839) Minerals and Rocks: A two Stamp set from Croatia issued by Hrvakska Posta (or Croatia Post) on 24.10.2018:

839) Minerals and Rocks: A two Stamp set   from Croatia issued by Hrvakska Posta (or Croatia Post) on 24.10.2018: 

About Minerals and Rocks:

The science that explores the creation and classification of rocks is called Petrology , while the science that explores what mineral rocks consists of is called Minerology.

Rocks make up the solid part of the Earth called Lithosphere and it encompasses the crust and the upper mantle reaching depths of around 100 kilometres. 

Below the Lithosphere there is 2,900 kilometres of thick outer core. 

The inner core reaches the depth of 6,370 kilometres to the centre of the Earth.

Rocks are aggregates of one or more types of minerals , while Minerals are the result of natural physical-chemical processes of a certain chemical composition and a crystal structure stable at certain temperature and pressure conditions.

Proper inner mineral structure is also reflected in their external shape. Such proper shapes are called Crystals. 

Rocks, according to their creation are classified as Igneous, Sedimentary & Metamorphic. Around 95% of the Lithosphere consists of Igneous rocks, while the remaining 5% consists of the other two types.

Rocks that consist of just a single mineral are called mono-mineral rocks, while rocks that consist of several minerals are called poly-mineral rocks.

Minerals (more than 4,700 are known today) are divided into 14 classes according to their chemical composition and structural characteristics - Native Elements, Sulphides, Sulphates, Halides, Oxides and Hydroxides, Carbonates, Nitrates, Borates, Iodates, Phosphates, Arsenates and Vanadates, Wolframites and Molybdates, Organic Compounds and a large group of Silicates that make up 95% of the Earth's crust.

QUARTZ "cadavac" - MOSLAVACKA GORA, CROATIA:

Quartz (SiO2) is one of the most common minerals found in the Earth's crust. 

Even though Quartz has been classified as an oxide based on its chemical composition, it belongs to the Silicate class because its regular internal structure matches the tecto-silicate structure. Quartz crystallises in a hexagon shape and often appears in the form of prismatic crystals whose ends have two rhomboidal surfaces. Also it can have a granular or microcrystal aggregates  (agate, chalcedony, carnelian, chrysoprase, onyx etc) and it is transparent to semi-transparent, colourless to multi-coloured, glassy to dimly bright & so on.

Transparent quartz which has nice colours is often used to make jewellery.

The permatite veins of the granite that make up the main section of the Moslavacka gora, there are crystals and conglomerate pieces of transparent yellow to brown quartz, particularly close to the Moslavina villages of Hruskovica, Gornie Jelenje, Mustafina Klada and Gornia Vlahinicka.

AUGITE DIABASE CONGLOMERATE - Island of Brusnik, Croatia:

Apart from the Island of Jabuka, Brusnik is the second island in the Adriatic Sea that was mostly built on magmatic rocks. Unlike the high Island of Jabuka, Brusnik is a low island located three kilometres South-east of the Island of Svetac.

Nearly the entire Brusnik Island and cliffs close to its shore are made up of conglomerates, while the cliffs of massive diabase (black eruptive stones used to make whetstones or "brus" which give the island its name), criss-crossed by cracks are found in its central section. The Brusnik conglomerates have a predominance of feldspars and augite and amphiboles and biotite as ancillary minerals. In addition epidote, titanite and prehnite are also found.

The rocks of both Brusnik & Jabuka were created by sub-volcanic magma of very similar/same  compositions, probably occurring at the same time. The conglomerates were later created by the long term effects of sea-waves from broken pieces of rocks suck as diabase.

Brusnik has been protected as a geological nature monument since 1951.

The Commemorative Stamp set:

The Miniature sheet (MS) bearing the two stamps, the first one at left portraying the Moslavacka Gora minerals and the second one at right portraying the Brusnik rocks conglomerates. The MS is titled "MINERALI STIJENE"

The First Day Cover (FDC) bearing the title "MINERALI STIJENE" ai left and the two stamps at top right. The Cancellation Handstamp is of PAVI DAN" and bears the date "24.10.2018"


Technical Details:

Stamp Issue Date: 24.10.2018

Designer: Dean Roksandic

Printer: AKD d.o.o., Zagreb

Process: Offset Printing

Colours: Multicolour

Sizes:
Stamp Size: 29.82 mm x 35.50 mm; 
Souvenir Sheet Size: 112 mm x 73 mm

Values: 5 HRK x 2