**1. Introduction**

Vaterite is the least thermodynamically stable anhydrous calcium carbonate polymorph and it easily transforms into more stable calcite or aragonite in the presence of water. This form of calcium carbonate mineral was named to honor the German chemist and mineralogist, Heinrich Vater, in 1903.

Because of its instability, the existence of vaterite is very rare in nature. It have been found in some sediments and rocks [1], e.g., as a major constituent of a carbonated calcium silicate hydrogel complex formed from larnite in Ballycraigy, Ireland [2]. Vaterite can be precipitated in some mineral springs when specific glacial conditions take place [3]. Also, vaterite crystals have been identified in materials produced by living organisms, e.g., otolith organs of fishes [4–6], spicules of the ascidian Herdmania momus [6,7], freshwater pearls, crustacean tissues, or bird eggs [6,8] as well as the chalky crust on the surface of leaves of the alpine plant, *Saxifraga scardica* [9].

Synthetic vaterite particles have been used as a carrier of active compounds for medical treatments [10–14]. They have been tested as a template for biodegradable polymer capsules, which can be used for applications in nanomedicine [10,11,15]. Also, vaterite particles are added to personal care products as abrasives, adsorbents, anticaking agents, buffers, or dyes [15]. Due to their unique optical properties, vaterite microspheres has been useful in microrheology and microfluidics [16]. Spherical vaterite particles have been used to generate flow within microfluidic channels that has allowed the creation of an optical driven pump [17]. This polymorphic CaCO3 form has been proposed as a coating pigment for ink jet paper [18]. The main advantages of vaterite particles are their easy and low-cost preparation, ability to design particles with defined characteristics, porous structure, mild conditions for decomposition, non-toxicity, and biocompatibility [11,15].

Recently, this unstable mineral was widely investigated to identify the favorable conditions for its production as well as to verify its usefulness for various applications. Therefore, the current opinions on vaterite synthesis as well as a discussion on the variables affecting its formation are overviewed. The issues raised in this review are presented in the diagram in Figure 1.

**Figure 1.** Diagrammatic representation of raised issues.
