·
CALCITE
·
MARBLE
·
DOLOMITE
·
TALC
CALCITE:
Named by Haidinger
in 1845 after a Greek root meaning to reduce a powder by heat ( also Latic
Calx, burnt lime )
Calcite CaCO3 Crystal system Trigonal. Habit Crystals common and very varied in habit, more
so than any other mineral. The commonest habits are tabular; prismatic; acute
or obtuse rhombohedral ; and scalenohedral (dog tooth aggregates, or as
granular, stalactite or massive aggregates. Twinning Common: there are two
laws, in the first the twin plane is the basal pinacoid,
and in the second the twin plane is a rhombohedra! Face Lamellar twinning may
also be produced by pressure. SG 2.7 (when pure) Hardness 3 Cleavage
Rhombohedral, perfect. Although the habit of calcite is so variable, it always
cleaves into rhombohedral cleavage fragments. The phenomenon known as double
refraction is well shown by calcite, in that if a clear cleavage rhomb of
calcite is placed over a dark spot on paper, two images are seen. If the
calcite rhomb is rotated, one image remains
stationary and the other rotates with the rhomb. Fracture Conchoidal but
rarely seen owing to perfection of cleavage. Color and transparency usually
colorless (Iceland spar) or white; also shades of grey, yellow, green, red,
purple, blue and even brown or black; transparent to translucent: some deeply colored
forms nearly opaque.
Streak White
Lustre Vitreous, sometimes pearly parallel to cleavage. Hardness, dissolves
readily with effervescence in cold dilute hydrochloric acid. Occurrence calcite
used as a common and widely distributed mineral. It is a rock forming mineral
that is a major constituent in calcareous sedimentary rocks (limestone) and
metamorphic rocks (marble). It may be precipitated directly from sea water, and
it forms the shells of many living organisms which, on death accumulate to form
limestone. metamorphosed limestone, when pure, forms pure, forms white granular
marble; the presence of other minerals results in colored, figured marble. It
occurs also as a primary mineral in Carbonatites,
which are calcareous igneous rock that form intrusive plugs. It is of common
occurrence in veins, either as the main constituent or as a gangue mineral
accompanying metallic ores. Secondary calcite sometimes replaces primary
minerals such as pyroxenes or feldspar in igneous rock.
In areas of hot springs it is deposited as
travertine or tufa, and stalactites and stalagmites of calcite are common in
caves in limestone areas. Calcite, in the form of limestone, is quarried on a
large scale for use in the making of cement, as a flux in the smelting of metallic
ores, as a fertilizer and as a building stone.
MARBLE :
Color white or
grey but a wider range of black, red and green occurs, often in streak and patches.
Texture medium -to coarse-grained; granular; often of a sugary appearance.
structure sedimentary structures abundant at low metamorphic grades, but with
increasing recrystalization they are destroyed. Mineralogy essentially calcite,
but may contain greater or lesser amounts of dolomite. Some brucite, olivine,
serpentine, tremolite, silicate rock and skarn. Marbles are readily scratched
with a knife, which serves to distinguish them from the much harder white
quartzites. Field Relations: Marbles are produced by the metamorphism of
limestone around igneous intrusions. They are thus found in the vicinity of
such intrusions but can usually be traced into unmetamorphosed limestone. They
are associated with rocks such as hornfelses.
DOLOMITE :
Dolomite Named
in 1791 after Deodat Guy Silvain
Tancrede Gratet de Dolomieu French Engineer and Mineralogist. Crystal
System Trigonal. Habit Crystals usually
rhombohedral with curved composite faces; also occurs in massive, granular
aggregates and as a rock- forming mineral in dolomitic
limestones. Twinning common. SG 2.8-2.9. Hardness 3.5
-4 Cleavage Rhombohedral, perfect. Fracture subconchoidal.
Colour and Transparency Usually white, though
sometimes colorless; also yellowish to brown, occasionally pink; transparent to
translucent. Streak White Lustre Vitreous to pearly. Distinguishing features
Dolomite is similar to calcite but dissolve only slowly in cold dilute
hydrochloric acid, but effervesces readily when warmed. Occurrence-Dolomite
occurs widely as a rock-forming mineral. It is usually of secondary occurrence,
having formed by the action of magnesium-bearing solutions on limestone. It
also occurs as a gangue mineral in hydrothermal veins, particularly those
containing galena and sphalerite. Dolomitic
limestones are used as building stones, and the
mineral is used in the manufacture of refractory bricks furnace linings.
Dolomite is named after Dolomieu (1750-1801), a
French mineralogist.
TALC:
Talc Mg3 Si4O10
(OH)2 the name has
been in use since antiquity and is derived from the arabic.
The name kerolite, applied to a hydrated and
layer-disordered variety, is from the Greek Keros,
Wax and Lithos stone.
Talc is
exclusively used as a filler in paints, rubber, soaps
and cosmetics and as a lubricating dusting powder and also as a ornamental
stone for carvings.
