Page 149 - A Practical Introduction to Optical Mineralogy

THE NON-SILICATES CARBONATES

as a secondary mineral on alteration of ferromagnesian minerals by late
Calcite CaCO, trigonal, cia 0.8543
stage hydrothermal solutions in which C02 is present.
no 1.658
n. 1.486 trigonal, cia 0.80
8 0.172 Dolomite CaMg(C0,)2
Uniaxial -ve 1.679
n 0
D = 2.715 H = 3 n. 1.500
8 0.179
COLOUR Colourless.
Uniaxial - ve
HABIT Often as shapeless grains (anhedral); occasional rhombohedral outline
D = 2.86 H = 31/2-4
seen in sedimentary limestones.
CLEAVAGE Perfect { 1010} rhombohedral cleavage- three cleavage traces seen in coLOUR Colourless.
some sections. *HABIT Usually subhedral, but dolomitisation of limestones often leads to
*RELIEF Moderate with extreme variation because of large birefringence. Note euhedral crystals occurring as rhombohedra with curved faces (baroque
that the refractive indices cover a range of values which 'bracket' 1.54. dolomite).
The crystal is said to' twinkle' during rotation. Prismatic crystals parallel cLEAVAGE Perfect { 1011} rhombohedral, as calcite.
to the c axis are length fast (n.). RELIEF Low to moderate (variable with optic orientation).
*BIREFRINGENCE} Extremely high with pale pinks and greens of fourth order and higher. *BIREFRINGENCE Extremely high, even higher than calcite (almost colourless but slight
INTERFERENCE Because of the large birefringence, grains show moderate order inter- iridescence gives indication of extreme birefringence).
FIGURE terence colours even when the optic axis is near vertical, and these can *TwiNNING Similar to calcite, i.e. multiple on {0221}. The twin lamellae show
be used to obtain a uniaxial interference figure. birefringence of a lower order than the crystal.
2
2
TWINNING { 01 i2} common, appearing as multiple twins, { 0001} common, simple zoNING Commonly encountered, Fe • substitution of Mg • (ankerite).
twin plane. occuRRENCE Note that dolomite is also a name given to rock consisting mainly of
occuRRENCE One of the most common non-silicate minerals. It is a principal con- dolomite. Dolomite occasionally occurs as a primary mineral in
stituent of sedimentary limestones, occurring as carbonate shell sedimentary rocks and is often associated with evaporite deposits. As a
material, as fine precipitates, and as clastic material. Shells generally are secondary mineral dolomite is formed during dolomitisation of a lime-
composed of calcite or aragonite. Aragonite usually occurs as the initial stone shortly after deposition, and before consolidation. Another type
carbonate precipitate but it eventually recrystallises to calcite. of dolomitisation occurs after consolidation of a limestone if Mg-rich
On metamorphism pure calcitic limestone recrystallises to marble in solutions enter the rock. Dolomite is currently forming in certain saline
which the calcite grains are welded together in a mosaic; in impure lakes. The formation of primary and secondary dolomite may be due to
limestones the calcite combines with impurites present to give new the marine environment changing from deep to shallow water with
minerals, the type of mineral depending upon the temperature and C0 2 increasing salinity. Dolomite can occur as a gangue mineral with fluorite,
pressure. The reaction barite, calcite, quartz or siderite in association with lead and zinc sul-
phides. Dolomite rock is commonly associated with serpentines and
calcite + silica-+ wollastonite (CaSiO,) + C0 2 other ultramafic rocks and it is common in ophiolite suites. During
metamorphism dolomitic marbles may crystallise from dolomitic lime-
occurs at = 600 oc at low pressures but the same reaction occurs at over stones. At higher grades of metamorphism the dolomite eventually
800 °C as the pressure increases. Calcite can also occur with calc-silicate breaks down to give periclase MgO, with brucite, Mg(OH)2 forming on
minerals such as diopside, garnet (Ca-rich) and idocrase (vesuvianite) in hydration.
metamorphic rocks.
Calcite may occur in vugs or cavities in igneous rocks, being deposited Siderite FeCO, trigonal, cia 0.8184
by late stage hydrothermal solutions. In hydrothermal veins, calcite is a
no 1.782
common gangue mineral, often being found with fluorite, barite or
n. 1.575
quartz and in association with the sulphide ore minerals. Calcite may
8 0.207
occur as a primary crystallising mineral in some igneous rocks, particu-
Uniaxial - ve
larly carbonatites and some nepheline-syenites. Calcite may also occur
D = 3.50 H = 4+
136 137