question 36

smithsonite hardness

Answer 36

4 to 4.5

Question 37

smithsonite specific gravity

Answer 37

4.3 to 4.45

Question 38

smithsonite color

Answer 38

- vitreous, translucent to colorless, white - dirty brown, pink, green, blue - yellow variety is called turkey-fat ore

Question 39

smithsonite occurence

Answer 39

(is a zinc ore of supergene origin) - in zinc deposits in limestone - associated with limonite, cerussite, calcite, galena, sphalerite, hemimorphite

Question 40

smithsonite remarks

Answer 40

- its reniform, botryoidal and stalactitic habit - powdered form reacts with hot HCl - distinguish from other carbonates with its 4 hardness and high specific gravity